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About the IEC, Vision and Mission. The IEC is one of three global sister organizations (IEC, ISO, ITU) that develop International Standards for the world.

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The pH scale is something we're all familiar with; most people will remember it from school chemistry lessons. It's the scale used to rank how strong an acid (or alkali) a solution is. The colours associated with each number correspond to the colour that universal indicator turns in solutions of that particular pH. A fair proportion of people probably don't know the chemistry behind the pH scale, though - where exactly do these numbers come from?

The clues are actually partly hidden in the scale's name. The 'H' in pH stands for the element, hydrogen. On a simple level, the pH scale can be thought of as a ranking of the amount of hydrogen ions in a solution: the more hydrogen ions, the lower the pH number. The 'p' in pH, to chemists at least, stands for the mathematical operation '-log ₁₀'. pH, then, is simply equal to -log ₁₀[H⁺], where [H⁺] is the hydrogen ion concentration in a particular solution. (Note that, strictly speaking, we'd usually use H ₃O⁺ to represent hydrogen ions, as this is the form they take in solutions. However, to keep things simple, we'll continue to use H⁺ here).

Looking at the graphic above, you can see that an increase in pH of one point actually involves a tenfold decrease in the concentration of hydrogen ions in a solution. The scale is what's known as a logarithmic scale. Why do we bother with this mathematical manipulation in the first place? Well, it's clearly a lot easier to state a single number when referring to the acidity (or alkalinity) of a solution than it is to quote the many-numbered hydrogen ion concentration. A pH of spot on 7 denotes a neutral solution (neither acidic or alkaline). Any pH below 7 is acidic, whilst any pH above 7 is termed alkaline.

Water molecules have the chemical formula H ₂O. However, these molecules are capable of splitting up slightly in solution, in H⁺ and OH ⁻ (hydroxide) ions. In a neutral solution, the concentrations of these two ions are equal. However, the addition of an acid or alkali can cause them to vary. Acids are a source of hydrogen ions, and adding them to water increases the concentration of hydrogen ions in solution, lowering the concentration of hydroxide ions. For alkalis, the opposite is true: they decrease the concentration of hydrogen ions, whilst increasing the concentration of hydroxide ions.

Something a lot of people don't realise is that pH is temperature dependent. Strictly speaking, pure water only has a pH of 7 at 'room temperature' (25˚C). Above and below this temperature, it can vary: for example, at 100˚C, the pH of pure water is 6.14, whilst at 0˚C, it's 7.47. This doesn't mean that the pure water is becoming acidic or alkaline, but that, at these temperatures, those particular pH numbers represent the neutral point. As a side note, it's also worth pointing out that the pH scale isn't limited to the usual 0-14 range shown here - some strong acids and alkalis can fall outside this range, into negative pH values or values higher than 14.

Another common misconception about pH concerns the human body. Diets such as the alkaline diet claim that it is possible to affect the pH value of your body by altering your diet to include 'alkalising' foodstuffs that make the pH of your body more alkaline. Whilst what's actually suggested, a diet heavy in fruits and vegetables, is certainly by no means unhealthy, it's pretty much impossible for what you eat to change the pH of your body. I'm not going to go into too much detail, as Kat from The Chronicle Flask blog has already done an excellent job of debunking alkaline diets, but it's worth reiterating a few key points.

The pH of the stomach can vary, between 1.5 and 3.5 on the pH scale. However, this has no effect on the pH of our body, or, more specifically, our blood. Human blood has a pH value that's always slightly alkaline, between 7.35-7.45. If we were able to purposefully change the pH of blood outside of this small range, we could actually cause ourselves a good deal of harm; even a pH change of 0.5 either way could result in irreversible cell damage. Luckily, the food we eat has next to no effect on blood pH.

What we eat can affect the pH of our urine. The pH of urine has an average value of around 6, but can range between 4.5 and 8. However, whilst you might be able to have the slightest effect on your urinary pH with your diet, it's unrelated to the pH of your blood; this stays in the range previously stated, regardless of any change in urinary pH.

Moving on, and whilst we're talking about acids, it's worth talking about the strength of acids. Strictly speaking, it's hard to place particular chemical compounds at specific points on the pH scale, as their positions vary depending on concentration. Concentration is a measure of how much of a substance is dissolved in a solution. If we have a lot of an acid dissolved in a relatively small amount of water, we'd say we have a solution with a high concentration. Similarly, if we had very little acid dissolved in lot of water, we'd have a solution with a low concentration.

Since concentration of solutions can easily be varied, solutions of varied concentration of the same acid can have different pH values. Some acids, however, are stronger than others. Hydrochloric acid, the same acid found in stomach acid, is a strong acid as it can easily split up into its component ions. On the other hand, acetic acid, the acid found in vinegar, is a comparatively weak acid - it doesn't split up into its component ions easily. Another example of a weak acid is hydrofluoric acid; contrary to what a certain popular chemistry-themed TV show would have you believe, it's actually a fairly weak acid, which certainly couldn't dissolve a body. It's an unpleasant compound for different reasons though. We can rank acids in terms of their strength (or how easily they split up into their component ions), but that's a topic for another post!
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References & Further Reading

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The pH scale is something we're all familiar with; most people will remember it from school chemistry lessons. It's the scale used to rank how strong an acid (or alkali) a solution is. The colours associated with each number correspond to the colour that universal indicator turns in solutions of that particular pH.

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A free game-based learning resource for practicing the chemical symbols of the Periodic Table of Elements. The game includes the first 92 elements on the periodic table.

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Actually, the pH scale is not limited to 0-14, but most common solutions fall into this range. The pH of a solution is calculated as the negative base-10 logarithm of the hydronium ion (H_3O^+) concentration in solution.

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About this unit

This unit is part of the Chemistry library. Browse videos, articles, and exercises by topic.

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Periodic table | Learn atomic structure & periodic trends | Khan Academy

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This unit is part of the Chemistry library. Browse videos, articles, and exercises by topic.

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Sulfur, Chemical Element - reaction, water, uses, elements, proteins, examples, metal, gas

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Sulfur exists in two allotropic forms. Allotropes are forms of an element with different physical and chemical properties. The two forms of sulfur are known as α-form and β-form (the Greek letters alpha and beta, respectively). Both allotropes are yellow, with the α-form a brighter yellow and the β-form a paler, whitish-yellow.

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Periodic Table of Elements and Chemistry

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Award winning periodic table with user-friendly element data and facts. Cool online chemistry videos, dictionary, tools, etc.

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Yes indeed. Helium does strange things to your voice because it is much less dense than air.

In your throat it is acting a bit like a musical instrument. You get sound waves vibrating backwards and forwards, up and down above your vocal cords. That gives rich timbre to your voice.

It picks which frequencies of your voice to amplify.

Helium is a much lower density gas than air, and that means that sound travels much faster in it.

Then your throat will vibrate at much higher frequencies. It will amplify the sounds at much higher frequencies.

To reverse the helium effect you'll need a much denser gas than air.

There are a couple of good ones. Xenon would work beautifully, which is a noble gas, very safe. Another good one to try is sulphur hexafluoride. Both of these are much denser than air so you'll amplify the much deeper sounds in your voice.

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Is there a gas that can do the opposite of helium to the voice?

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Yes indeed. Helium does strange things to your voice because it is much less dense than air.In your throat it is acting a bit like a musical instrument. You get sound waves vibrating backwards and forwards, up and down above your vocal cords.

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A "cesium(-beam) atomic clock" (or "cesium-beam frequency standard") is a device that uses as a reference the exact frequency of the microwave spectral line emitted by atoms of the metallic element cesium, in particular its isotope of atomic weight 133 ("Cs-133").

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Four major facts underlie the accuracy of Atomic clocks :

1. The output frequency is locked to the energy level difference of free atoms.
2. The atoms are isolated from their surroundings (minimum interaction/ collisions with other atoms, photons, fields, etc.)
3. Long interrogation time reduces frequency uncertainty (by Heisenberg principle: (f ~ 1/(t).
4. The atoms' thermal motion is reduced as much as possible (eliminating Doppler shift of the 1st and 2nd order).

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What is a Rubidium Atomic Clock?

A rubidium clock is an atomic clock which uses rubidium atoms and an "optical pumping" technique to measure the atomic transition frequency. A rubidium Atomic clock includes 87Rb isotope confined in a vapor cell.
The atoms are excited to a high energy level (in figure #1) by light emitted by other 87Rb atoms. The light is filtered to achieve the required wavelength,thereby increasing the population of level 2 compare to level 1.

An injected microwave frequency of ~6.84GHz induces transition from level 2 to level 1 increasing the number of atoms in level 1, and yielding more absorption of light. A photodiode detects the reduction in the light intensity. This reduction occurs only when the injected microwave frequency equals precisely the transition frequency of the atoms. The injected frequency is derived from a crystal oscillator. The photodiode detection signal is then used to tune the crystal oscillator frequency. In other words, the crystal oscillator frequency is locked to the frequency of the waves emitted by the atoms.

1. Light from a 87Rb lamp contains both 3-1 and 2-1 lines.
2. A 85Rb cell filters the 2-1 line out, and transmits the 3-1 line only.
3. The filtered light "optically pumps" the 87Rb atoms in the resonance
4. cell from level 1 to 3.
5. A microwave signal at a frequency of 6.84 GHz is injected into the cell and induces transition from level 2 to 1.
6. When the injected wave frequency matches the difference of the 2-1 line, the
7. photo-diode detects a reduction in the intensity of the transmitted light.

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rubidium clock, rubidium, atomic clock | Accubeat GPS

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Four major facts underlie the accuracy of Atomic clocks : 1. The output frequency is locked to the energy level difference of free atoms. 2. The atoms are isolated from their surroundings (minimum interaction/ collisions with other atoms, photons, fields, etc.) 3. Long interrogation time reduces frequency uncertainty (by Heisenberg principle: (f ~ 1/(t).

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A phase-locked loop (PLL) is an electronic circuit with a voltage or voltage-driven oscillator that constantly adjusts to match the frequency of an input signal. PLLs are used to generate, stabilize, modulate, demodulate, filter or recover a signal from a "noisy" communications channel where data has been interrupted.

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A phase-locked loop (PLL) is an electronic circuit with a voltage or voltage-driven oscillator that constantly adjusts to match the frequency of an input signal. PLLs are used to generate, stabilize, modulate, demodulate, filter or recover a signal from a "noisy" communications channel where data has been interrupted.

Where phase-locked loops are used

PLLs are used in telecommunications, computers, radio and other electronic applications. They are frequently used in wireless communication, primarily on frequency modulation ( FM) or phase modulation ( PM) transmissions. Phase-locked loops are more commonly used for digital data transmission than for analog transmission and are more commonly manufactured as integrated circuits, although discrete circuits are used for microwave signal processing.

How phase-locked loops work

PLLs work by constantly adjusting a voltage or current-driven oscillator to match (lock onto) the phase and frequency of an input signal, which typically consists of a voltage-controlled oscillator (VCO) tuned using a special semiconductor diode called a varactor. The VCO is initially tuned to a frequency close to the desired receiving or transmitting frequency. A circuit called a phase comparator causes the VCO to seek and lock onto the desired frequency, which is set via a crystal-controlled reference oscillator. When the VCO frequency differs from the reference frequency, the phase comparator produces an error voltage. The comparator output is usually run through a low-pass filter (a signal filter that reduces the strength of high-frequency waves) to further reduce noise. The filtered output is fed back to the varactor to continually push the VCO toward the reference frequency.

The filtered output of the comparator also provides the output of the circuit -- the signal found in the transmission (the voice, video or data). Since the signal is encoded by modulating a carrier wave, it can be thought of as the difference between the carrier waveform and the actual transmitted waveform, and can therefore be found in the output of the comparator.

Together, the phase-locked loop, VCO, reference oscillator and phase comparator comprise a frequency synthesizer -- an electronic system that produces a range of frequencies from a single fixed oscillator. Wireless equipment that uses this type of frequency control is said to be frequency-synthesized. Other frequency-synthesized devices include mobile phones, satellite receivers and GPS systems.

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What is phase-locked loop? - Definition from WhatIs.com

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A phase-locked loop (PLL) is an electronic circuit with a voltage or voltage-driven oscillator that constantly adjusts to match the frequency of an input signal. PLLs are used to generate, stabilize, modulate, demodulate, filter or recover a signal from a "noisy" communications channel where data has been interrupted.

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A diode is a specialized electronic component with two electrodes called the anode and the cathode. Most diodes are made with semiconductor materials such as silicon, germanium, or selenium. Some diodes are comprised of metal electrodes in a chamber evacuated or filled with a pure elemental gas at low pressure. Diodes can be used as rectifiers, signal limiters, voltage regulators, switches, signal modulators, signal mixers, signal demodulators, and oscillators.

The fundamental property of a diode is its tendency to conduct electric current in only one direction. When the cathode is negatively charged relative to the anode at a voltage greater than a certain minimum called forward breakover, then current flows through the diode. If the cathode is positive with respect to the anode, is at the same voltage as the anode, or is negative by an amount less than the forward breakover voltage, then the diode does not conduct current. This is a simplistic view, but is true for diodes operating as rectifiers, switches, and limiters. The forward breakover voltage is approximately six tenths of a volt (0.6 V) for silicon devices, 0.3 V for germanium devices, and 1 V for selenium devices.

The above general rule notwithstanding, if the cathode voltage is positive relative to the anode voltage by a great enough amount, the diode will conduct current. The voltage required to produce this phenomenon, known as the avalanche voltage, varies greatly depending on the nature of the semiconductor material from which the device is fabricated. The avalanche voltage can range from a few volts up to several hundred volts.

When an analog signal passes through a diode operating at or near its forward breakover point, the signal waveform is distorted. This nonlinearity allows for modulation, demodulation, and signal mixing. In addition, signals are generated at harmonics, or integral multiples of the input frequency. Some diodes also have a characteristic that is imprecisely termed negative resistance. Diodes of this type, with the application of a voltage at the correct level and the polarity, generate analog signals at microwave radio frequencies.

Semiconductor diodes can be designed to produce direct current (DC) when visible light, infrared transmission (IR), or ultraviolet (UV) energy strikes them. These diodes are known as photovoltaic cells and are the basis for solar electric energy systems and photosensors. Yet another form of diode, commonly used in electronic and computer equipment, emits visible light or IR energy when current passes through it. Such a device is the familiar light-emitting diode (LED).

See an introductory tutorial on diodes and semiconductors:

Continue Reading About diode

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What is diode? - Definition from WhatIs.com

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A diode is a specialized electronic component with two electrodes called the anode and the cathode. Most diodes are made with semiconductor materials such as silicon, germanium, or selenium. Some diodes are comprised of metal electrodes in a chamber evacuated or filled with a pure elemental gas at low pressure.

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Diagram of a copper cathode in a galvanic cell (e.g., a battery). Positively charged cations move towards the cathode allowing a positive current i to flow out of the cathode.

A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic CCD for Cathode Current Departs. A conventional current describes the direction in which positive charges move. Electrons have a negative electrical charge, so the movement of electrons is opposite to that of the conventional current flow. Consequently, the mnemonic cathode current departs also means that electrons flow into the device's cathode from the external circuit. For example, the end of a household battery marked with a + (plus) is the cathode.

The electrode through which conventional current flows the other way, into the device, is termed an anode.

Conventional current flows from cathode to anode outside the cell or device (with electrons moving in the opposite direction), regardless of the cell or device type and operating mode.

Cathode polarity with respect to the anode can be positive or negative depending on how the device is being operated. Inside a device or a cell, positively charged cations always move towards the cathode and negatively charged anions move towards the anode, although cathode polarity depends on the device type, and can even vary according to the operating mode. Whether the cathode is negatively polarized (such as recharging a battery) or positively polarized (such as a battery in use), the cathode will draw electrons into it from outside, as well as attract positively charged cations from inside.

A battery or galvanic cell in use has a cathode that is the positive terminal since that is where conventional current flows out of the device. This outward current is carried internally by positive ions moving from the electrolyte to the positive cathode (chemical energy is responsible for this "uphill" motion). It is continued externally by electrons moving into the battery which constitutes positive current flowing outwards. For example, the Daniell galvanic cell's copper electrode is the positive terminal and the cathode.

A battery that is recharging or an electrolytic cell performing electrolysis has its cathode as the negative terminal, from which current exits the device and returns to the external generator as charge enters the battery/ cell. For example, reversing the current direction in a Daniell galvanic cell converts it into an electrolytic cell where the copper electrode is the positive terminal and also the anode.

In a diode, the cathode is the negative terminal at the pointed end of the arrow symbol, where current flows out of the device. Note: electrode naming for diodes is always based on the direction of the forward current (that of the arrow, in which the current flows "most easily"), even for types such as Zener diodes or solar cells where the current of interest is the reverse current. In vacuum tubes (including cathode-ray tubes) it is the negative terminal where electrons enter the device from the external circuit and proceed into the tube's near-vacuum, constituting a positive current flowing out of the device.

The word was coined in 1834 from the Greek κάθοδος ( kathodos), 'descent' or 'way down', by William Whewell, who had been consulted by Michael Faraday over some new names needed to complete a paper on the recently discovered process of electrolysis. In that paper Faraday explained that when an electrolytic cell is oriented so that electric current traverses the "decomposing body" (electrolyte) in a direction "from East to West, or, which will strengthen this help to the memory, that in which the sun appears to move", the cathode is where the current leaves the electrolyte, on the West side: " kata downwards, `odos a way ; the way which the sun sets".

The use of 'West' to mean the 'out' direction (actually 'out' → 'West' → 'sunset' → 'down', i.e. 'out of view') may appear unnecessarily contrived. Previously, as related in the first reference cited above, Faraday had used the more straightforward term "exode" (the doorway where the current exits). His motivation for changing it to something meaning 'the West electrode' (other candidates had been "westode", "occiode" and "dysiode") was to make it immune to a possible later change in the direction convention for current, whose exact nature was not known at the time. The reference he used to this effect was the Earth's magnetic field direction, which at that time was believed to be invariant. He fundamentally defined his arbitrary orientation for the cell as being that in which the internal current would run parallel to and in the same direction as a hypothetical magnetizing current loop around the local line of latitude which would induce a magnetic dipole field oriented like the Earth's. This made the internal current East to West as previously mentioned, but in the event of a later convention change it would have become West to East, so that the West electrode would not have been the 'way out' any more. Therefore, "exode" would have become inappropriate, whereas "cathode" meaning 'West electrode' would have remained correct with respect to the unchanged direction of the actual phenomenon underlying the current, then unknown but, he thought, unambiguously defined by the magnetic reference. In retrospect the name change was unfortunate, not only because the Greek roots alone do not reveal the cathode's function any more, but more importantly because, as we now know, the Earth's magnetic field direction on which the "cathode" term is based is subject to reversals whereas the current direction convention on which the "exode" term was based has no reason to change in the future.

Since the later discovery of the electron, an easier to remember, and more durably technically correct (although historically false), etymology has been suggested: cathode, from the Greek kathodos, 'way down', 'the way (down) into the cell (or other device) for electrons'.

In chemistry, a cathode is the electrode of an electrochemical cell at which reduction occurs. The cathode can be negative like when the cell is electrolytic (where electrical energy provided to the cell is being used for decomposing chemical compounds); or positive as when the cell is galvanic (where chemical reactions are used for generating electrical energy). The cathode supplies electrons to the positively charged cations which flow to it from the electrolyte (even if the cell is galvanic, i.e., when the cathode is positive and therefore would be expected to repel the positively charged cations; this is due to electrode potential relative to the electrolyte solution being different for the anode and cathode metal/electrolyte systems in a galvanic cell).

The cathodic current, in electrochemistry, is the flow of electrons from the cathode interface to a species in solution. The anodic current is the flow of electrons into the anode from a species in solution.

In an electrolytic cell, the cathode is where the negative polarity is applied to drive the cell. Common results of reduction at the cathode are hydrogen gas or pure metal from metal ions. When discussing the relative reducing power of two redox agents, the couple for generating the more reducing species is said to be more "cathodic" with respect to the more easily reduced reagent.

In a galvanic cell, the cathode is where the positive pole is connected to allow the circuit to be completed: as the anode of the galvanic cell gives off electrons, they return from the circuit into the cell through the cathode.

When metal ions are reduced from ionic solution, they form a pure metal surface on the cathode. Items to be plated with pure metal are attached to and become part of the cathode in the electrolytic solution.

Glow from the directly heated cathode of a 1 kW power tetrode tube in a radio transmitter. The cathode filament is not directly visible

In a vacuum tube or electronic vacuum system, the cathode is a metal surface which emits free electrons into the evacuated space. Since the electrons are attracted to the positive nuclei of the metal atoms, they normally stay inside the metal and require energy to leave it; this is called the work function of the metal. Cathodes are induced to emit electrons by several mechanisms:

Thermionic emission: The cathode can be heated. The increased thermal motion of the metal atoms "knocks" electrons out of the surface, an effect called thermionic emission. This technique is used in most vacuum tubes.

Field electron emission: A strong electric field can be applied to the surface by placing an electrode with a high positive voltage near the cathode. The positively charged electrode attracts the electrons, causing some electrons to leave the cathode's surface. This process is used in cold cathodes in some electron microscopes, and in microelectronics fabrication,

Secondary emission: An electron, atom or molecule colliding with the surface of the cathode with enough energy can knock electrons out of the surface. These electrons are called secondary electrons. This mechanism is used in gas-discharge lamps such as neon lamps.

Photoelectric emission: Electrons can also be emitted from the electrodes of certain metals when light of frequency greater than the threshold frequency falls on it. This effect is called photoelectric emission, and the electrons produced are called photoelectrons. This effect is used in phototubes and image intensifier tubes.

Cathodes can be divided into two types:

Hot cathode

Schematic symbol used in circuit diagrams for vacuum tube, showing cathode

A hot cathode is a cathode that is heated by a filament to produce electrons by thermionic emission. The filament is a thin wire of a refractory metal like tungsten heated red-hot by an electric current passing through it. Before the advent of transistors in the 1960s, virtually all electronic equipment used hot-cathode vacuum tubes. Today hot cathodes are used in vacuum tubes in radio transmitters and microwave ovens, to produce the electron beams in older cathode-ray tube (CRT) type televisions and computer monitors, in x-ray generators, electron microscopes, and fluorescent tubes.

There are two types of hot cathodes:

Directly heated cathode: In this type, the filament itself is the cathode and emits the electrons directly. Directly heated cathodes were used in the first vacuum tubes, but today they are only used in fluorescent tubes, some large transmitting vacuum tubes, and all X-ray tubes.

Indirectly heated cathode: In this type, the filament is not the cathode but rather heats the cathode which then emits electrons. Indirectly heated cathodes are used in most devices today. For example, in most vacuum tubes the cathode is a nickel tube with the filament inside it, and the heat from the filament causes the outside surface of the tube to emit electrons. The filament of an indirectly heated cathode is usually called the heater. The main reason for using an indirectly heated cathode is to isolate the rest of the vacuum tube from the electric potential across the filament. Many vacuum tubes use alternating current to heat the filament. In a tube in which the filament itself was the cathode, the alternating electric field from the filament surface would affect the movement of the electrons and introduce hum into the tube output. It also allows the filaments in all the tubes in an electronic device to be tied together and supplied from the same current source, even though the cathodes they heat may be at different potentials.

In order to improve electron emission, cathodes are treated with chemicals, usually compounds of metals with a low work function. Treated cathodes require less surface area, lower temperatures and less power to supply the same cathode current. The untreated tungsten filaments used in early tubes (called "bright emitters") had to be heated to 1400 °C (~2500 °F), white-hot, to produce sufficient thermionic emission for use, while modern coated cathodes produce far more electrons at a given temperature so they only have to be heated to 425-600 °C (~800-1100 °F) () There are two main types of treated cathodes:

Cold cathode (lefthand electrode) in neon lamp

Coated cathode - In these the cathode is covered with a coating of alkali metal oxides, often barium and strontium oxide. These are used in low-power tubes.

Thoriated tungsten - In high-power tubes, ion bombardment can destroy the coating on a coated cathode. In these tubes a directly heated cathode consisting of a filament made of tungsten incorporating a small amount of thorium is used. The layer of thorium on the surface which reduces the work function of the cathode is continually replenished as it is lost by diffusion of thorium from the interior of the metal.[11]

Cold cathode

This is a cathode that is not heated by a filament. They may emit electrons by field electron emission, and in gas-filled tubes by secondary emission. Some examples are electrodes in neon lights, cold-cathode fluorescent lamps (CCFLs) used as backlights in laptops, thyratron tubes, and Crookes tubes. They do not necessarily operate at room temperature; in some devices the cathode is heated by the electron current flowing through it to a temperature at which thermionic emission occurs. For example, in some fluorescent tubes a momentary high voltage is applied to the electrodes to start the current through the tube; after starting the electrodes are heated enough by the current to keep emitting electrons to sustain the discharge.

Cold cathodes may also emit electrons by photoelectric emission. These are often called photocathodes and are used in phototubes used in scientific instruments and image intensifier tubes used in night vision goggles.

In a semiconductor diode, the cathode is the N-doped layer of the PN junction with a high density of free electrons due to doping, and an equal density of fixed positive charges, which are the dopants that have been thermally ionized. In the anode, the converse applies: It features a high density of free "holes" and consequently fixed negative dopants which have captured an electron (hence the origin of the holes).

When P and N-doped layers are created adjacent to each other, diffusion ensures that electrons flow from high to low density areas: That is, from the N to the P side. They leave behind the fixed positively charged dopants near the junction. Similarly, holes diffuse from P to N leaving behind fixed negative ionised dopants near the junction. These layers of fixed positive and negative charges are collectively known as the depletion layer because they are depleted of free electrons and holes. The depletion layer at the junction is at the origin of the diode's rectifying properties. This is due to the resulting internal field and corresponding potential barrier which inhibit current flow in reverse applied bias which increases the internal depletion layer field. Conversely, they allow it in forwards applied bias where the applied bias reduces the built in potential barrier.

Electrons which diffuse from the cathode into the P-doped layer, or anode, become what are termed "minority carriers" and tend to recombine there with the majority carriers, which are holes, on a timescale characteristic of the material which is the p-type minority carrier lifetime. Similarly, holes diffusing into the N-doped layer become minority carriers and tend to recombine with electrons. In equilibrium, with no applied bias, thermally assisted diffusion of electrons and holes in opposite directions across the depletion layer ensure a zero net current with electrons flowing from cathode to anode and recombining, and holes flowing from anode to cathode across the junction or depletion layer and recombining.

Like a typical diode, there is a fixed anode and cathode in a Zener diode, but it will conduct current in the reverse direction (electrons flow from anode to cathode) if its breakdown voltage or "Zener voltage" is exceeded.

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A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic CCD for Cathode Current Departs. A conventional current describes the direction in which positive charges move. Electrons have a negative electrical charge, so the movement of electrons is opposite to that of the conventional current flow.

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Electronics, Circuitry, Element Interactions, Communications Flashcards | Quizlet

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Start studying Electronics, Circuitry, Element Interactions, Communications. Learn vocabulary, terms, and more with flashcards, games, and other study tools.

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Resistor values are often indicated with color codes. Practically all leaded resistors with a power rating up to one watt are marked with color bands. The coding is defined in the international standard IEC 60062. This standard describes the marking codes for resistors and capacitors. It includes also numerical codes, as for example often used for SMD resistors. The color code is given by several bands. Together they specify the resistance value, the tolerance and sometimes the reliability or failure rate. The number of bands varies from three till six. As a minimum, two bands indicate the resistance value and one band serves as multiplier. The resistance values are standardized, these values are called preferred value.

Resistor color code chart

The chart below shows how to determine the resistance and tolerance for resistors. The table can also be used to specify the color of the bands when the values are known. An automatic resistor calculator can be used to quickly find the resistor values.

Tips for reading resistor codes

In the sections below examples are given for different numbers of bands, but first some tips are given to read the color code:

The reading direction might not always be clear. Sometimes the increased space between band 3 and 4 give away the reading direction. Also, the first band is usually the closest to a lead. A gold or silver band (the tolerance) is always the last band.

It is a good practice to check the manufacturer's documentation to be sure about the used coding system. Even better is to measure the resistance with a multi-meter. In some cases this might even be the only way to figure out the resistance; for example when the color bands are burnt off.

4 band resistor

The four band color code is the most common variation. These resistors have two bands for the resistance value, one multiplier and one tolerance band. In the example on the left these bands are green, blue, red and gold. By using the color code chart, one finds that green stands for 5 and blue for 6. The value is thus 56·100 =5600 Ω. The golden band means that the resistor has a tolerance of 5%. The resistance value lies therefore between 5320 and 5880 Ω.

If the tolerance band would be left blank, the result is a 3 band resistor. This means that the resistance value remains the same, but the tolerance is 20%.

5 band resistor

Resistors with high precision have an extra band to indicate a third significant digit. Therefore, the first three bands indicate the significant digits, the fourth band is the multiply factor and the fifth band represents the tolerance. There are exceptions to this. For example, sometimes the extra band indicates failure rate (military specification) or temperature coefficient (older or specialized resistors). Please read the section "Color code exceptions" for more information.

Shown example: brown (1), yellow (4), purple (7), black (x1), green (0.5%): 147 Ω 0.5%.

6 band resistor

Resistors with 6 bands are usually for high precision resistors that have an additional band to specify the temperature coefficient (ppm/K). The most common color for the sixth band is brown (100 ppm/K). This means that for a temperature change of 10 ˚C, the resistance value can change 0.1%. For special applications where temperature coefficient is critical other colors

Shown example: orange (3), red (2), brown (1), green (x10), brown (1%), red(50 ppm/K): 3.21 k Ω 1% 50 ppm/K.

Color code exceptions

Reliability band

Resistors that are produced according to military specifications, sometimes include an extra band to indicate reliability. This is specified in failure rate (%) per 1000 hours of service. This is rarely used in commercial electronics. Mostly the reliability band can be found on four band resistors. More information about the reliability can be found in the US military handbook MIL-HDBK-199.

Single black band or zero-ohm resistor

A resistor with a single black band is called a zero-ohm resistor. Principally it is a wire link with only function of connecting traces on a PCB. Using the resistor package has the advantage of being able to use the same automated machines to place components on a circuit board.

5 band resistor with a 4th band of gold or silver

Five band resistors with a fourth band of gold or silver form an exception, and are used on specialized and older resistors. The first two bands represent the significant digits, the 3th the multiply factor, the 4th the tolerance and the 5th the temperature coefficient (ppm/K).

Deviating colors

For high voltage resistors often the colors gold and silver are replaced with yellow and gray. This is to prevent having metal particles in the coating.

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Resistor values are often indicated with color codes. Practically all leaded resistors with a power rating up to one watt are marked with color bands. The coding is defined in the international standard IEC 60062. This standard describes the marking codes for resistors and capacitors.

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A basic background in nuclear physics for those who want to start at the beginning.
Some of the terms used in this factsheet can be found in IEER's on-line glossary.

A. Structure of the Atom

The atoms of which every element of matter is composed have a nucleus at the center and electrons whirling about this nucleus that can be visualized as planets circling around a sun, though it is impossible to locate them precisely within the atom. The nuclei of atoms are composed of protons, which have a positive electrical charge, and neutrons, which are electrically neutral. Electrons are electrically negative and have a charge equal in magnitude to that of a proton.

The number of electrons in an atom is normally equal to the number of protons in the nucleus. As a result, atoms of elements are normally electrically neutral. The mass of an atom lies almost entirely in its nucleus since protons and neutrons are far heavier than electrons.

Free neutrons are unstable particles which decay naturally into a proton and electron, with a half-life of about 12 minutes.

neutron ===> proton + electron + a neutrino

However, it is remarkable that neutrons, when they exist together with protons in the nucleus of atoms, are stable. Protons are about 1,836 times heavier than electrons, and neutrons are about 1,838 times heavier than electrons. The energy balance in the decay of a neutron is achieved by the
anti-neutrino, a neutral particle that carries off surplus energy as the
neutron decays. The nominal mass of an atom of an element is measured by the sum of the protons and neutrons in it. This integer is called the mass number. The nominal mass of an atom is not affected by the number of electrons, which are very light. Hence the nominal mass, based on the mass number, approximates the actual atomic mass. The number of protons in the nucleus, which determines the chemical properties of an element, is called the atomic number. Elements are arranged in ascending order of atomic number in an arrangement called the periodic table. The term derives from the tendency to periodicity of chemical properties deriving from arrangements of electrons in atoms.

B. Radioactive Decay

The nuclei of some elements are not stable. These nuclei are radioactive, in that they emit energy and particles, collectively called "radiation." All elements have at least some isotopes that are radioactive. All isotopes of heavy elements with mass numbers greater than 206 and atomic numbers greater than 83 are radioactive.

There are several ways in which unstable nuclei undergo radioactive decay:

Alpha decay, which the emission of a helium-4 nucleus containing two protons and two neutrons. This is the least penetrating form of radiation. It is stopped by the dead layer of skin and so does no harm when outside the body. But it is the most damaging form of radiation when deposited inside the body.

Beta decay, which the emission of an electron or a positron (a particle identical to an electron except that it has a positive electrical charge).

Electron capture, which is the capture by the nucleus of an electron from among the ones whirling around it. In effect, the electron combines with a proton to yield a neutron.

Spontaneous fission, which is the fission of a heavy element without input of any external particle or energy.

Often, there is still excess residual energy in the nucleus after the emission of a particle or after electron capture. Some of this residual energy after radioactive decay can be emitted in the form of high-frequency electromagnetic radiation, called gamma rays. Gamma rays are essentially like X-rays and are the most penetrating form of radiation. It should be noted that the emission of gamma rays does not change the mass number or atomic number of the nucleus - that is, unlike radioactive decay by emission of particles, spontaneous fission, or electron capture, it does not cause the transmutation of the nucleus into another element.

Each quantum, or unit, of a gamma ray (or other electromagnetic energy) is called a photon. Gamma rays are like light, except that they are much higher frequency electromagnetic rays. Photon energy is directly proportional to the frequency of the electromagnetic radiation. Photons of gamma rays can damage living cells by splitting molecules apart or ionizing elements in them.

Many heavy nuclei emit an energetic alpha particle when they decay. For instance uranium-238 decays into thorium-234 with a half-life of almost 4.5 billion years by emitting an alpha particle:

92-uranium-238 ====> 90-thorium-234 + alpha particle (nucleus of 2-helium-4)

The mass number of uranium-238 declines by four and its atomic number by two when it emits an alpha particle. The number before the element name is the atomic number and that after the element name is the mass number. The totals of the atomic numbers and the mass numbers, respectively, on both sides of the nuclear reaction must be the same. (This is like balancing a chemical equation, in which the number of atoms of each element on both sides of the reaction must be equal)

In beta decay, the atomic number increases by one if an electron is emitted or decreases by one if a positron is emitted. For instance thorium-234, which is the decay product of uranium-238, in turn beta-decays into protactinium-234 by emitting an electron:

90-thorium-234 ====> 91-protactinium-234 + beta particle (electron)

The nuclei that result from radioactive decay may themselves be radioactive. Therefore, some radioactive elements have decay chains that may contain many radioactive elements, one derived from the other. (See Uranium Factsheet for a diagram of the decay chain of uranium-238.)

The radioactive decay of nuclei is described probabilistically. Within any given time period, a particular unstable nucleus has a fixed probability of decay. As a result, each radioactive element is characterized by a "half-life," which is the time it takes for half the initial atoms to decay (or transmute into another element or nuclear state). At the end of one half-life, half the original element is left, while the other half is transformed into another element. After two half-lives, one fourth of the original element is left; after three half-lives one eighth is left, and so on. This results in the build-up of decay products. If the decay products themselves decay into other elements, a whole host of radioactive materials come into being. The decay products of radioactive elements are also called daughter products or progeny.

C. Binding Energy

Nuclei are tightly bound together by the strong nuclear force and each nucleus has a characteristic binding energy. This is the amount of energy it would take to completely break up a nucleus and separate all the neutrons and protons in it. Typically, binding energy increases by several megaelectron-volts (MeV) for every proton or neutron added to a nucleus. (Since protons and neutrons are constituent particles of nuclei, they are known collectively as nucleons.) The release of nuclear energy derives from the differences in binding energy between the initial nucleus (or nuclei) and relative to the end-products of the nuclear reaction, such as fission or fusion.

The electrons that whirl around the nucleus are held together in their orbits by electrical forces. It takes on the order of a few electron-volts to dislodge an electron from the outer shell of an atom. The "binding energy" of a nucleon is on the order of a million times greater. Electrons are the particles the enable chemical reactions; nucleons take part in nuclear reactions. The huge differences in binding energy are one measure of the differences in the quantities of energy derived from nuclear compared to chemical reactions.

It must be stressed that the binding energy is the amount of energy that would have to be added to the nucleus to break it up. It can be thought of (approximately) as the amount of energy liberated when a nucleon is drawn into the nucleus due to the short range nuclear attractive force. Since energy and mass are equivalent, nuclei with higher binding energy per nucleon have a lower atomic weight per nucleon.

The key to release of nuclear energy from fission of heavy elements and fusion of light elements is that elements in the middle of the periodic table of elements, with intermediate mass numbers have a higher binding energy per nucleon (that is a lower atomic weight per nucleon). Therefore when a heavy nucleus is fissioned, the resultant products of the nuclear reaction have a slightly smaller combined nuclear mass. This mass difference is converted to energy during nuclear fission.

D. Nuclear Fission

Nuclear energy is produced by the conversion of a small amount of the mass of the nucleus of an atom into energy. In principle, all mass and energy are equivalent in a proportion defined by Albert Einstein's famous equation

where E stands for energy, m for mass and c for the speed of light. Since the speed of light is a very large number-300 million meters per second-a small amount of mass is equivalent to a very large amount of energy. For instance, one kilogram (about 2.2 pounds) of matter is equivalent to

This is a huge of amount of energy, equivalent to the energy content of over three million metric tons of coal.

Heavy atoms such as uranium or plutonium can be split by bombarding them with neutrons. The resultant fragments, called fission products, are of intermediate atomic weight, and have a combined mass that is slightly smaller than the original nucleus. The difference appears as energy. As explained in the previous section, this mass difference arises from the binding energy characteristics of heavy elements compared to elements of intermediate atomic weight. Since the binding energy of the fission products per nucleon is higher, their total nucleonic mass is lower. The net result is that fission converts some of the mass of the heavy nucleus into energy.

The energy and mass aspects of the fission process can be explained mathematically as follows. Let the total binding energy of the heavy nucleus and the two fission products be B h, B f1, and B f2, respectively. Then:

Amount of energy released per fission E r = (B f1 + B f2) - B h

This energy appears in various forms: the kinetic energy of the neutrons, the vibrational energy of the fission fragments, and gamma radiation. All of these forms of energy are converted to heat by absorption in with the surrounding media in the reactor, mainly the coolant and the moderator (for thermal reactors).
The most basic fission reaction in nuclear reactors involves the splitting of the nucleus of uranium-235 when it is struck by a neutron. The uranium-235 first absorbs the neutron to yield uranium-236, and most of these U-236 nuclei split into two fission fragments. Fission reactions typically also release two to four neutrons (depending on the speed on the neutrons inducing the fission and probabilistic factors). One of these neutrons must trigger another fission for a sustained chain reaction. The fission reactions in a nuclear reactor can be written generically as follows:

U-235 + n ==> U-236

U-236 ===> fission fragments + 2 to 4 neutrons + 200 MeV energy (approx.)

The uranium-236 nucleus does not split evenly into equal fission fragments. Rather, the tendency, especially with fission induced by thermal neutrons, is for one fragment to be considerably lighter than the other. Figure 9 (not available in on-line version of report) shows the distribution of fission products due to fission with the slow neutrons and fast neutrons. It can be seen that the fission product atomic numbers are concentrated in the ranges from about 80 to 105 and from about 130 to 150 in thermal reactors. An example of a fission reaction is:

92-U-235 + n ==> 92-U-236

92-U-236 ===> 38-strontium-90 + 54-xenon-144 + 2 neutrons + energy

While many heavy nuclei can be fissioned with fast neutrons, only a few can be fissioned with "slow" neutrons. It turns out that, with some exceptions, like plutonium-240, only nuclei that can be fissioned with slow neutrons can be used for sustaining chain reactions. Isotopes with nuclei that can be fissioned with zero energy neutrons (in practice neutrons with low energy, or "slow neutrons") are called fissile materials. Generally these are the odd-numbered isotopes, such as uranium-233, uranium-235, plutonium-239, and plutonium-241. Other heavy nuclei, like uranium-238, can be fissioned with fast neutrons, and so are fissionable, but not fissile.

There are only three fissile isotopes of practical importance: uranium-233, uranium-235, and plutonium-239. Of these, only uranium-235 occurs naturally in significant quantities. The other two occur in trace quantities only.

E. Fertile Materials

To obtain plutonium-239 and uranium-233 in amounts useful for nuclear energy production, they must be manufactured from materials that occur in relative abundance. Plutonium-239 is produced from reactions following the absorption of a neutron by uranium-238; uranium-233 is produced by neutron absorption in thorium-232. Uranium-238 and thorium-232 are called fertile materials, and the production of fissile materials from them is called breeding.
The reactions for plutonium-239 are

92-U-238 + n ===> 92-U-239

92-U-239 ====> 93-Np-239 + beta particle (electron)

93-Np-239 ====> 94-Pu-239 + beta particle (electron)

For uranium-233 the reactions are:

90-Th-232 + n ===> 90-Th-233

90-Th-233 ===> 91-Pa-233 + beta particle (electron)

91-Pa-233 ====> 92-U-233 + beta particle.

The symbol Pa stands for the element protactinium.

(See Chart of Chemical Names on IEER's On-Line classroom page for this and other chemical names and their symbols.)

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A basic background in nuclear physics for those who want to start at the beginning. Some of the terms used in this factsheet can be found in IEER's on-line glossary. A. Structure of the Atom The atoms of which every element of matter is composed have a nucleus at the center and electrons whirling about [...]

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As discussed in a previous section of Lesson 1, atoms are the building blocks of matter. There are different types of atoms, known as elements. Atoms of each element are distinguished from each other by the number of protons that are present in their nucleus. An atom containing one proton is a hydrogen atom (H). An atom containing 6 protons is a carbon atom. And an atom containing 8 protons is an oxygen atom.

The number of electrons that surround the nucleus will determine whether or not an atom is electrically charged or electrically neutral. The amount of charge on a single proton is equal to the amount of charge possessed by a single electron. A proton and an electron have an equal amount but an opposite type of charge. Thus, if an atom contains equal numbers of protons and electrons, the atom is described as being electrically neutral. On the other hand, if an atom has an unequal number of protons and electrons, then the atom is electrically charged (and in fact, is then referred to as an ion rather than an atom). Any particle, whether an atom, molecule or ion, that contains less electrons than protons is said to be positively charged. Conversely, any particle that contains more electrons than protons is said to be negatively charged.

Charged versus Uncharged Particles

Positively Charged

Negatively Charged

Uncharged

Possesses more protons than electrons

Possesses more electrons than protons

Equal numbers of protons and electrons

Charged Objects as an Imbalance of Protons and Electrons

In the previous section of Lesson 1, an atom was described as being a small and dense core of positively charged protons and neutral neutrons surrounded by shells of negatively charged electrons. The protons are tightly bound within the nucleus and not removable by ordinary measures. While the electrons are attracted to the protons of the nucleus, the addition of energy to an atom can persuade the electrons to leave an atom. Similarly, electrons within atoms of other materials can be persuaded to leave their own electron shells and become members of the electrons shells of other atoms of different materials. In short, electrons are migrants - constantly on the move and always ready to try out a new atomic environment.

All objects are composed of these atoms. The electrons contained within the objects are prone to move or migrate to other objects. The process of an electron leaving one material object to reside (perhaps only temporarily) in another object is a common everyday occurrence. Even as you read the words of this web page, some electrons are likely moving through the monitor and adhering to your clothing (assuming that you are using this resource online) (and wearing clothes). If you were to walk across the carpeting towards the door of the room, electrons would likely be scuffed off the atoms of your shoes and moved onto the atoms of the carpet. And as clothes tumble in the dryer, it is highly likely that electrons on one piece of clothing will move from the atoms of the clothing onto the atoms of another piece of clothing. In general, for electrons to make a move from the atoms of one material to the atoms of another material, there must be an energy source, a motive, and a low-resistance pathway.

The cause and mechanisms by which this movement of electrons occurs will be the subject of Lesson 2. For now, it is sufficient to say that objects that are charged contain unequal numbers of protons and electrons. Charged objects have an imbalance of charge - either more negative electrons than positive protons or vice versa. And neutral objects have a balance of charge - equal numbers of protons and electrons. The principle stated earlier for atoms can be applied to objects. Objects with more electrons than protons are charged negatively; objects with fewer electrons than protons are charged positively.

In this discussion of electrically charged versus electrically neutral objects, the neutron has been neglected. Neutrons, being electrically neutral play no role in this unit. Their presence (or absence) will have no direct bearing upon whether an object is charged or uncharged. Their role in the atom is merely to provide stability to the nucleus, a subject not discussed in The Physics Classroom. When it comes to the drama of static electricity, electrons and protons become the main characters.

Charge as a Quantity

Like mass, the charge of an object is a measurable quantity. The charge possessed by an object is often expressed using the scientific unit known as the Coulomb. Just as mass is measured in grams or kilograms, charge is measured in units of Coulombs (abbreviated C). Because one Coulomb of charge is an abnormally large quantity of charge, the units of microCoulombs (µC) or nanoCoulombs (nC) are more commonly used as the unit of measurement of charge. To illustrate the magnitude of 1 Coulomb, an object would need an excess of 6.25 x 10 ¹⁸ electrons to have a total charge of -1 C. And of course an object with a shortage of 6.25 x 10 ¹⁸ electrons would have a total charge of +1 C.

The charge on a single electron is -1.6 x 10 -19 Coulomb. The charge on a single proton is +1.6 x 10 -19 Coulomb. The quantity of charge on an object reflects the amount of imbalance between electrons and protons on that object. Thus, to determine the total charge of a positively charged object (an object with an excess of protons), one must subtract the total number of electrons from the total number of protons. This operation yields the number of excess protons. Since a single proton contributes a charge of +1.6 x 10 -19 Coulomb to the overall charge of an atom, the total charge can be computed by multiplying the number of excess protons by +1.6 x 10 -19 Coulomb. A similar process is used to determine the total charge of a negatively charged object (an object with an excess of electrons), except that the number of protons is first subtracted from the number of electrons.

This principle is illustrated in the following table.

Object

# of Excess Protons/Electrons

Quantity and Kind of Charge (Q) on Object in Coulombs (C)

In conclusion, an electrically neutral object is an object that has a balance of protons and electrons. In contrast, a charged object has an imbalance of protons and electrons. Determining the quantity of charge on such an object involves a counting process; the total number of electrons and protons are compared to determine the difference between the number of protons and electrons. This difference is multiplied by 1.6 x 10 -19 Coulombs to determine the overall quantity of charge on the object. The type of charge (positive or negative) is determined by whether the protons or the electrons are in excess.

We Would Like to Suggest ...

Sometimes it isn't enough to just read about it. You have to interact with it! And that's exactly what you do when you use one of The Physics Classroom's Interactives. We would like to suggest that you combine the reading of this page with the use of our Charging Interactive. You can find it in the Physics Interactives section of our website. The Charging Interactive is an electrostatics "playground" that allows a learner to investigate a variety of concepts related to charge, charge interactions, charging processes, and grounding. Once you get the hang of the concepts, put your game-face on tap the Play button.

Check Your Understanding

Use your understanding of charge to answer the following questions. When finished, click the button to view the answers.

1. TRUE or FALSE: An object that is positively charged contains all protons and no electrons.

2. TRUE or FALSE: An object that is negatively charged could contain only electrons with no accompanying protons.

3. TRUE or FALSE: An object that is electrically neutral contains only neutrons.

4. Identify the following particles as being charged or uncharged. If charged, indicate whether they are charged positively or negatively. (n = neutron, p = proton, e = electron)

5. Consider the graphic at the right of a neutral oxygen atom.

a. Explain what must happen in order for the oxygen atom to become negatively charged.
b. Explain what must happen in order for the oxygen atom to become positively charged.

6. Determine the quantity and type of charge on an object that has 3.62 x 10 ¹² more protons than electrons.

7. Complete the following statements:

After some rather exhausting counting (and a rather tall tale), a physics teacher determines that a very small sample of an object contains ...

a. ... 8.25749 x 10 ¹⁷ protons and 5.26 x 10 ¹⁴ electrons; the charge on this object is ____ Coulombs.

b. ... 3.12 x 10 ¹⁴ protons and 4.5488 x 10 ¹⁶ electrons; the charge on this object is ____ Coulombs.

c. ... 2.40277 x 10 ¹⁹ protons and 9.88 x 10 ¹⁶ electrons; the charge on this object is ____ Coulombs.

d. ... 2.6325 x 10 ¹⁵ protons and 2.6325 x 10 ¹⁵ electrons; the charge on this object is ____ Coulombs.

8. The amount of charge carried by a lightning bolt is estimated at 10 Coulombs. What quantity of excess electrons is carried by the lightning bolt?

9. Respond to the following student statement:

"A positively charged object is an object that has an excess of positive electrons."

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As discussed in a previous section of Lesson 1, atoms are the building blocks of matter. There are different types of atoms, known as elements. Atoms of each element are distinguished from each other by the number of protons that are present in their nucleus. An atom containing one proton is a hydrogen atom (H).

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The Royal Society of Chemistry's interactive periodic table features history, alchemy, podcasts, videos, and data trends across the periodic table. Click the tabs at the top to explore each section. Use the buttons above to change your view of the periodic table and view Murray Robertson's stunning Visual Elements artwork. Click each element to read detailed information.

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Periodic Table - Royal Society of Chemistry

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Interactive periodic table with element scarcity (SRI), discovery dates, melting and boiling points, group, block and period information.

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Electrical Construction & Maintenance (EC&M) Magazine

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The convergence of information technology (IT) equipment and telecommunications networks has brought about the need to power equipment with a variety of input requirements. Designers of today's telecommunications facilities are faced with the challenge of configuring a power system that supports these requirements as reliably and economically as possible.

Traditional telecommunications equipment generally requires -48VDC input power. Such power systems consist of multiple parallel-redundant rectifiers that convert AC power to -48VDC power, charge lead-acid storage batteries, and supply power to critical-load equipment. Converters or inverters are used to derive other required voltages from the -48VDC power plant. Long battery support times are necessary to support the equipment during AC power or rectifier failures. Sometimes plant personnel use engine-generator systems to supplement the AC power during sustained power failures. Battery support times range from a minimum of 1 hr to more than 24 hr, with typical ones lasting between 3 hr and 8 hr. Fig. 1 shows a typical telecommunications power system using rectifiers and -48VDC battery systems to support the critical load equipment.

Traditional IT equipment requires AC input power that generally matches the available AC power source configurations - typically 120V, 208V, or 240V single-phase AC in 60-Hz countries and 220V to 240V single-phase AC in 50-Hz countries. These power configurations use AC UPS systems with battery systems sized to provide necessary time for an orderly shutdown or to start standby engine-generators. Virtually all critical IT facilities include permanently sited engine-generator systems (and associated automatic transfer switches) to protect against sustained AC power failures. Fig. 2, on page 23, shows a typical IT power configuration using AC UPS systems to support critical loads.

Both traditional telecommunications and IT facilities generally include other critical support equipment, such as lighting and air conditioning. This equipment generally tolerates brief interruptions in power without adverse consequences on the operation of the telecommunications or IT equipment.

For telecommunications systems without permanently sited engine generators, it is common to power support equipment with inverters connected to the -48VDC power system. IT systems run such equipment with AC power. An IT system without permanently sited engine generators provides orderly shutdown time, while one with engine generators provides extended run time.

With the convergence of telecommunications and IT equipment, it is likely that critical electronic loads in any one facility will require both -48VDC power and one or more of the AC power voltages. Often a site's system cannot operate unless both the DC- and AC-powered loads are operational. This codependence demands equally high reliability and availability for the DC and AC power systems.

Why have 8 hr of battery backup for the DC-powered loads and only 15 min for the AC loads? The answer lies in the historical biases of AC and DC electrical power system designers. DC power engineers believe batteries are more reliable, but AC power engineers believe power conversion equipment, engine-generators, and AC power are more dependable. Understanding each perspective requires an analysis of both systems' application.

The AC/DC Debate

DC power systems use a multitude of parallel rectifiers to derive -48VDC from AC power. For example, a 4000A rectifier system is often built using 21 or more 200A rectifier modules. The typical mean time between failure (MTBF) of these rectifiers is between 100,000 and 200,000 hr. With so many rectifiers used in parallel, the observed failure rate of the power conversion equipment is substantially increased.

From the AC power engineer's perspective, the most reliable system would use N+1 parallel redundant rectifiers, with N as small as possible, to reduce parts and the probability of a rectifier failure. In other words, the more components, the higher probability of any one component failing ( Fig. 3).

DC power systems use -48VDC battery plants with long-duration backup times (i.e., low rate of discharge). They use only 24 cells in series and apply them at a relatively low discharge rate. Furthermore, the end-of-discharge and float voltages are carefully controlled.

On the other hand, AC UPS systems typically use 120 to 240 cells in series and apply them at very high discharge rates (i.e., 10- to 20-min backup times), with deep discharge voltages down to 1.65V per cell or less. With these striking application differences, it's no wonder that DC power engineers believe batteries are reliable, while AC power engineers believe they are the most unreliable component of the AC UPS system.

DC systems don't always include permanently sited engine-generator systems. If they do, they often lack regular maintenance and testing. In addition, a serial dependency with reduced observed reliability exists because both the rectifier and engine-generator must be operational.

High-availability AC power systems routinely include large, permanently sited, redundant engine-generators that receive careful maintenance and regular testing to assure availability when needed. The AC load equipment can use the output of the engine generator even when the AC power conversion equipment is in bypass.

Fig. 4, below, and Table 1, on page 24, show first-order reliability calculations comparing the relative calculated unavailabilities of DC and AC powering options. The unavailability of an AC UPS system (with 10-min battery backup) is 3.5×10⁻¹⁰ . For a DC power system (with 8-hr battery backup), the unavailability is approximately the same at 9×10⁻¹⁰ .

The unavailability of the DC power system is dependent on the size of the battery plant. For example, if you use a 3-hr battery, the unavailability of the DC power system increases to 3.45×10⁻⁷ . The AC UPS power system is relatively unaffected by battery size. Increasing the battery backup time to 3 hr yields the same unavailability as the 10-min battery. These first-order calculations don't include real-world issues such as failures in the power distribution system, lack of maintenance, and human error. They do, however, point out the relative dependencies on the various subcomponents.

One of the arguments often posed in favor of DC over AC power is that it is more straightforward with fewer power conversions. The source of AC-powered electronic equipment is the public utility. The UPS converts AC power to DC power to charge the batteries and power the inverter. The inverter converts DC power back to AC power. This AC power supplies the load equipment, which in turn converts the power back to DC.

In DC power systems, the rectifiers convert the AC power to DC and directly supply the DC-powered load equipment. But an often-ignored fact is the load equipment uses power converters to convert the -48VDC power to other voltages required by the electronics. The expected efficiency advantage of a DC powering approach is often not realized.

The AC-to-DC efficiency of the multiple, smaller rectifiers that typify the DC system averages 89%, while the AC-to-AC efficiency of large UPS systems generally ranges from 92% to 94%. Since the efficiencies of the different power supplies are approximately the same, the overall efficiencies of the two systems produce nearly identical measurements ( Fig. 5).

In the case of DC power systems, the mitigating reliability factor is the battery is directly connected to the load bus. For AC UPS systems, the mitigating factor is the ability to provide an alternate source of power with the UPS bypass circuit. Despite the philosophical differences in powering perspectives, both approaches are feasible. And when implemented properly, both demonstrate high reliability.

Input Power Requirements

When you select a power system configuration, the input power requirement of the loads is the most important consideration. Other factors include size, installation costs, operating efficiency, and maintenance costs. Most IT equipment requires AC input power, just as most telecom equipment requires -48VDC input power. The mix of the two generally dictates the configuration. For example, a central office that uses -48VDC power for most of its critical equipment will lean toward a conventional DC approach. Data centers with equipment that mainly uses AC power will follow a conventional AC UPS approach.

If the manufacturer offers a choice of input power, the percentage mix becomes a variable, depending on the particular preferences. For typical Internet hosting sites and server-based telecommunications sites, the mix of AC-powered loads (in kW) ranges from 85% to 95%, and DC-powered loads are 5% to 15%. Typical site sizes tend to be in the 10,000-to-100,000-sq-foot range, with power densities of the load equipment measuring 35W to 80W per square foot. With these requirements, the critical power system sizes tend to fall between 350 kW and 8MW. At -48VDC, these power levels range from 7000A to an astounding 160,000A. At 480VAC 3-phase, these same power levels fall within a more manageable 420A to 9600A. Controlling voltage drop in -48VDC power systems at higher power levels becomes extremely difficult and expensive. Clearly, larger power systems favor AC power distribution systems or smaller DC power systems located close to the load equipment.

Distributed DC Power Configuration

Typical IT rooms use packaged, commercially available power distribution units (PDUs) to distribute AC power to the load equipment. The PDU performs the conditioning, distribution, and monitoring of the load equipment power. The 480VAC output of large UPS systems is distributed to a number of PDUs located throughout the data center. A PDU typically contains an isolation transformer, which provides voltage stepdown to 208/120VAC, common-mode noise isolation, local voltage adjustment, and ground referencing. It also contains output distribution panelboards with output circuit breakers, cables, and receptacles to match load equipment requirements. PDUs should be located close to the load equipment to minimize the output distribution circuit length and voltage drop.

For -48VDC power systems, a similar PDU approach is possible. Point-of-use N+1 redundant rectifier systems (without batteries and with integral output power distribution) are located close to the load equipment. The rectifiers are powered from the same AC UPS systems as the other associated electronic load equipment. The footprint of these batteryless rectifier systems (DC PDUs) is approximately the same as the secondary DC distribution bays normally required with conventional centralized DC power systems.

To mitigate potential failures and allow maintenance within the AC or DC systems without load shutdown, dual UPS systems with redundant power paths, dual rectifier systems, and dual-input load equipment are used ( Fig. 6). This high-availability approach is known as a "hybrid distributed redundant power system." On the surface, this sounds expensive, but reduced installation costs and a smaller footprint make it cost-effective.

Summary

Telecom and IT equipment that requires AC and -48VDC power provides new challenges for system designers. Conventional configurations may not be the most appropriate choice for powering this type of equipment, particularly for Internet applications with high-load power densities, codependency of the AC- and DC-powered load equipment, and a mixture of loads that require AC input power.

Alternative, cost-effective configurations can create systems that have the necessary reliability, availability, and maintainability to meet today's nonstop processing requirements.

References:

1. Gruzs, T.M., and James Hall, "AC, DC or Hybrid Power Solutions for Today's Telecommunications Facilities," INTELEC® 2000 Conference, Phoenix, AZ, Sept. 10-14, 2000.

2. "Powering the Internet, Datacom Equipment in Telecom Facilities: The Need for a DC Powering Option," Technical Subgroup on Telecommunications Energy Systems of the Power Electronics Society of the IEEE, INTELEC Web site: http://www.pels.org/Comm/Telecom/Telecom.html.

Cost Comparison - A Case Study

A large Internet hosting company was building a number of sites nationwide. Each site was approximately 10,000 sq ft with a planned load density of up to 75W per square foot. The exact mix of AC- and DC-powered load equipment was unknown since it was dependent on the tenant's selected equipment. However, the design criteria allowed DC-powered equipment to comprise up to 10% of the total load. To assure maximum availability, the design required redundant, dual-distribution power systems to support load equipment having dual-input power connections.

The selected AC UPS configuration was distributed redundant, 750kVA systems (two independent systems with three 375kVA modules, each for N+1 redundancy) with 15 min of battery backup at full load. Permanently sited, redundant (N+1) standby engine generators were included. The ultimate maximum total -48VDC power load to be accommodated was 1600A. The initial design required the ability to support half the ultimate load. All of the loads were expected to be dual-input DC- or AC-powered equipment. The company considered several different DC power-system configurations.

Two designs were selected for engineering and cost analysis. One configuration used centralized 1600A DC power plants. The alternative configuration used distributed batteryless DC rectifier systems powered from the AC UPS systems. These rectifier systems were deployed in capacities of 400A each. The rectifiers and output distribution were combined and located in the same footprint as the centralized system's secondary distribution bays. The dual redundant AC UPS systems and redundant standby generator systems provided outage protection, thus eliminating the 1-hr DC system battery requirement. The AC UPS size was the same for either alternative because both the AC- and DC-powered load equipment required accommodation. Furthermore, the incremental increase in UPS capacity to accommodate the additional 10% DC load did not cause the UPS module size to change.

The hybrid distributed DC power system configuration met the high-availability requirements of the Internet hosting company by using the dual redundant AC UPS with redundant power distribution paths. A high degree of fault tolerance was obtained without any single failure points in the AC or DC system. Any component within the AC or DC system could fail or be maintained without disrupting the critical load equipment operation.

A cost comparison of the two designs is shown in Table 2. Significant savings come with the distributed batteryless DC rectifier approach, mainly due to the lower engineering costs, reduced installation materials, and lower overall installation costs. Other benefits include reduced footprint, ease of expansion, and greater flexibility in accommodating varying mixes of AC- and DC-powered load equipment.

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Powering Telecom and Info Technology Systems

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The convergence of information technology (IT) equipment and telecommunications networks has brought about the need to power equipment with a variety of input requirements. Designers of today's telecommunications facilities are faced with the challenge of configuring a power system that supports these requirements as reliably and economically as possible.

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A semiconductor is a substance, usually a solid chemical element or compound, that can conduct electricity under some conditions but not others, making it a good medium for the control of electrical current. Its conductance varies depending on the current or voltage applied to a control electrode, or on the intensity of irradiation by infrared ( IR), visible light, ultraviolet (UV), or X rays.

The specific properties of a semiconductor depend on the impurities, or dopants, added to it. An N-type semiconductor carries current mainly in the form of negatively-charged electrons, in a manner similar to the conduction of current in a wire. A P-type semiconductor carries current predominantly as electron deficiencies called holes. A hole has a positive electric charge, equal and opposite to the charge on an electron. In a semiconductor material, the flow of holes occurs in a direction opposite to the flow of electrons.

Elemental semiconductors include antimony, arsenic, boron, carbon, germanium, selenium, silicon, sulfur, and tellurium. Silicon is the best-known of these, forming the basis of most integrated circuits ( ICs). Common semiconductor compounds include gallium arsenide, indium antimonide, and the oxides of most metals. Of these, gallium arsenide ( GaAs) is widely used in low-noise, high-gain, weak-signal amplifying devices.

A semiconductor device can perform the function of a vacuum tube having hundreds of times its volume. A single integrated circuit (IC), such as a microprocessor chip, can do the work of a set of vacuum tubes that would fill a large building and require its own electric generating plant.

See also atom, transistor, bipolar transistor, and field-effect transistor.

Continue Reading About semiconductor

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What is semiconductor? - Definition from WhatIs.com

# data[].attributes.attachment_link.description
A semiconductor is a substance, usually a solid chemical element or compound, that can conduct electricity under some conditions but not others, making it a good medium for the control of electrical current. Its conductance varies depending on the current or voltage applied to a control electrode, or on the intensity of irradiation by infrared ( IR), visible light, ultraviolet (UV), or X rays.

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See also field-effect transistor (field-effect transistor) and transistor.

A bipolar transistor is a semiconductor device commonly used for amplification. The device can amplify analog or digital signals. It can also switch DC or function as an oscillator. Physically, a bipolar transistor amplifies current, but it can be connected in circuits designed to amplify voltage or power.

There are two major types of bipolar transistor, called PNP and NPN. A PNP transistor has a layer of N-type semiconductor between two layers of P-type material. An NPN transistor has a layer of P-type material between two layers of N-type material. In P-type material, electric charges are carried mainly in the form of electron deficiencies called holes. In N-type material, the charge carriers are primarily electrons.

The bipolar transistor has advantages and disadvantages relative to the field-effect transistor (field-effect transistor). Bipolar devices can switch signals at high speeds, and can be manufactured to handle large currents so that they can serve as high-power amplifiers in audio equipment and in wireless transmitters. Bipolar devices are not especially effective for weak-signal amplification, or for applications requiring high circuit impedance.

Bipolar transistors are fabricated onto silicon integrated circuit (IC) chip. A single IC can contain many thousands of bipolar transistors, along with other components such as resistors, capacitors, and diodes.

Continue Reading About bipolar transistor

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What is bipolar transistor? - Definition from WhatIs.com

# data[].attributes.attachment_link.description
See also field-effect transistor (field-effect transistor) and transistor. A bipolar transistor is a semiconductor device commonly used for amplification. The device can amplify analog or digital signals. It can also switch DC or function as an oscillator. Physically, a bipolar transistor amplifies current, but it can be connected in circuits designed to amplify voltage or power.

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Conductance is an expression of the ease with which electric current flows through a substance. In equations, conductance is symbolized by the uppercase letter G. The standard unit of conductance is the siemens (abbreviated S), formerly known as the mho.

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When a current of one ampere (1 A) passes through a component across which a voltage of one volt (1 V) exists, then the conductance of that component is 1 S. The siemens is, in fact, equivalent to one ampere per volt. If G is the conductance of a component (in siemens), I is the current through the component (in amperes), and E is the voltage across the component (in volts), then:

G = I/ E

In general, when the applied voltage is held constant, the current in a direct-current ( DC) circuit is directly proportional to the conductance. If the conductance is doubled, the current is also doubled; if the conductance is cut to 1/10 its initial value, the current also becomes 1/10 as great. This rule also holds for most low-frequency alternating-current (AC) systems, such as household utility circuits. In some AC circuits, especially at high frequencies, the situation is more complex, because some components in these systems store and release energy, as well as dissipating or converting it.

Conductance is inversely related to resistance. If R is the resistance of a component or device (in ohms), then the conductance G (in siemens) is given by:

G = 1/ R

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What is conductance? - Definition from WhatIs.com

# data[].attributes.attachment_link.description
Conductance is an expression of the ease with which electric current flows through a substance. In equations, conductance is symbolized by the uppercase letter G. The standard unit of conductance is the siemens (abbreviated S), formerly known as the mho.

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Voltage, also called electromotive force, is a quantitative expression of the potential difference in charge between two points in an electrical field.

The greater the voltage, the greater the flow of electrical current (that is, the quantity of charge carriers that pass a fixed point per unit of time) through a conducting or semiconducting medium for a given resistance to the flow. Voltage is symbolized by an uppercase italic letter V or E. The standard unit is the volt, symbolized by a non-italic uppercase letter V. One volt will drive one coulomb (6.24 x 10 ¹⁸) charge carriers, such as electrons, through a resistance of one ohm in one second.

Voltage can be direct or alternating. A direct voltage maintains the same polarity at all times. In an alternating voltage, the polarity reverses direction periodically. The number of complete cycles per second is the frequency, which is measured in hertz (one cycle per second), kilohertz, megahertz, gigahertz, or terahertz. An example of direct voltage is the potential difference between the terminals of an electrochemical cell. Alternating voltage exists between the terminals of a common utility outlet.

A voltage produces an electrostatic field, even if no charge carriers move (that is, no current flows). As the voltage increases between two points separated by a specific distance, the electrostatic field becomes more intense. As the separation increases between two points having a given voltage with respect to each other, the electrostatic flux density diminishes in the region between them.

Also see current, resistance, and Ohm's Law for DC Circuits.

Continue Reading About voltage

# data[].attributes.attachment_link.title
What is voltage? - Definition from WhatIs.com

# data[].attributes.attachment_link.description
Voltage, also called electromotive force, is a quantitative expression of the potential difference in charge between two points in an electrical field. The greater the voltage, the greater the flow of electrical current (that is, the quantity of charge carriers that pass a fixed point per unit of time) through a conducting or semiconducting medium for a given resistance to the flow.

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Not having the proper technology powering an IoT deployment can lead to unsatisfactory results. Learn about the types of batteries that work best in ...

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BBC - Standard Grade Bitesize Physics - Communication using waves : Revision, Page 5

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A BBC Bitesize secondary school revision resource for Standard Grade Physics on communication using waves: transverse, longitudinal, frequency, amplitude.

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IEEE 5G is a new IEEE initiative that strives to address the challenges of next generation wireless technologies.

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Please ensure you have JavaScript enabled in your browser. If you leave JavaScript disabled, you will only access a portion of the content we are providing. Here's how.

What does it mean for a solution to be acidic or basic (alkaline)?

It all has to do with hydrogen ions (abbreviated with the chemical symbol H⁺). In water (H ₂O), a small number of the molecules dissociate (split up). Some of the water molecules lose a hydrogen and become hydroxide ions (OH −). The "lost" hydrogen ions join up with water molecules to form hydronium ions (H ₃O⁺). For simplicity, hydronium ions are referred to as hydrogen ions H⁺. In pure water, there are an equal number of hydrogen ions and hydroxide ions. The solution is neither acidic or basic.

An acid is a substance that donates hydrogen ions. Because of this, when an acid is dissolved in water, the balance between hydrogen ions and hydroxide ions is shifted. Now there are more hydrogen ions than hydroxide ions in the solution. This kind of solution is acidic.

A base is a substance that accepts hydrogen ions. When a base is dissolved in water, the balance between hydrogen ions and hydroxide ions shifts the opposite way. Because the base "soaks up" hydrogen ions, the result is a solution with more hydroxide ions than hydrogen ions. This kind of solution is alkaline.

Acidity and alkalinity are measured with a logarithmic scale called pH. Here is why: a strongly acidic solution can have one hundred million million, or one hundred trillion (100,000,000,000,000) times more hydrogen ions than a strongly basic solution! The flip side, of course, is that a strongly basic solution can have 100,000,000,000,000 times more hydroxide ions than a strongly acidic solution. Moreover, the hydrogen ion and hydroxide ion concentrations in everyday solutions can vary over that entire range.

In order to deal with these large numbers more easily, scientists use a logarithmic scale, the pH scale. Each one-unit change in the pH scale corresponds to a ten-fold change in hydrogen ion concentration. The pH scale is theoretically open-ended but most pH values are in the range from 0 to 14. It's a lot easier to use a logarithmic scale instead of always having to write down all those zeros! By the way, notice how one hundred million million is a one with fourteen zeros after it? It is not coincidence, it is logarithms!

To be more precise, pH is the negative logarithm of the hydrogen ion concentration:

The square brackets around the H⁺ automatically mean "concentration" to a chemist. What the equation means is just what we said before: for each 1-unit change in pH, the hydrogen ion concentration changes ten-fold. Pure water has a neutral pH of 7. pH values lower than 7 are acidic, and pH values higher than 7 are alkaline (basic). Table 1 has examples of substances with different pH values (Decelles, 2002; Environment Canada, 2002; EPA, date unknown).

Table 1. The pH Scale: Some Examples

How Do You Measure the pH of a Solution?

The pH of a liquid or solution is often an important piece of information in science. Measuring pH can be done simply and quickly using pH test paper, pH indicator sticks, or a pH meter. pH test paper and indicator sticks are pieces of paper or stiffer sticks that contain pH indicators (chemicals that change color depending on how acidic or basic a solution is). To measure pH, a piece of pH test paper or an indicator stick is dipped into the liquid. The color of the dipped paper/stick is then matched to a color key that comes with the container of pH test paper or indicator sticks. Each color on the key represents a different pH. An example of a used pH indicator stick and the corresponding color key is shown below in Figure 1. pH meters are electronic devices that used to measure pH. They consist of a probe that is dipped in a solution, and a digital readout. pH meters are even more precise than pH test paper or indicator sticks. Table 2 below discusses what types of pH measuring devices are best for different science project applications, and offers a quick link to purchasing different pH test papers and indicator sticks.

Figure 1. pH test paper (not shown) and pH indicator sticks (shown here) are dipped into a solution then matched against a color key to determine the solution's approximate pH (Michael Krahe, 2005).

pH meters

0-14

0.1 or less depending on the meter

High quality pH meters can be expensive. We recommend checking if there is one available at your local high school chemistry laboratory before purchasing. Various models are available from Amazon.com.

Suitable for more advanced research where the exact pH of a solution matters. For example, when creating buffers for a biotechnology project.

Table 2. The items above can be used to measure pH for science projects and other hobby and home applications. Clicking the purchasing links will take you directly to the product at www.amazon.com.

To get accurate pH readings always remember to:

Wait a minute or two after you add an acid or a base to a solution. This will allow the reaction (ions being either donated [acid] or accepted [base]) to complete before you measure.

Swirl or mix a solution well before measuring. This will help ensure that the solution is uniform.

When using pH test paper/indicator sticks you should also:

Make sure to only use paper/sticks that have not been previously wetted.

Wait for the color to stop changing (1-2 minutes maximum) before matching the paper/stick to the color key. Do not wait more than 5 minutes after the color has stabilized or it may start to fade and affect the accuracy of your reading.

When using a pH meter you should also:

Carefully read the manual for the pH meter before using it.

Rinse the pH meter probe with distilled water before every reading.

Use solutions with known pH values, see Table 2, to make sure the pH meter is accurately calibrated.

Make sure the pH meter probe is properly submerged in the solution before taking a reading.

Bibliography

For more information about the pH scale, try these references:

Decelles, P. (2002). "The pH Scale," Virtually Biology Course, Basic Chemistry Concepts, Johnson County Community College. Retrieved July 24, 2006, from http://staff.jccc.net/pdecell/chemistry/phscale.html.

Khan Academy. (2009, September 7). Acid Base Introduction: Arrhenius, Bronsted Lowry, and Lewis Acids and Bases. Retrieved May 1, 2012, fromhttp://www.khanacademy.org/science/chemistry/v/acid-base-introduction

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Please ensure you have JavaScript enabled in your browser. If you leave JavaScript disabled, you will only access a portion of the content we are providing. Here's how. It all has to do with hydrogen ions (abbreviated with the chemical symbol H+). In water (H 2O), a small number of the molecules dissociate (split up).

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It's not enough to do good research. You have to communicate it-not just to the other people in your academic department, but to anyone. That can be difficult if, for example, you work in an abstract branch of mathematics. A long talk full of equations won't cut it. Sometimes you need to pull out science's most powerful and top-secret communication tool: interpretive dance.

That's exactly what Nancy Scherich did. Her Ph.D. research at the University of California, Santa Barbara, is in topology, the study of geometry in which shape and size don't matter. Her focus is on braid theory; she spends her days "with paper and pencil" to find the rules that determine the unique representations of twists and knots in high-dimensional spaces. So naturally, she created a dance to explain it with aerial silk acrobatics and glowing hula hoops. Look for the mathematical plot twist. (Spoiler alert: It involves linear algebra and murder!)

Scherich is the overall winner of this year's " Dance Your Ph.D." contest. This is the 10th year of a challenge laid down by Science and AAAS for researchers to explain their work with dance moves. Scherich is joined by three other researchers who won in their scientific categories with dances explaining their work on sea star ecology, the psychology of creativity, and the biochemistry of criminal forensics. That last one, by Natália Oliveira at the Federal University of Pernambuco in Recife, Brazil, is also the winner of our online audience favorite award. In all, 53 scientists from around the world submitted dances.

Why do they do it? The $2500 in cash prizes probably doesn't hurt. But the main reasons, according to a survey of the contestants, range from edging out their nondancing peers to winning scholarships to finding love. Whatever the motivation, we all end up with some truly stunning videos.

Chemistry, People's Choice

Natália Oliveira from the Federal University of Pernambuco in Recife, Brazil, with "Development of biosensors for forensic sciences applications."

Social sciences

Judit Pétervári from Queen Mary University of London with "The evaluation of creative ideas-analyzing the differences between expert and novice judges."

Biology

Monica Moritsch from the University of California, Santa Cruz, with "Intertidal community consequences of sea star wasting syndrome."

This year's judges:

Katrien Kolenberg, Astrophysicist at the University of Leuven in Belgium and inaugural Ph.D. dancer

Carl Flink, Choreographer and director of the Black Label Movement and professor at the University of Minnesota in Minneapolis

Alexa Meade, Artist and visual engineer

Kieran Gourley, Mathematician and professional ballet dancer in Sydney, Australia

Matt Kent, Emily Kent, Renée Jaworski, Pilobolus

doi:10.1126/science.aar3697

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Semiconductors are used extensively in electronic circuits. As its name implies, a semiconductoris a material that conducts current, but only partly. The conductivity of a semiconductor is somewhere between that of an insulator, which has almost no conductivity, and a conductor, which has almost full conductivity. Most semiconductors are crystals made of certain materials, most commonly silicon.

To understand how semiconductors work, you must first understand a little about how electrons are organized in an atom. The electrons in an atom are organized in layers. These layers are called shells. The outermost shell is called the valence shell.

The electrons in this shell are the ones that form bonds with neighboring atoms. Such bonds are called covalent bonds. Most conductors have just one electron in the valence shell. Semiconductors, on the other hand, typically have four electrons in their valence shell.

If all the neighboring atoms are of the same type, it's possible for all the valence electrons to bind with valence electrons from other atoms. When that happens, the atoms arrange themselves into structures called crystals. Semiconductors are made out of such crystals, usually silicon crystals.

Here, each circle represents a silicon atom, and the lines between the atoms represent the shared electrons. Each of the four valence electrons in each silicon atom is shared with one neighboring silicon atom. Thus, each silicon atom is bonded with four other silicon atoms.

Pure silicon crystals are not all that useful electronically. But if you introduce small amounts of other elements into a crystal, the crystal starts to conduct in an interesting way.

The process of deliberately introducing other elements into a crystal is called doping. The element introduced by doping is called a dopant. By carefully controlling the doping process and the dopants that are used, silicon crystals can transform into one of two distinct types of conductors:

N-type semiconductor: Created when the dopant is an element that has five electrons in its valence layer. Phosphorus is commonly used for this purpose.

The phosphorus atoms join right in the crystal structure of the silicon, each one bonding with four adjacent silicon atoms just like a silicon atom would. Because the phosphorus atom has five electrons in its valence shell, but only four of them are bonded to adjacent atoms, the fifth valence electron is left hanging out with nothing to bond to.

The extra valence electrons in the phosphorous atoms start to behave like the single valence electrons in a regular conductor such as copper. They are free to move about. Because this type of semiconductor has extra electrons, it's called an N-type semiconductor.

P-type semiconductor: Happens when the dopant (such as boron) has only three electrons in the valence shell. When a small amount is incorporated into the crystal, the atom is able to bond with four silicon atoms, but since it has only three electrons to offer, a hole is created. The hole behaves like a positive charge, so semiconductors doped in this way are called P-type semiconductors.

Like a positive charge, holes attract electrons. But when an electron moves into a hole, the electron leaves a new hole at its previous location. Thus, in a P-type semiconductor, holes are constantly moving around within the crystal as electrons constantly try to fill them up.

When voltage is applied to either an N-type or a P-type semiconductor, current flows, for the same reason that it flows in a regular conductor: The negative side of the voltage pushes electrons, and the positive side pulls them. The result is that the random electron and hole movement that's always present in a semiconductor becomes organized in one direction, creating measurable electric current.

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A micrometer (sometimes expressed using the obsolete term, micron ), is one-millionth of a meter and can also be expressed as:

10 ⁻⁶ meter

One thousandth of a millimeter

One 25-thousandth of an inch

The micrometer is a unit of measure for the core in optical fiber , for which the most common diameter is 62.5 micrometers. It is also used to measure the line width on a microchip . Intel's Pentium 4 microprocessors are built using 0.18 or 0.13 micrometer line widths. AMD's Athlon uses a 0.18 line width.

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In chemistry, neutralization or neutralisation (see spelling differences), is a chemical reaction in which an acid and a base react quantitatively with each other. In a reaction in water, neutralization results in there being no excess of hydrogen or hydroxide ions present in the solution.

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A neutralization reaction is when an acid and a base react to form water and a salt and involves the combination of H⁺ ions and OH⁻ ions to generate water. The neutralization of a strong acid and strong base has a pH equal to 7. The neutralization of a strong acid and weak base will have a pH of less than 7, and conversely, the resulting pH when a strong base neutralizes a weak acid will be greater than 7.

When a solution is neutralized, it means that salts are formed from equal weights of acid and base. The amount of acid needed is the amount that would give one mole of protons (H⁺) and the amount of base needed is the amount that would give one mole of (OH⁻). Because salts are formed from neutralization reactions with equivalent concentrations of weights of acids and bases: N parts of acid will always neutralize N parts of base.

Strong Acids

Strong Bases

HCl

LiOH

HBr

NaOH

HI

KOH

HCIO ₄

RbOH

HNO ₃

CsOH

Ca(OH) ₂

Sr(OH) ₂

Ba(OH) ₂

Strong Acid-Strong Base Neutralization

\[\underset{acid}{HCl_{(aq)}} + \underset{base}{NaOH_{(aq)}} \leftrightharpoons \underset{salt}{NaCl_{(aq)}} + \underset{water}{H_2O_{(l)}}\]

\[H^+_{(aq)} + \cancel{Cl^-_{​(aq)}} + \cancel{Na^+_{​(aq)}} + OH^-_{​(aq)} → \cancel{Na^+_{​(aq)}} + \cancel{Cl^-_{​(aq)}} + H_2O_{(l)}\]

When the spectator ions are removed, the net ionic equation shows the \(H^+\) and \(OH^-\) ions forming water in a strong acid, strong base reaction:

\(H^+_{(aq)} + OH^-_{(aq)} \leftrightharpoons H_2O_{(l)} \)

When a strong acid and a strong base fully neutralize, the pH is neutral. Neutral pH means that the pH is equal to 7.00 at 25 degrees Celsius. At this point of neutralization, there are equal amounts of \(OH^-\) and \(H_3O^+\). There is no excess \(NaOH\). The solution is \(NaCl\) at the equivalence point. When a strong acid completely neutralizes a strong base, the pH of the salt solution will always be 7.

Weak Acid-Weak Base Neutralization

A weak acid, weak base reaction can be shown by the net ionic equation example:

\(H^+ _{(aq)} + NH_{3(aq)} \leftrightharpoons NH^+_{4 (aq)} \)

The equivalence point of a neutralization reaction is when both the acid and the base in the reaction have been completely consumed and neither of them are in excess. When a strong acid neutralizes a weak base, the resulting solution's pH will be less than 7. When a strong base neutralizes a weak acid, the resulting solution's pH will be greater than 7.

Strength of Acid and Base

pH Level

Strong Acid-Strong Base

7

Strong Acid-Weak Base

<7

Weak Acid-Strong Base

>7

Weak Acid-Weak Base

pH <7 if \(K_a > K_b\)
pH =7 if \(K_a = K_b\)
pH >7 if \(K_a< K_b\)

Titration

One of the most common and widely used ways to complete a neutralization reaction is through titration. In a titration, an acid or a base is in a flask or a beaker. We will show two examples of a titration. The first will be the titration of an acid by a base. The second will be the titration of a base by an acid.

Example: Titrating a Weak Acid
SOLUTION
\[Ka=(7)(10^{-3})\]

\[K_a=(7)(10^{-3})=\dfrac{(x^2)M}{(0.1-x)M}\]

\[x=[H_3O^+]=0.023\;M\]

Solve for pH:

\[pH=-\log_{10}[H_3O^+]=-\log_{10}(0.023)=1.64\]

pH=pKa+log[mmol Base/mmol Acid]

pH=pKa+ log[0.65mmol/0.65mmol]

pH=pKa+log(1)

\[pH=pKa\]

Kb=(x^2)M/(0.05-x)M

\[x=[OH^-]=(2.67)(10^{-7})\]

\[pOH=-\log_{10}((2.67)(10^{-7}))=6.57\]

\[pH=14-6.57=7.43\]

\[ (14 mL)(0.1M)=1.4\; mmol OH^-\]

\[(10\; mL)(0.1\;M)= 1\;mmol \;HX\]

\[[OH-]=\frac{0.4\;mmol}{10\;mL+14\;mL}=0.17\;M\]

\[pOH=-log_{10}(0.17)=1.8\]

\[pH=14-1.8=12.2\]

Example 2

\[M_1V_1 = M_2V_2\]

\[(0.0835)(0.015) = M_2(0.0105)\]

Problems

1. Will the salt formed from the following reaction have a pH greater than, less than, or equal to seven?

\(CH3COOH_{(aq)} + NaOH_{(s)} \leftrightharpoons Na^+ + CH3COO^- + H2O_{(l)}\)

2. How many mL of .0955 M Ba(OH) ₂ solution are required to titrate 45.00 mL of .0452 M HNO ₃?

3. Will the pH of the salt solution formed by the following chemical reaction be greater than, less than, or equal to seven?

\(NaOH + H_2SO_4 \leftrightharpoons H_2O + NaSO_4\)

4. We know that it takes 31.00 mL of an unknown solution to neutralize 25.00 mL of .135 M KOH solution. What is the molarity of the unknown solution?

Solutions

1. After looking at the net ionic equation,

\[CH_3CO_2H_{(aq)} + OH^- \leftrightharpoons CH_3COO^- + H_2O_{(l)}\]

we see that a weak acid, \(CH_3CO_2H\), is being neutralized by a strong base, \(OH^-\). By looking at the chart above, we can see that when a strong base neutralizes a weak acid, the pH level is going to be greater than 7.

2. By plugging the numbers given in the problem in the the equation:

\[M_1V_1= M_2V_2\]

we can solve for \(V_2\).

\[V_2= \dfrac{M_1V_1}{M_2} = \dfrac{(0.0452)(0.045)}{0.0955} = 21.2\; mL\]

Therefore it takes 21.2 mL of \(Ba(OH)_2\) to titrate 45.00 mL \(HNO_3\).

3. We know that NaOH is a strong base and H ₂SO ₄ is a strong acid. Therefore, we know the pH of the salt will be equal to 7.

4. By plugging the numbers given in the problem into the equation:

\[M_1V_2 = M_2V_2\]

we can solve for M ₂.

(0.135)(0.025) = M ₂(0.031)

M ₂ = 0.108 M. Therefore, the molarity of the unknown solution is .108 M.

References

1. Petrucci, et al. General Chemistry: Principles & Modern Applications. 9th ed. Upper Saddle River, New Jersey: Pearson/Prentice Hall, 2007.

2. Criddle, Craig and Larry Gonick. The Cartoon Guide to Chemistry. New York: HarperCollins Publishers, 2005.

Contributors

Katherine Dunn (UCD), Carlynn Chappell (UCD)

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A neutralization reaction is when an acid and a base react to form water and a salt and involves the combination of H+ ions and OH- ions to generate water. The neutralization of a strong acid and strong base has a pH equal to 7.

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Biochemical Society | Advancing Molecular Bioscience

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The Biochemical Society exists to advance the molecular and cellular biosciences. We achieve this through publications, events, education, policy, awards and grants.

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Statistics and probability archive containing a full list of statistics and probability questions and answers from April 30 2014.

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Statistical Analysis Handbook - (c) 2015 Dr M J de Smith

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Miner3D - Smart Data Visualization in full 3D

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Data Visualization in 3D for Interactive Data Analysis

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Narrow-mindedness - It's not Them, It's Us | Maura O'Neill | TEDxRainier

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Introduction to Quantum Computing (17) - Grover's Algorithm: Problem Statement

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Volatility surprises arise in removing excess hydrogen

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Volatility surprises arise in removing excess hydrogen

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Hydrogen Ion Concentration Changes in Mitochondrial Membranes

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Hydrogen Ion Concentration Changes in Mitochondrial Membranes

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"The hydrogen ion concentration of the blood is normally controlled within narrow limits, in health rarely exceeding 46 nmol/L or falling below 35 nmol/L. This regulation is achieved in spite of the continuous production of hydrogen ions as a result of the normal processes of metabolism."

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Hydronium - an overview | ScienceDirect Topics

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Hydrogen ions are ubiquitous in the body and maintenance of appropriate concentrations is critical to normal function.

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"Studies have shown that physicians commonly order more laboratory tests than required, use them for the wrong purposes, and ignore or misinterpret their results. While these errors have obvious implications for the quality of patient care, there are large socioeconomic implications as well. Health care expenditures in the United States in the 1980s exceeded $400 billion yearly, approximately 10% of the gross national product. Of this cost, a significant proportion can be attributed to laboratory tests"

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Ordering and interpreting diagnostic tests are fundamental skills. Surprisingly, however, evidence indicates that many of us are poorly trained in this vital area. Studies have shown that physicians commonly order more laboratory tests than required, use them for the wrong purposes, and ignore or misinterpret their results.

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I want to provide an algorithm for converting an N-digit number, written in base K, to base 10. To begin, let's define:digit.
Usually, a digit refers to a value written in base 10, between 0 and 9. Non-negative numbers written in base 10 are a sequence of one or more digits.

However, I want to use a more general definition, which allows a digit to be a single value written in base K, instead of simply in base 10.

Definition A digit is a single symbol used to represent a value in base K.
Just so you don't get confused, I will write the base of a number as a subscript. For example, 723 ₈ is a 3-digit value written in base 8. It consists of the digits: '7', '2', and '3'.
Suppose a number has N digits. To make it easier to talk about each digit, I will subscript the digits from right to left, starting with the subscript 0. Therefore, an N digit value will look like: dN-1 dN-2 ... d₁ d₀.

For example, for the number 723, the digits are d₂=7, d₁=2, and d₀=3. I'll explain why the digits are subscripted right to left rather than left to right later on.

Range of Values for a Single Digit
Think about the following question: what is the range of valid values for a single digit in base K?
To answer this, let's pick a specific base to work in. What is the valid range of digits in base 10? The valid digits range from '0' through '9' inclusive.

In binary (base 2), the valid values are 0 and 1.

Do you see a pattern? Notice that the range of the values for a single digit always fall short of the base itself. Thus, for a arbitrary base, K, the valid range of values for a single digit are between 0 and K - 1, inclusive.

That raises the question: what happens when you go past base 10. For example, what about base 11? If you write out the valid range of digits, you'd get '0', '1', ... ,'9', but what comes after '9'? Should you write '10'? That doesn't seem right, because it uses TWO digits. We'd like a way to represent the value 10 with one "symbol".

It's common to use alphabetic characters, where 'A' stands for 10, B for 11, and so forth. Using alphabetic characters only goes so far. You can go up to 'Z' and represent numbers up to base 36.

(In case you're wondering, the base is equivalent to the number of symbols used for a single digit. There are 10 digits from '0' to '9', plus 26 alphabetic characters, so 36 total symbols means you can go up to base 36).

You might wonder what happens if you have go to a higher base. Do you start making up even more symbols?

Here's where we cheat a little. You don't really have to invent even more symbols to represent other digits. What you can do is to make up a symbol like vi where i is a value written in base 10. For example, suppose you need to write some numbers in base 100. One possible way to write this is: v₂₃ v₉₈ v₄.

We still think of this as a 3 digit value. The v's help us determine where the individual digits are.

Number of possible values for N digit number in base K
Computers store numbers using a finite number of digits. Obviously, if it were infinite, you would need an infinite amount of memory to store a number. However, it's even more restrictive. Computers typically store numbers using very few bits. Numbers are often stored using 32 or 64 bits. This means there is both a maximum and a minimum value. This also means there are a finite number of representable numbers.
This is very important! There are some numbers that can't be natively represented on a computer (there are some inefficient ways around this problem, though).

To explore this further, let's consider an N digit number in base K. How many possible different N-digit patterns are there? (I occasionally call the numbers, patterns, so don't let that bother you).

The answer requires simple combinatorics. Since each digit is in base K, there are K choices for the first digit, K choices for the second digit, and so forth. The answer is to multiply the choices together to get K multiplied to itself N times, or simply, K N.

Again, let's consider a simple example. How many 4-digit base-3 patterns are there? The answer is 3 ⁴ = 81. Since these values are contiguous starting at 0, we can represent numbers from 0 up to 80 in base 3 using exactly 4 base-3 digits.

Converting from base 10 to base 10
After all that introduction, we're ready to convert numbers. We're going to do this in a similar way that you were taught in elementary school (assuming you did your education in the US---if not, hopefully, this won't be too hard to follow).
In elementary school days, you may have been that that digits have places. For example, there's the 1's place, the 10's place, the 100's place and so forth.

Here's an example of "1976" written. The first row indicates the "place" and the second row are the individual digits of 1976.

1000's

100's

10's

1's

1

9

7

6

You were taught that you could write these numbers out as:

   1976 = (1 * 1000) + (9 * 100) + (7 * 10) + (6 * 1)
        = 1976

In words, you multiplied the digit by its place, and summed the result together. For example, 9 is in the hundred's place, so you multiply 9 by 100.
This elementary school exercise must have seem utterly trivial to you, and perhaps you wondered why you were taught that 1976 happens to equal 1976.

Only this is not nearly as easy as you might think. In elementary school, they may have also taught you Roman numerals. You hardly ever see Roman numerals outside of some copyright years on films (and that's starting to be rare) or the current incarantion of the Super Bowl (which seems to the be last major display of Roman numerals).

Consider a number like DCLXXIX. If memory serves, this Roman numerals for 679 in base 10. Roman numerals have bizarre rules. For example, 7 is written VII. 8 is written VIII. 9 is written IX. When the I comes before the X, it means to subtract 1. Roman numerals have some odd combination of unary values (III meaning 3) and subtraction (such as IX). The rules are more complex than Arabic numerals, the numbers you and I are most familiar with.

Instead of calling it the thousands place, the hundreds places, and so forth, we're going to modify the scheme just a little by using powers of 10.

Thus, we can write:

Recall that any positive number raised to the power 0 is 1.
Thus, we can rewrite 1976 as:

   1976 = (1 * 10³) + (9 * 10²) + (7 * 10¹) + (6 * 10⁰) = 1976
Let's use that notation with the d and the subscripts.

d ₃

d ₂

d ₁

d ₀

10 ³

10 ²

10 ¹

10 ⁰

1

9

7

6

What do you notice in common between the green rows and the red ones? You'll notice that the subscript of the green rows match the superscript of the red one. For example, d₃ refers to 10³.
Earlier on, I said that I'd explain why we number right to left, starting at index 0. This is the reason. The subscripts refer to the power of 10 you multiply the value.

This leads us to the following formula:

You multiply the digit's value by 10 raised to the power of the digit's subscript.
Converting from base K to base 10
While the above formula may have seemed trivially stupid in elementary school, it's important when it comes to other bases. For example, what if I ask you "what is 212 ₃? in base 10". How would you answer?
The summation formula given above works in other bases, but we have to modify it somewhat.

The main difference is that 10 has been replaced by K, where K is the base.
Let's try a simple example:

Here, K = 3, so we just use the summation formula:

   212₃ = (2 * 3²) + (1 * 3¹) + (2 * 3⁰) = (2 * 9) + (1 * 3) + (2 * 1) = 18 + 3 + 2 = 23₁₀
If K (the base) is greater than 10, then the problem is a little trickier. You must convert the alphabetic characters to their base 10 values. For example,

d ₂

d ₁

d ₀

11 ²

11 ¹

11 ⁰

1

1

A

We wish to convert11A₁₁ to base 10.

   11A₁₁ = (1 * 11²) + (1 * 11¹) + (A * 11⁰) = (1 * 121) + (1 * 11) + (10 * 1) = 121 + 11 + 10 = 142₁₀

Converting Fractional Values From Base 10 to Base K
You can write fractional values in base 10 by including a decimal point and values afterwards. Since decimal point suggest "base 10", we'll use the more generalradix point as our terminology. In base 10, the radix point is called the decimal point.
We only need to extend the summation formula given earlier to handle a finite number of digits left of the radix point and right of the radix point. Assume that we have a N digits left of the radix point and M digits right of the radix point.

The summation formula given in previous sections can be generalized to:

Let's look at an example of this. Suppose you want to convert 101.011₂ to base 10. You'd write:

d ₂

d ₁

d ₀

d₋₁

d₋₂

d₋₃

2 ²

2 ¹

2 ⁰

2⁻¹

2⁻²

2⁻³

1

0

1

0

1

1

Using the summation formula, where N=3 and M=3, we get:

   101.011₂ = (1 * 2²) + (0 * 2¹) + (A * 2⁰) + (0 * 2⁻¹) + (1 * 2⁻²) + (1 * 2⁻³) = (1 * 4) + (0 * 2) + (1 * 1) + (0 * .5) + (1 * .25) + (1 * .125) = 4 + 0 + 1 + 0 + .25 + .125 = 5.375₁₀
Notice that the summation still works. We now add negative subscripts for digits right of the radix point, and just like before, we raise the base to the negative power, based on the subscript.
Summary
To convert a number written in base K, with N digits left of the radix point and M digits right of the radix point, use the following summation:

If there are no values right of the radix point. That is, if M=0, then the number of numeric "patterns" (i.e., the number of possible values) of an N digit base-K number isKN.
Those are probably the two most pertinent facts presented here.

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I want to provide an algorithm for converting an N-digit number, written in base K, to base 10. To begin, let's define:digit.Usually, a digit refers to a value written in base 10, between 0 and 9. Non-negative numbers written in base 10 are a sequence of one or more digits.

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In stark contrast to earlier findings, adults do not produce new nerve cells in a brain area important to memory and navigation, scientists conclude after scrutinizing 54 human brains spanning the age spectrum.

The finding is preliminary. But if confirmed, it would overturn the widely accepted and potentially powerful idea that in people, the memory-related hippocampus constantly churns out new neurons in adulthood. Adult brains showed no signs of such turnover in that region, researchers reported November 13 at a meeting of the Society for Neuroscience in Washington, D.C.

Previous studies in animals have hinted that boosting the birthrate of new neurons, a process called neurogenesis, in the hippocampus might enhance memory or learning abilities, combat depression and even stave off the mental decline that comes with dementia and old age ( SN: 9/27/08, p. 5). In rodents, exercise, enriched environments and other tweaks can boost hippocampal neurogenesis - and more excitingly, memory performance. But the new study may temper those ambitions, at least for people.

Researchers studied 54 human brain samples that ranged from fetal stages to age 77, acquired either postmortem or during brain surgery. These samples were cut into thin slices and probed with molecular tools that can signal dividing or young cells, both of which are signs that nerve cells are being born.

As expected, fetal and infant samples showed evidence of both dividing cells that give rise to new neurons and young neurons themselves in the hippocampus. But with age, these numbers declined. In brain tissue from a 13-year-old, the researchers spotted only a handful of young neurons. And in adults, there were none.

"We were unable to detect young neurons in adult human hippocampus," study coauthor Shawn Sorrells of the University of California, San Francisco said in his presentation.

The researchers also looked for physical signs of dividing cells and young neurons, such as small, elongated nuclei, using electron microscopy and light microscopy. Again, there was no evidence of newborn neurons in adults, Sorrells said in his presentation. (Because the results have been submitted for potential publication in a journal, Sorrells declined to comment for this story.)

That absence of young hippocampal neurons conflicts with some earlier research. A definitive answer has eluded scientists in part because there's no easy way to study these cells in living people. Several of the earlier studies relied on unusual circumstances to spot cells born in adulthood.

A landmark study, published in Nature Medicine in 1998, found newborn neurons in the hippocampi of people who, as part of their cancer treatment, had been dosed with an imaging molecule called BrdU that gets incorporated into the DNA of newly formed neurons.

A different approach, published in 2013 in Cell, led to the same conclusion. By looking at people exposed to radioactive carbon released during Cold War bomb testing as adults, Jonas Frisén and colleagues found a surprisingly large number of newborn hippocampal cells - an estimated 700 new cells per day in each hippocampus (brains have one hippocampus on each side).

The methods used to study tissue from people after they've died can be finicky, particularly that from older people. Molecular markers of neurogenesis in adult brain tissue can be difficult to see, making the method "extremely challenging," says Frisén, of the Karolinska Institute in Stockholm.

It's also possible that the birth rate of neurons varies widely from person to person. "There are huge differences between different mouse strains," Frisén says, and it's not unreasonable that similar differences might exist in people. In the new study, the subjects' health could have declined before their deaths, reducing neurogenesis at the very end of their lives.

Experts say the preliminary study is not the last word. "I would love to see more data," says Gerd Kempermann of Technische Universität Dresden in Germany. In the meantime, the body of evidence in favor of human neurogenesis in the hippocampus, including results from his lab, is quite strong, Kempermann says.

"I am convinced enough that an abstract at a meeting claiming otherwise does not shake up my concept," Kempermann says. But he thinks it will be valuable to explore other explanations for the results, even if they don't overturn the idea of human hippocampus neurogenesis. "Let's be open and wait," he says.

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Study casts doubt on whether adult brain's memory-forming region makes new cells

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In stark contrast to earlier findings, adults do not produce new nerve cells in a brain area important to memory and navigation, scientists conclude after scrutinizing 54 human brains spanning the age spectrum. The finding is preliminary. But if confirmed, it would overturn the widely accepted and potentially powerful idea that in people, the memory-related hippocampus constantly churns out new neurons in adulthood.

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Researchers have successfully incorporated washable, stretchable and breathable electronic circuits into fabric, opening up new possibilities for smart textiles and wearable electronics. The circuits were made with cheap, safe and environmentally friendly inks, and printed using conventional inkjet printing techniques.

The researchers, from the University of Cambridge, working with colleagues in Italy and China, have demonstrated how graphene -- a two-dimensional form of carbon -- can be directly printed onto fabric to produce integrated electronic circuits which are comfortable to wear and can survive up to 20 cycles in a typical washing machine.

The new textile electronic devices are based on low-cost, sustainable and scalable inkjet printing of inks based on graphene and other two-dimensional materials, and are produced by standard processing techniques. The results are published in the journal Nature Communications.

Based on earlier work on the formulation of graphene inks for printed electronics, the team designed low-boiling point inks, which were directly printed onto polyester fabric. Additionally, they found that modifying the roughness of the fabric improved the performance of the printed devices. The versatility of this process allowed the researchers to design not only single transistors but all-printed integrated electronic circuits combining active and passive components.

Most wearable electronic devices that are currently available rely on rigid electronic components mounted on plastic, rubber or textiles. These offer limited compatibility with the skin in many circumstances, are damaged when washed and are uncomfortable to wear because they are not breathable.

"Other inks for printed electronics normally require toxic solvents and are not suitable to be worn, whereas our inks are both cheap, safe and environmentally-friendly, and can be combined to create electronic circuits by simply printing different two-dimensional materials on the fabric," said Dr Felice Torrisi of the Cambridge Graphene Centre, the paper's senior author.

"Digital textile printing has been around for decades to print simple colorants on textiles, but our result demonstrates for the first time that such technology can also be used to print the entire electronic integrated circuits on textiles," said co-author Professor Roman Sordan of Politecnico di Milano. "Although we demonstrated very simple integrated circuits, our process is scalable and there are no fundamental obstacles to the technological development of wearable electronic devices both in terms of their complexity and performance."

"The printed components are flexible, washable and require low power, essential requirements for applications in wearable electronics," said PhD student Tian Carey, the paper's first author.

The work opens up a number of commercial opportunities for two-dimensional material inks, ranging from personal health and well-being technology, to wearable energy harvesting and storage, military garments, wearable computing and fashion.

"Turning textile fibres into functional electronic components can open to an entirely new set of applications from healthcare and wellbeing to the Internet of Things," said Torrisi. "Thanks to nanotechnology, in the future our clothes could incorporate these textile-based electronics, such as displays or sensors and become interactive."

The use of graphene and other related 2D material (GRM) inks to create electronic components and devices integrated into fabrics and innovative textiles is at the centre of new technical advances in the smart textiles industry. The teams at the Cambridge Graphene Centre and Politecnico di Milano are also involved in the Graphene Flagship, an EC-funded, pan-European project dedicated to bringing graphene and GRM technologies to commercial applications.

The research was supported by grants from the Graphene Flagship, the European Research Council's Synergy Grant, The Engineering and Physical Science Research Council, The Newton Trust, the International Research Fellowship of the National Natural Science Foundation of China and the Ministry of Science and Technology of China. The technology is being commercialised by Cambridge Enterprise, the University's commercialisation arm.

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electronics; technology; wearable technology; textiles and clothing; spintronics; graphene; engineering and construction; consumer electronics

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Fully integrated circuits printed directly onto fabric

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Researchers have successfully incorporated washable, stretchable and breathable electronic circuits into fabric, opening up new possibilities for smart textiles and wearable electronics. The circuits were made with cheap, safe and environmentally friendly inks, and printed using conventional inkjet printing techniques.

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Learn more about Gordon Moore and Moore's Law at the Intel Museum's permanent exhibit dedicated to Gordon Moore and Moore's Law, located in Intel's Robert Noyce Building, Santa Clara, California.

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Learn more about Gordon Moore and Moore's Law at the Intel Museum's permanent exhibit dedicated to Gordon Moore and Moore's Law, located in Intel's Robert Noyce Building, Santa Clara, California.

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A new type of smart fabric developed at the University of Washington could pave the way for jackets that store invisible passcodes and open the door to your apartment or office.

The UW computer scientists have created fabrics and fashion accessories that can store data -- from security codes to identification tags -- without needing any on-board electronics or sensors.

As described in a paper presented Oct. 25 at the Association for Computing Machinery's User Interface Software and Technology Symposium (UIST 2017), they leveraged previously unexplored magnetic properties of off-the-shelf conductive thread. The data can be read using an instrument embedded in existing smartphones to enable navigation apps.

"This is a completely electronic-free design, which means you can iron the smart fabric or put it in the washer and dryer," said senior author Shyam Gollakota, associate professor in the Paul G. Allen School of Computer Science & Engineering. "You can think of the fabric as a hard disk -- you're actually doing this data storage on the clothes you're wearing."

Most people today combine conductive thread -- embroidery thread that can carry an electrical current -- with other types of electronics to create outfits, stuffed animals or accessories that light up or communicate.

But the UW researchers realized that this off-the-shelf conductive thread also has magnetic properties that can be manipulated to store either digital data or visual information like letters or numbers. This data can be read by a magnetometer, an inexpensive instrument that measures the direction and strength of magnetic fields and is embedded in most smartphones.

"We are using something that already exists on a smartphone and uses almost no power, so the cost of reading this type of data is negligible," said Gollakota. In one example, they stored the passcode to an electronic door lock on a patch of conductive fabric sewn to a shirt cuff. They unlocked the door by waving the cuff in front of an array of magnetometers.

The UW researchers also created fashion accessories like a tie, belt, necklace and wristband and decoded the data by swiping a smartphone across them.

They used conventional sewing machines to embroider fabric with off-the-shelf conductive thread, whose magnetic poles start out in a random order. By rubbing a magnet against the fabric, the researchers were able to physically align the poles in either a positive or negative direction, which can correspond to the 1s and 0s in digital data.

Like hotel card keys, the strength of the magnetic signal weakens by about 30 percent over the course of a week, though the fabric can be re-magnetized and re-programmed multiple times. In other stress tests, the fabric patch retained its data even after machine washing, drying and ironing at temperatures of up to 320 degrees Fahrenheit.

This is in contrast to many smart garments today that still require on-board electronics or sensors to work. That can be problematic if you get caught in the rain or forget to detach those electronics before throwing them in the washing machine -- a potential barrier to widespread adoption of other wearable technology designs.

The team also demonstrated that the magnetized fabric could be used to interact with a smartphone while it is in one's pocket. Researchers developed a glove with conductive fabric sewn into its fingertips, which was used to gesture at the smartphone. Each gesture yields a different magnetic signal that can invoke specific actions like pausing or playing music.

"With this system, we can easily interact with smart devices without having to constantly take it out of our pockets," said lead author Justin Chan, an Allen School doctoral student.

In the team's tests, the phone was able to recognize six gestures -- left flick, right flick, upward swipe, downward swipe, click and back click -- with 90 percent accuracy. Future work is focused on developing custom textiles that generate stronger magnetic fields and are capable of storing a higher density of data.

The research was funded by the National Science Foundation, the Alfred P. Sloan Foundation and Google.

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wearable technology; textiles and clothing; consumer electronics; electronics; hacking; information technology; encryption; computers and internet

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How to store information in your clothes invisibly, without electronics

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A new type of smart fabric developed at the University of Washington could pave the way for jackets that store invisible passcodes and open the door to your apartment or office. The UW computer scientists have created fabrics and fashion accessories that can store data -- from security codes to identification tags -- without needing any on-board electronics or sensors.

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These proceedings of the October 2007 symposium includes papers reflecting the diversity of this dynamic field, with no less than 11 major topics. These include high dielectric constant ("high-k") materials (based or not based on Hf), defects and interfaces, characterization (electrical and physical/chemical) reliability, metal gate electrodes, process integration, high mobility substrates and high-k memories. Topics of invited papers include the tight distribution of dielectric characteristics of HfSiON in metal gate devices, dynamical properties of orthorhombic phases of Group IVb transition metal oxides, interface characterization in nanoelectronics, theoretical studies on Fermi level pining of 11f-based high-k gate stacks based on thermodynamics, the role of ionicity in defect formation in Hf-based dielectrics, challenges in gate stack etching and cleaning, crystalline rare-earth oxides as high-k materials for future CMOS technologies, high-k characterization by RFCV, uses for germanium, and the essence of VFB shifts in high-k gate stacks. Includes author index.

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Physics and Technology of High-k Gate Dielectrics 5

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These proceedings of the October 2007 symposium includes papers reflecting the diversity of this dynamic field, with no less than 11 major topics. These include high dielectric constant ("high-k") materials (based or not based on Hf), defects and interfaces, characterization (electrical and physical/chemical) reliability, metal gate electrodes, process integration, high mobility substrates and high-k memories.

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Interactive periodic table showing names, electrons, and oxidation states. Visualize trends, 3D orbitals, isotopes, and mix compounds. Fully descriptive writeups.