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Electrostatic Phenomena

Electrostatics is the study of non-moving electric charges, sometimes called static electricity. A simple experiment will demonstrate the phenomena.

1. Take a polythene rod and place one end of it near some pieces of paper. Does anything happen?

2. Rub the rod with a cloth and again place it near some pieces of paper as shown in the diagram below.

Does this experiment tell us that the friction produced by rubbing the rod must have affected the rod in some way. We can do further experiments to discover the properties of such rods.

Further Experiments

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Questions: Q1. What do you think would happen if we brought two charged ebonite rods next to each other? Q2. There appears to be more than one type of charge. How can we tell this from the above experiments?

Conclusion: From the experiments we can conclude that:

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The Atom

To understand electrostatics it is first important to understand the basic structure of an atom. An atom is made up of three different particles. This is demonstrated in the following diagram showing an atom of beryllium

Protons - having a _______________ charge and present in the nucleus.

Neutrons - having ______ charge and present in the nucleus.

Electrons - having a _______________ charge and orbit the nucleus.

Q1. How many electrons are there present in the beryllium atom? Q2. How many protons are there present in the beryllium atom? Q3. Is the beryllium atom positively charged, negatively charged or neutral? This is true for all elements they are electrically neutral, having an equal number of electrons and protons.

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Insulators and Conductors

Insulators Materials that do not allow electrons to move about inside them are called electrical insulators. Examples of insulators:

• Plastic • Rubber • Wood

The method of charging by friction will only work when two insulators are rubbed against each other. When an insulator is charged by the friction method the charge remains on the surface of the material. This is because the charge cannot move through the insulator. A charged insulator can be discharged by passing it quickly through a Bunsen flame. Conductors Some materials allow electrons to move about easily inside them. These are called electrical conductors. Examples of conductors:

• Copper is the best conductor • All metals are good conductors and the only non-metal conductor is carbon

A way to charge a conducting object is by induction. Once a conductor is charged the charge will move throughout the object. Semiconductors Not all materials are so easy to classify. Some of materials allow only a very few electrons to move around. These are called semiconducting materials. Superconductors Some materials, at very low temperatures (about -270 ºC), have zero resistance. There is a lot of interest in these materials as they could be used to transport energy with zero loss, and save a lot of money.

Charging and Discharging

Method 1 — Charging by Friction

The polythene rod starts off being neutral - having an equal number of positive charges

(____________) and negative charges (_____________). When a soft cloth rubs polythene the

friction causes some of the electrons in the atoms to break free of their bonds and move from one

object to the other.

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The polythene rod ends up with a negative charge. This means that the electrons have moved from

the _______________ to the ________________. The soft cloth will also be left with a charge. It

will be _______________ charged.

Q. List two materials that will gain electrons when rubbed. Q. Why can conductors not be charged using the friction method? Method 2 — Charging by Induction Charge Two Conductors with Equal and Opposite Charges Step 1

Step 2

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Step 3

Step 4

Charge One Conductor The metal sphere is left with a _______________ charge. The charge left on an object such as this will always be _______________ to that of the charged rod used. Earthing In the above example (step 3) the positive charge was removed from the sphere.

This was achieved through a process called ______________ or _____________.

This means giving a path for the charges to flow between the charged object and the Earth.

Step 1

Step 2

Step 3

Step 4

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Earthing a Negatively Charged Conductor

Earthing a Positively Charged Conductor

In both cases we end up with a neutral sphere.

Discharging a Charged Insulator Q1. Why will touching the end of a charged rod not discharge it?

Q2. Explain how such a charged insulator rod can be discharged quickly.

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Quick Revision Questions: 1 Two metal balls, P and Q, each hangs from a nylon thread as shown below. A negative charged rod is then placed between them. While P is repelled by the rod, Q is attracted to the rod. What are the charges of P and Q?

2 What will happen if a positive charge of static electricity is placed in an electric field? A The charge will remain positive but become smaller. B The charge will remain positive but become larger. C The charge will become negative. D A force is produced on the charge.

3 A piece of metal foil, which is initially uncharged, is picked up by a charged rod. Which one of the following diagrams is correct?

The Electroscope

An electroscope is used to test for charge and to test the sign of a charge (positive or negative).

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Detecting a Charge When the electroscope is uncharged it may be used to see if a material is charged or uncharged.

Q. Why does the gold leaf move? Q. Can we tell if the charge is negative or positive?

Electric Fields

Just like magnets have fields around them charges also have fields surrounding them. Electric fields are similar to magnetic fields in many ways.

• Electric fields are represented by lines. • Electric fields travel from a positive charge towards a negative charge. • Electric fields cannot cross each other or touch. • Electric field lines represent the path a small positive charge would follow is set free in the electric field.

Complete the following 3 diagrams to show the electric field lines around the following charges.

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Uses and Dangers of Electrostatics

Uses for Electrostatics Flue-ash removal

Flue ash is a mixture of dust and smoke produced by many factories and power stations.

The dust particles are negatively charged as they pass through the negatively charged grid.

These particles are attracted by the positively charged metal plates in the chimney as they rise up due to the density difference in the gas.

These particles and removed from the exhaust gases to prevent environment pollution.

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Photocopying

1. You place the document you want to copy upside down on the glass

2. An extremely bright light scans across the document. Much more light reflects off the white areas (where there is no ink) than off the black, inked areas.

3. An "electrical shadow" of the page forms on the photoconductor. The photoconductor in a photocopier is a rotating conveyor belt coated with chemical called selenium.

4. As the belt rotates, it carries the electrical shadow around with it.

5. An ink drum touching the belt coats it with tiny particles of powdered ink (toner).

6. The toner has been given an electrical charge, so it sticks to the electrical shadow and makes an inked image of the original page on the belt.

7. A sheet of paper from a hopper on the other side of the copier feeds up toward the first belt on another conveyor belt. As it moves along, the paper is given a strong electrical charge.

8. When the paper moves near the upper belt, its strong charge attracts the charged toner particles away from the belt. The image is rapidly transferred from the belt onto the paper.

9. The inked paper passes through two hot rollers (the fuser unit). The heat and pressure from the rollers fuse the toner particles permanently onto the paper.

10. The final copy emerges from the side of the copier. Thanks to the fuser unit, the paper is still warm. It may still have enough static electric charge to stick to your pullover. Try it (but make sure the ink is dry first).

Spray Painting

Many mass produced objects such as cars are spray painted.

To increase efficiency and reduce paint usage the paint particles and the car body are given opposite charges.

The paint droplets repel each other as they are all charged of same type and spread out which covers more area evenly.

The metal plate which is given an opposite charge will attract the paint droplets by reducing the wastage.

High Voltage Generators A Van de Graff generator can be used to produce very large voltages up to 14 million volts.

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Dangers of Electrostatics Lightning Lightning is caused by charging produced by the friction between water droplets and air molecules. When the charge built up is large enough, the air will ionise allowing the charge to discharge to the ground. Fires and Explosions

Charge can build up on many objects such as planes and petrol tankers. If not discharged carefully a spark, similar to that produced by lightning, can start a fire or cause an explosion.