# data[].attributes.body
where k e is Coulomb's constant (k e ≈ 9×10⁹ N⋅m²⋅C−2), q ₁ and q ₂ are the signed magnitudes of the charges, and the scalar r is the distance between the charges. The force of the interaction between the charges is attractive if the charges have opposite signs (i.e., F is negative) and repulsive if like-signed .T his formula gives the intensity of the force, NOT the direction, which depends on the charges' sign.
The force is along the straight line joining them. If the two charges have the same sign, the electrostatic force between them is repulsive; if they have different signs, the force between them is attractive.

# data[].attributes.body
"The electric force between two charges at rest is directly proportional to the charges themselves and inversely proportional to the square of the distance"
This is the main postulate of the Coulomb's law that is summarized in this formula:

# data[].attributes.body
He used a device invented by himself called "torsion balance". Torsion balance, device used to measure the gravitational acceleration at the Earth’s surface. Other such devices, using different methods to obtain the same result, are pendulums and gravimeters. The torsion balance consists essentially of two small masses at different elevations that are supported at opposite ends of a beam. The latter is suspended from a wire that undergoes torsion because the masses are affected differently by the force of gravity. When the wire is twisted, an optical system indicates the angular deflection (θ), and the torque (T), or twisting force, involved can be calculated from the simple expression Fd = kθ, in which k is a constant that depends on the properties of the instrument.

# data[].attributes.body
Millikan oil drop experiment, first direct and compelling measurement of the electric charge of a single electron. It was performed originally in 1909 by the American physicist Robert A. Millikan, who devised a straightforward method of measuring the minute electric charge that is present on many of the droplets in an oil mist. The force on any electric charge in an electric field  is equal to the product of the charge and the electric field. Millikan was able to measure both the amount of electric force and magnitude of electric field on the tiny charge of an isolated oil droplet and from the data determine the magnitude of the charge itself.
Millikan’s original experiment or any modified version, such as the following, is called the oil-drop experiment. A closed chamber with transparent sides is fitted with two parallel metal plates, which acquire a positive or negative charge when an electric current is applied. At the start of the experiment, an atomizer sprays a fine mist of oil droplets into the upper portion of the chamber. Under the influence of gravity and air resistance, some of the oil droplets fall through a small hole cut in the top metal plate. When the space between the metal plates is ionized by radiation (e.g., X-rays), electrons from the air attach themselves to the falling oil droplets, causing them to acquire a negative charge. A light source, set at right angles to a viewing microscope, illuminates the oil droplets and makes them appear as bright stars while they fall. The mass of a single charged droplet can be calculated by observing how fast it falls. By adjusting the potential difference, or voltage, between the metal plates, the speed of the droplet’s motion can be increased or decreased; when the amount of upward electric force equals the known downward gravitational force, the charged droplet remains stationary. The amount of voltage needed to suspend a droplet is used along with its mass to determine the overall electric charge on the droplet. Through repeated application of this method, the values of the electric charge on individual oil drops are always whole-number multiples of a lowest value—that value being the elementary electric charge itself (about 1.602 10−19 coulomb). From the time of Millikan’s original experiment, this method offered convincing proof that electric charge exists in basic natural units. All subsequent distinct methods of measuring the basic unit of electric charge point to its having the same fundamental value.

Millikan was able to calculate the mass of each droplet:
m = 6πrηV/g

When an electron jumps on a droplet, this one becomes negatively charged:
kq = mg       SO     q = mg/k

# data[].attributes.body
A Van De Graaff generator is an electrostatic generator, which uses a moving belt to accumulate electric charge on a hollow metal globe on the top of an insulated column, creating very high electric potentials.
It produced very high voltage direct current electricity at low current levels.
The potential difference achieved in modern Van De Graaff generators can reach 5 megaVolts.
In order to double the voltage, two generators are often used together: one generating positive and the other negative potential; this is called a tandem Van De Graaff accelerator.

# data[].attributes.body
Invented by Volta, this instrument is the first example of an electrostatic machine based on the induction principle.

The earthed shield is placed on the plate which has previously been charged by rubbing it with a cloth. The shield is earthed by touching it with one's finger. Once the earth is removed, the shield is lifted and becomes charged by electrostatic induction. Without needing to rub the plate again, the shield can be repeatedly put on the plate and removed, thus getting quite a high charge each time. This machine could continually supply a lasting electric charge. It therefore was known as the "perpetual electrophorus".

# data[].attributes.attachment_link.title
3 experiments of electrostatic induction

# data[].attributes.body
Polarization is the process of separating opposite charges within an object.
The positive charge is separated from the negative one. inducting the movement of electrons within an object, one side is left with an excess of positive charge and the other is left with an excess of negative charge.
The polarization process always involves the use of a charged object to induce electron movement or its rearrangement.
In a conducting object, electrons are induced into movement from one side of the object to the other/opposite one. In an insulator, electrons simply redistribute themselves within the atom or molecules the nearest to its outer surface.
Electrons are not free to move across the surface of an insulator

# data[].attributes.attachment_link.title
The Gold Leaf Electroscope - Current Electricity (CBSE Grade 08 Physics)

# data[].attributes.body
The gold leaf electroscope is an instrument for detecting and measuring static electricity or voltage.
A metal disc is connected to a narrow metal plate and a thin piece of gold leaf is fixed to the plate. The whole of this part of the electroscope is insulated from the body of the instrument. A glass front prevents air draughts but allows you to watch the behaviour of the leaf.
When a charge is put on the disc at the top it spreads down to the plate and leaf. This means that both the leaf and plate will have the same charge. Similar charges repel each other and so the leaf rises away from the plate - the bigger the charge the more the leaf rises.

The leaf can be made to fall again by touching the disc - you have earthed the electroscope. An earth terminal prevents the case from becoming live. The electroscope can be charged in two ways:

(a) by contact - a charged rod is touched on the surface of the disc and some of the charge is transferred to the electroscope. This is not a very effective method of charging the electroscope.

(b) by induction - a charged rod is brought up to the disc and then the electroscope is earthed, the rod is then removed.

The two methods give the gold leaf opposite charges.

# data[].attributes.body
Ernest Rutherford, while he was using the Crookes Tube for his experiments on x-rays, found  that these rays could be divided in three types of particles:  alpha, beta, and gamma particles, and he classified them by their ability to penetrate matter. Because alpha particles carry more electric charge, are more massive, and move slowly compared to beta and gamma particles, they interact much more easily with matter. Beta particles are much less massive and move faster, but are still electrically charged. A sheet of aluminum one millimeter thick or several meters of air will stop these electrons and positrons. Because gamma rays carry no electric charge, they can penetrate large distances through materials before interacting–several centimeters of lead or a meter of concrete is needed to stop most gamma rays. in 1897 Thomson using The plum pudding model explained an atom as a positive charge body which contains small negatively charged particles which are called electrons. He also described that the negative charge in atom is balanced with the equal amount of positive charge to maintain the neutrality of atom. But there were some faults in this model of Thomson. He did not give the complete structure of atom which was then given by Rutherford in his Gold Foil Experiment in 1898 which was published in 1911.

Rutherford discovered the concept of nucleus in atom. His research is based on the experiment with alpha particles. Alpha particles are helium atom particles. He did bombardment of positive alpha particles on thin foil of gold approx. 8.6 x 10 -6 centimetres thick and took the observations on the screen of zinc sulphide which was behind the gold foil. He observed the deflection of these bombarded alpha particles on the photographic film. This experiment was determined to find out the structure of an atom. By this time it was discovered by J.J. Thomson that electrons are present in an atom and that they are negatively charged. So it was assumed that since an atom is neutral and electrons present are negatively charged, there should be some positive charge inside it that makes it neutral. So Rutherford worked under the discoveries and assumption of J.J. Thomson. He accepted J.J. Thomson’s model of an atom which was plum pudding model.

According to the plum pudding model, there were negatively charged particles i.e. electrons embedded or suspended in a sphere of positive charge (electrons presented as plums inside the bowl of pudding).
Ernest Rutherford set up an apparatus and did an experiment that could confirm JJ Thomson’s model of an atom. But he ended up with some new facts in the structure of the atom.

Construction of his experiment:

It has a radioactive source rich in positively charged heavy alpha particles inside a cube shaped thick lead box with a narrow opening.
The alpha particles were confined to a narrow beam by passing then through a lead sheet through a slit. An extremely thin gold foil was bombarded with the narrow beam of fast moving alpha particles. On bombarding the alpha particles were scattered in different directions with different angles and were detected by florescent rotatable detector which has a microscope and a screen coated with zinc sulphide. The whole experimental setup was placed in an evacuated chamber to prevent scattering by the air molecules. These particles after striking on the screen caused scintillations. Before performing this experiment it was assumed by Rutherford that most of the alpha particles would pass through the gold foil with less deflection. He assumed this on the basis of theory proposed by JJ Thomson. This was assumed because the alpha particles are heavy and the negative charge in the "plum pudding model" is widely spread.

After performing his experiment he made observations:

Almost all the alpha particles did pass through the foil but

Some alpha particles were deflected off at different angles as observed at the screen of the detector.

Very few of the alpha particles (one or two) even bounced backwards after hitting the gold foil.

On the basis of these observations Rutherford made the following conclusions:

Since most of the alpha particles passed straight through the gold foil without any deflection, most of the space within the atoms is empty.

Since some of the alpha particles (which are big in size) were deflected by large angles or bounced backwards, they must have approached some positively charged region responsible for the deflection. This positively charged region is now called the nucleus.

As very few alpha particles undergone the deflection, it was concluded that the volume occupied by the central region ( nucleus ) is very small.

Since alpha particles which are relatively denser, were deflected by the central volume of charge, it shows that almost the complete mass of the atom must be within the central volume.

Upon analysis of his experiment in 1911, he suggested that the plum pudding model of J.J. Thomson of the atom had drawbacks. Rutherford's new model for the atom, based on the experimental results, contained the new features as follows:

High central charge was concentrated into a very small volume in comparison to the rest of the atom and

This central volume also contained most of the atomic mass of the atom. This region was named the "nucleus" of the atom in later years.

Rutherford's model did not make any new headway in explanation of the electron-structure of the atom.

# data[].attributes.body
In 1896 Henri Becquerel was using naturally fluorescent minerals to study the properties of x-rays, which had been discovered in 1895 by Wilhelm Roentgen. He exposed potassium uranyl sulfate to sunlight and then placed it on photographic plates wrapped in black paper, believing that the uranium absorbed the sun’s energy and then emitted it as x-rays. To his surprise, the images were strong and clear, proving that the uranium emitted radiation without an external source of energy such as the sun. Becquerel had discovered radioactivity. The term radioactivity was actually coined by Marie Curie, who together with her husband Pierre, began investigating the phenomenon recently discovered by Becquerel. The Curies extracted uranium from ore and to their surprise, found that the leftover ore showed more activity than the pure uranium. They concluded that the ore contained other radioactive elements. This led to the discoveries of the elements polonium and radium.

# data[].attributes.body
In late 1895, a German physicist, W. C. Roentgen was working with a cathode ray tube in his laboratory. He was working with tubes similar to our fluorescent light bulbs. He evacuated the tube of all air, filled it with a special gas, and passed a high electric voltage through it. When he did this, the tube would produce a fluorescent glow. Roentgen shielded the tube with heavy black paper, and found that a green colored fluorescent light could be seen coming from a screen setting a few feet away from the tube. He realized that he had produced a previously unknown "invisible light," or ray, that was being emitted from the tube; a ray that was capable of passing through the heavy paper covering the tube. Through additional experiments, he also found that the new ray would pass through most substances casting shadows of solid objects on pieces of film. He named the new ray X-ray, because in mathematics "X" is used to indicated the unknown quantity.

In his discovery Roentgen found that the X-ray would pass through the tissue of humans leaving the bones and metals visible. One of Roentgen's first experiments late in 1895 was a film of his wife Bertha's hand with a ring on her finger (photo down below). The news of Roentgen's discovery spread quickly throughout the world. Scientists everywhere could duplicate his experiment because the cathode tube was very well known during this period. In early 1896, X-rays were being utilized clinically in the United States for such things as bone fractures and gun shot wounds. one year later Henri bequerl

# data[].attributes.body
"Everything is light, everything is warmth, everything is electricity, everything is a magnetic field, everything is you."

LESSON 1
Electricity is everywhere around us, even inside ourselves. In fact in our bodies there are a lot of electric impulses (like our brain impulse).

# data[].attributes.body
But if you think that electricity is a recent discovery, you are absolutely wrong!
In fact ancient egyptian was the first people to begin studies on electricity with an experiment called "torpedo".
Also Greek people studied electricty: Thales of Miletus analyzed a special stone, the amber, finding out that touching it with hair it provokes an attraction to little bodies.
Then, William Gilbert, in the 17th century, affirmed that if amber got scratched it would have got electrified and this phenomenon is called electricity by friction. Gilbert defined this discovery "electricity", which derives from the greek word "electron".

# data[].attributes.body
The first case where two objects interact using electricity is called electricity by friction. For example, in the first case, when we put one to each other two things made of the same material, then we can notice that there is no phenomena. If we scratch them and  put them near each other, they'll make a repulsion effect.
if  we put near each other two objects made of different materials and one of the two rods is made of glass or strands of hair there is an attraction.
thnaks to these informations B. Franklin was the first to divide the materials in two types:  the resinous material(which  conventionally represent negative electricity) and the vitreous material (which conventionally represent the positive electricity).
Today we know that two like charges repells each other while two unlike charges attract each other and the greater is the charge, the greater is the force.

# data[].attributes.body
The electric force becomes smaller with the progression of the distance, for example, we have metal sphere on a wood support: at first the metal sphere is neutral, then we take a plastic rod that has an attractive force. If we touch the metal sphere with a rod, the sphere becomes negatively charged. This phenomenon is called electricity by contact. It concerns only metal. If we put near each other two metal spheres, one with no charge and another with electric charge and both are connected thanks to a plastic rod, the plastic will not let flow the charge from a sphere to another because it acts like an insulator. But, if we put a metal rod this one will act like a conductor.
Remember: our body is a really nice conductor. If you don't trust me try to put two fingers into the electric socket.

# data[].attributes.body
In 1897, for the first time scientist understood electricity (before 1897 electricity was considered as a fluid.). The first experiment was made by Joseph Thomson. In 1897, using a Crookes Tube, he set out to prove that the cathode rays produced from the cathode were actually a stream of negatively charged particles called electrons. If a gas sample is introduced into the region between two charged plates, a current flow can be observed, suggesting that the atoms have been broken down into charged constituents. The source of these charged particles is a heated cathode that, in fact, causes the atoms of the sample to ionize. These were known as cathode rays. Then he brought a positive charge near the tube. The ray started changing its direction, and also the same thing happened with a negative charge. He found out that there is a relation between negative charges and mass which is called charge to mass ratio (e/m). He was able to find out the mass of the particles, which are 2000 times smaller than hydrogen atom. while dalton said that in his atomic model the atoms were indivisible, Thomson was able to understand that the truth is that atoms are made of smaller  negative particles and as a consequence, there must be also some positive particles. this new atomic model was called Plum Pudding model.

# data[].attributes.body
In the 19th century, John Dalton, an English chemist, meteorologist and physicist, began a series of experiments which would culminate in him proposing the first atomic model (which would be known as Dalton’s Atomic Theory). Dalton came up with his theory of atoms as a result of his research into gases that he has done in 1800.

In the course of this research into gases, Dalton found out that certain gases could only be combined in certain proportions, even if two different compounds shared the same common element or group of elements.

These experiments were based on two laws that had emerged near the end of the 18th century which dealt with chemical reactions. The first law was the law of conservation of mass, formulated by Antoine Lavoisier in 1789, which states that the total mass in a chemical reaction remains constant. The second law was the law of definite proportions, formulated by the French chemist Joseph Louis Proust in 1799. This law states that if a compound is broken down into its constituent elements, then the masses of the constituents will always have the same proportions, regardless of the quantity or source of the original substance.

Studying these laws and building on them, Dalton developed his “law of multiple proportions”. This law states that the elements combine at the atomic level in fixed ratios which naturally differ based on the compounds being combined, due to their unique atomic weights. The findings became the basis of of Dalton’s Atomic Laws or Model, which focuses on five basic theorems:

1. The state that elements, in their purest state, consist of particles called atoms;

2. That atoms of a specific element are all the same, down to the very last atom;

3. That atoms of different elements can be told apart by their atomic weights;

4. That atoms of elements unite to form chemical compounds;

5. That atoms can neither be created or destroyed in chemical reaction, only the grouping ever changes.

Moreover, Dalton proposed that each chemical element is composed of atoms of a single, unique type, and though they cannot be altered or destroyed by chemical means, they can combine to form more complex structures (chemical compounds). This marked the first truly scientific theory of the atom, since Dalton reached his conclusions by experimentation and examination of the results. The unique flaw in Dalton’s atomic model was that the atom wasn’t the smallest part of matter. In fact it could be subdivided into even smaller elementary particles. And whereas Dalton conceived as atoms as a single entity with no separation between positive, negative, and neutral charges, later the experiments made by J.J. Thomson, Ernest Rutherford, and Neils Bohr revealed a more complex structure to the atom.