Magnetism 1

Danger from sparks

Making a spark
Look at the diagram. The belt on the electrostatic generator transfers charge to the dome. As the belt turns, the charge on the dome increases.
1. Copy and complete the sentences.
The greater the charge on the dome, the ____________ the potential difference between it and ____________.
When the potential difference is high enough, a _________ jumps through the air to a nearby ___________.
Sparks big and small
Static electricity can produce sparks. Often these sparks are very small. But sometimes they are huge.
2. Look at the pictures.
television screen,lightning and taking off a sweater
Then complete the following
Size of sparks (large or small)and How the static charge is produced in each case.

Preventing explosion at flour mills
Flour is made by grinding wheat to a very fine powder. The bits of flour make the air very dusty. This mixture of flour and air can be very dangerous. Just a tiny spark can make it explode.
3. Look at the diagrams.
There is a danger of a spark between the pipe and the container. Explain why.
4. How can such a spark be avoided?
Fire hazards with petrol tankers
Petrol is very flammable. A tiny spark can cause a serious fire when petrol or petrol vapor is open to the air.
At filling stations, petrol is transferred from tankers to underground tanks. There must be no sparks while this is being done.
5. In what two ways can petrol tankers produce static electricity?
6. Why does static electricity create a hazard when pumping petrol?
7. Write down two things that are done to prevent sparks.

Handle chips with care
Technicians who put electronic chips into circuits have special mats on their workbenches. These mats conduct electricity and are connected to earth. The technicians sometimes also wear wrist straps connected to earth.
8. Why are these precautions needed?
9. Explain how these precautions prevent damage to microchips.

E-Activity 16

Making use of static electricity

We can use static electricity in many different ways to do useful jobs.
1. Write down two different ways that static electricity is often used in schools and factories.
How a photocopier works
We take photocopiers for granted, but schools and offices could not manage without them. The diagrams above show, step by step, how a photocopy is made.
How to remove the dirt from smoke
Smoke contains lots of tiny bits of dirt. This dirt falls on houses and gardens. If people breathe in the dirt, it can damage their lungs.
The diagram shows how the dirt can be removed from factory chimneys using a smoke precipitator.
3. Copy and complete the sentences.
A metal grid and antenna are connected to the positive side of a high _______ supply.
The metal chimney is connected to the ______ side of the supply.
Tiny bits of dirt pick up a _______ charge as they pass through the grid. They are then repelled by the antenna and _____ by the chimney.
So the dirt collects (precipitates) on the inside of the __________ and is removed every so often.
How to make a spray hit its target
When a liquid is sprayed out of a nozzle, it becomes electrically charged. Sprays can be made to charge the droplets as much as possible. The diagrams show how this helps the spray to find its target.
4. Explain why a pesticide spray works better if it charges the droplets of pesticide.
5. Car makers want most of the paint from a spray to end up on a car body. What can they do to the car body to make sure this happens?

E-Activity 15

1. When you rub two different materials together, they become charged. Why?
2. What type of electrical charge does each electron carry?
3. Copy and complete the sentence.
When you rub a piece of polythene with a cloth, you make electrons move from the ____________ to the _______________.
4. Explain the following, as fully as you can.
(a) The polythene ends up with a negative charge.
(b) The cloth ends up with a positive charge.
5. The negative charge on the polythene is exactly equal to the positive charge on the cloth. Why is this?
6. (a) What type of charge do you get on a cloth when you rub apiece of acetate with it?
(b) What happens to the electrons as you rub the acetate with a cloth?
7. Copy and complete the sentence.
When you peel different materials apart, one of them becomes__________ charged and the other becomes _____________ charged.
8. When you peel some sticky tape from a roll, it often tends to go back on again. Explain, as fully as you can, why it does this.

Why rubbing things together produces electricity

It’s all down to electrons
When you rub two different materials together, electrons are rubbed off one material and onto the other. These electrons are very tiny. Each electron carries a small electrical charge.
If you rub two pieces of the same material together, electrons don’t move.
The materials that you rub together must not only be different. At least one of them, and usually both of them, must be an electrical insulator.

The diagrams show what happens when you rub polythene with a cloth and when you rub acetate with a cloth.
Peeling two different materials apart has the same effect as rubbing them together. Electrons are transferred from one material to the other.

E-Activity 14

1. Combing your hair can produce static electricity.
(a) Why does this happen?
(b) Write down two ways in which you can tell when it happens.
2. Copy the sentences. Then use the words attract and repel to complete them.
Two charged strips made from the same plastic __________each other.
A charged polythene strip and a charged acetate strip ______________ each other.
Both charged strips will also ___________ light objects that do not have a charge.
Charged objects sometimes attract and sometimes repel. This means that there must be different types of electrical charge.
3. Copy and complete the sentences.
Objects that have the __________ type of electrical charge repel each other. So if two charged objects attract each other they must have ___________ electrical charges.
4. You can use an electrostatic machine to make hair really stand on end. Explain how this happens, as fully as you can.