1 NCEA Physics Electricity and Magnetism 2 Amber is the fossilized resin from some trees. Used in jewellery it was often vigorously polished with a soft.

1

NCEA Physics

Electricity and Magnetism

2

Amber is the fossilized resin from some trees. Used in jewellery it was often vigorously polished with a soft cloth. Ancient Greeks discovered a strange property; when it is rubbed with a cloth, it can attract nearby bits of straw or grain. From the Latin word for amber, electrum, we get the word electricity.

3

This mysterious attractive or repulsive property could also be passed to other materials. In the classic experiment two tiny balls of pith were hung next to each other without touching. A glass rod was rubbed with silk and touched to each pith ball. The pith balls would suddenly move away from each other.

4

The traditional unit of charge is the Coulomb, named after French physicist, Charles Coulomb (1738-1806) He was a French physicist and military engineer. Because of his expertise with simple machines, he was able to build an apparatus to measure the electrical force between two charged objects.

5

It is possible to see a similar effect of an electric field. A charged object placed in fine oil with tiny bits of thin thread will cause the thread to line up in the field. It might look like the picture below.

6

When two opposite charges are placed near each other in fine oil with fine bits of thread, the situation looks like the picture below.

7

The drawing below represents what is happening between the plates. The top plate has an excess of positive charge and the bottom plate has an excess of negative charge. The electric field (E) is directed downward (from positive to negative). The drawing shows the direction of force (F) the field exerts on a positive charge and on a negative charge. If the charges were free to move, they would accelerate in the direction of the force.

8

Electric fields can, therefore, cause charges to move. The movement of charges is an important concept in electricity. Physicists call the net movement of charges an electric current. In the late 1700's scientists chose the direction of electric current to be the direction in which positive charges move in an electric field. This was before scientists knew that electrons and protons were the negative and positive charge particles, and that the electron moved more easily than the proton.

9

Van de Graaff

•A Van de Graaff generator can produce a very high static charge.

•Unfortunately the air will conduct some of this charge.

•A Van de graaff can only work well at low humidity.

10

How it works

The generator produces a large positive voltage.Electrons are

drawn off from the belt towards

the ground.

Electrons then jump from the

metal dome to the positive belt.

The dome is left with a large

positive charge.

11

Van de graff

12

Static hair

When you are charged by a van de Graaff machine each part of your body repels every other part. The strands of your hair repel each other and spread out from each other.

13

Van de graff hair shot

14

Uses and dangers

15

16

Lightning conductors•Lightning conductors can protect tall buildings during electrical storms.

•A long copper strip connects a spiked object on the roof to a large metal plate in the ground.

17

Lightning conductors•As the storm increases the bottom of the cloud becomes negatively charged.

•These negative charges repel the electrons in the lightning conductor, leaving it with a positive charge.

•The electrons in the cloud will be attracted to the copper strip rather than the building.

18

Lightning conductors•During the lightning strike the charge will pass through the copper strip to the ground.

•The building will not be damaged as long as the copper strip is in good condition.

19

Refuelling

20

Warning sign

21

Vapours are the danger

22

After a fire

23

How big is the problem ?

In 2000 there were 81 refuelling fires in the USA, in 2001 there were more than 130.

24

Year 2000 reportsRemember we said that static electricity likes low humidity. When it is cold the humidity drops and there is a greater

chance of fire.

25

Rules for safe refuelling

26

Filling a gas can

27

Special Pipes

•In NZ many petrol refuelling hoses are made from a special kind of plastic that can conduct static, this stops the build up of charge that could cause a fire.

•Unfortunately there are still many fires caused by smoking during refuelling.

•Some cars have strips fitted to the rear of the car to discharge the build up of static charge.

•Making a safe tyre that will not charge a car is important.

28

Planes and helicopters

•When a passenger jet refuels a large amount of petrol travels from one tank to another. This movement can cause a huge build up of charge. Before the fuelling starts cables can be attached to the plane to stop this build up.

•Helicopter blades cause a lot of static to build up. Sea rescue workers often have metal cables trailing below them to discharge any static before they have to touch the sea or boats below them.

29

Refuelling planes

During refuelling the fuel gains electrons from the pipe, making the pipe positive and the fuel negative. The resulting voltage may cause a spark – bad news!

Solution: Either earth the fuel tank with a copper rod or connect the tanker to the plane by a copper conductor.

30

Photocopiers

31

Static

•The key to a photocopier is that the powder used in the machines is attracted to static charge.

•How we use this piece of physics to make a copy of a piece of black and white paper is a little more complicated.

32

Charging the drum

•A photocopier has a drum that is charged with static in the dark.

•Light is reflected off the original paper onto the drum.

•When light hits the drum that part of the drum loses its charge.

33

Spraying the toner

•The drum is sprayed with a dark dust called toner.

•The toner sticks to the charged areas – the dark parts of the picture where the text was.

•Toner is very fine and extremely dangerous if breathed in.

34

Drum to paper

•We now have a copy of the original on the drum.

•Next a charged piece of plain paper is rolled against the drum.

•The plain paper picks up the toner from the drum.

35

Melting the toner

•The paper is now covered in toner powder.

•The toner is then heated to fix it into position.

•This is why the paper is hot when it comes out of the machine.

•This is how a photocopy is made.

36

The complete picture

37

Photocopying reviewPhotocopiers use static electricity. They work by: 1) Copying an ______ of the page onto a ______ plate, 2) Light then causes the charge to ____ away, leaving an “electrostatic impression” of the page, 3) The charges left on the plate ______ small drops of black powder, 4) The powder is transferred from the plate onto the _____, 5) The paper is _____ to “fix” the powder.

Photocopiers use static electricity. They work by: 1) Copying an image of the page onto a charged plate, 2) Light then causes the charge to leak away, leaving an “electrostatic impression” of the page, 3) The charges left on the plate attract small drops of black powder, 4) The powder is transferred from the plate onto the paper, 5) The paper is heated to “fix” the powder.