GR9 ELECTROSTATICS by Lizelle Swanepoel

ELECTROSTATICS

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Can you explain how the polystyrene chips stick to this cat without adhesive?

Why do these twins' hair stand on end like this?

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Via a force called electrostatic force acting between static charges.

DEFINITION OF STATIC ELECTRICITY

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DEFINITION:
Static electricity is stationary (still-standing) charge that builds up on a material’s surface by friction (due to rubbing).

ELECTROSTATICS - The study of stationary electric charge:

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QUESTIONS

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Filled out answers to questions:

DEVICES USED TO STUDY CHARGE: ELECTROSCOPE

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CHARGING AN ELECTROSCOPE BY INDUCTION

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2. VAN DE GRAAFF GENERATOR

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HOW THE VAN DE GRAAFF GENERATOR WORKS

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ROBERT VAN DE GRAAFF (1901 – 1967)

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He was a Dutch-American physicist, noted for his design and construction of high-voltage Van de Graaff generators.

3. Wimshurst machine

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LAW/PRINCIPLE OF CONSERVATION OF CHARGE

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DEFINITION: PRINCIPLE OF CONSERVATION OF CHARGE

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The principle of conservation of charge states:
The net charge of an isolated system remains constant during any physical process (e.g. two charged objects making contact and separating).

CHARGE ON INSULATORS VS CONDUCTORS

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Metals are good conductors, so charge is free to move around in and on it.
Insulators (e.g. plastic and rubber) don't allow electric charges to move through them.

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REGULAR VS IRREGULAR SHAPED CONDUCTORS

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EVEN VS UNEVEN SPREAD OF CHARGE:

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THE CHARGE ON AN ELECTRON

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The elementary charge, usually denoted by e or sometimes q_e, is the electric charge carried by a single electron.

Elementary charge = charge on a single electron = -1,6 x 10^-19C.
Robert A. Millikan's oil drop experiment first measured the size of the elementary charge in 1909.

QUANTISATION OF CHARGE

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What is quantization?

SHARING OF CHARGE

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PROBLEM: CHARGE ON CONDUCTING SPHERES AFTER TOUCHING AND SEPARATION

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GROUNDING AN ELECTROSCOPE

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The grounding process involves:

A transfer of electrons between the charged electroscope and the conducting object to which it is touched.
When a negatively charged electroscope is touched, electrons exit the electroscope to the ground.
When a positively charged electroscope is touched, electrons enter the electroscope from the ground.

POLARISATION OF WATER IN A STREAM BY CHARGED ROD

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Induction occurs in conductors,
- where electrons move freely and redistribute along the surface due to an external electric field.
Polarisation happens in insulators,
- where electrons shift slightly within their molecules, creating a dipole effect.

ELECTROSTATIC CHARGE DEMONSTRATIONS!

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REVISION QUESTION

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DEFINITION: Electrostatics is the study of electrically charged objects.

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HOW STATIC ELECTRICITY IS GENERATED

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Static electricity results when this charge is isolated on an object by insulating material.

Excess electrons – object will be charged negatively.

Deficit of electrons – objects will be positively charged.

When one object is rubbed by another electrons are often transferred. If a one or both objects are insulators or insulated then static electricity results.

BUILDING UP CHARGE

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IN A VAN DE GRAAFF GENERATOR

ELECTROSTATIC FORCE

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This is:

a non-contact (distance) force
the force between CHARGES.
an attractive or repulsive.
following the principle: "Unlike charges attract, like charges repel".

VAN DER GRAAFF GENERATOR – HOW DOES IT WORK?

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VAN DER GRAAFF GENERATOR – HOW DOES IT WORK?

· consists of a large hollow metallic dome supported by an insulating column
· A rubber belt runs on pulleys
· At the top and bottom of the column are two electrodes E₁ and E₂ which end in sharply pointed combs. They allow electrons to transfer.
· Electrons transfer onto the belt at the bottom electrode, and transfer off the belt at the top electrode.
· The electrons collect on the dome, and given an opportunity, will move off and discharge by sparking across a gap.
· The belt rotates, allowing a continuous buildup of charge.

HOW IS STATIC CHARGE GENERATED?

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JOHN TRAVOLTAGE

HOW IS CHARGE POLARISED IN NEUTRAL OBJECTS?

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CHARGING PERSPEX

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Static electricity results when charge is isolated on an object by an insulating material.

Excess electrons – object will be charged negatively.
Deficit of electrons – object will be positively charged.
When one object is rubbed by another, electrons are transferred.
If one or both objects are insulators or insulated, then static electricity results.

CHARGING POLYTHENE (PLASTIC)

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HOW THE VAN DE GRAAFF GENERATOR WORKS

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HOW THE VAN DE GRAAFF GENERATOR WORKS

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ELECTRIC FIELDS

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An electrically charged object always generates an electric field around it.

Definition: An electric field is a region of space in which an electric charge experiences a force.

The electric field is continuous around an object but we use electric field lines to represent the field.

An electric field line is a line drawn in such a way that a small positively charged object placed on the line will experience a resultant force at a tangent to the line at that point.

Properties of electric field lines:

1. They start on positive charges and end on negative
charges.

2. They never cross.

3. They are 3-dimensional.

4. The denser the field lines the stronger the field.

5. They enter and leave charged object at right angles.

6. They are continuous (although drawn as separate
lines).

PHOTOCOPIER - HOW IT WORKS

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Charge the Drum: A high voltage applies a negative charge to the drum's surface.
Exposure to Light: Light photons hit the drum, removing charge where light strikes, creating a charged image of the document.
Toner Application: Charcoal graphite and plastic toner particles are applied and stick to the charged areas of the drum.
Transfer to Paper: The toner image is transferred from the drum to the positively charged paper.
Fusing: Heat and pressure fuse the toner onto the paper, making the image permanent.

TYPICAL EXAM QUESTION

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ANSWER:

Stage 2

Light falls onto the plate, reflected off the scanned image.
Charge leaks away where the photons of light fall.

Stage 3

– Negatively charged toner particles (carbon and plastic mix)

- are attracted to the positively charged areas on the plate.

LIGHTNING HOW IT WORKS

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SCIENCE OF LIGHTNING https://youtu.be/fzNk4w2k2h0

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HOMEWORK - WORKSHEET

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CLEANING UP INDUSTRIAL SMOKE USING ELECTROSTATICS

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Smoke is produced when fossil fuels burn.
Smoke is made of tiny solid particles, such as carbon.
To remove these particles from the waste gases an electrostatic precipitator is used.

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ELECTROSTATICS DEMOS

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ELECTROSTATICS INTRO DEMOS

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TRIBOELECTRIC SERIES - When rubbed together, higher-ranked materials lose electrons, becoming positively charged, while lower-ranked materials gain electrons, becoming negatively charged.

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