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KS3 Physics

8J Magnets and Electromagnets



Contents

Magnetic materials

Magnetic fields

Electromagnets

Summary activities


Which of the metals below are magnetic metals?

aluminium (Al)

silver (Ag)

iron

(Fe)

gold (Au)nickel (Ni)

cobalt (Co)

copper (Cu) zinc (Zn)

magnesium (Mg)

Magnetic materials


N

S

Al ZnAg

Cu

Au

Mg

Fe

Ni

Co

A magnetic material is attracted to a magnet.

Magnetic materials

Only iron (Fe), nickel (Ni) and cobalt (Co) are magnetic.



Contents

Magnetic materials

Magnetic fields

Electromagnets

Summary activities


Forces between magnets – experiment


NS

The iron filings feel the effect of the magnetic field and show the direction of the forces in this region.

The region around a magnet where it has a magnetic effect is called its magnetic field.

When a magnetic material is placed in a magnetic field it will experience a force.

What is a magnetic field?


weakest field further away from poles

What is the shape and direction of the lines of force in the magnetic field around a bar magnet?

N S

Shape of a magnetic field

Where is the magnetic field strongest?

strongest field

at poles

strongest field

at poles


Bring the north poles of two bar magnets together.

What happens to the magnets?

Viewing magnetic fields: N poles together

N SS N

Next, bring the two north poles as close to each other as possible and place a piece of paper on top of the magnets.

Carefully scatter iron filings onto the paper.

Draw the pattern created by the iron filings.


NNS S

Magnetic field pattern: N poles together

What do you notice about the pattern of the lines of force in the region between the two north poles?


Bring the north and south poles of two bar magnets together.

What happens to the magnets?

Viewing magnetic fields: N and S poles together

S N S N

Next, put the north and south poles close to each other, without letting them touch, and place a piece of paper on top.

Carefully scatter iron filings onto the paper.

Draw the pattern created by the iron filings.


NN

How does this pattern compare with the pattern between the two north poles?

Magnetic field pattern: N and S poles together

What do you notice about the pattern of the lines of force in the region between the north and south poles?

S S


1. When two like poles (e.g. two north poles or two south poles) are put together, they repel each other.

Magnetic fields – summary

2. When two unlike poles (e.g. a north and a south pole) are put together, they attract each other.

3. Scattering iron filings around a bar magnets makes it possible to see the lines of force of the magnetic field.


A magnet can be made by magnetizing a material which is attracted to a magnet, e.g. a paper clip.

Stroke a magnet along the paperclip from one end to the other and then starting from the same place, repeat the movement. The more times this is done, the more magnetic the clip becomes.

Hold a nail in a magnetic field and hit it with a hammer.

Put a magnetic material in a strong magnetic field.

Making a magnet

There are three methods that can be used to make a magnet:



Contents

Magnetic materials

Magnetic fields

Electromagnets

Summary activities


Making an electromagnet

When electricity is passed through a coil of wire, the coil has a magnetic field around it. This is called an electromagnet.

If the coil of wire is wrapped around a piece of iron, such as an iron nail, the magnetic field gets stronger.


An iron core at the centre of a coil of wire, increases the strength of an electromagnet.

Two experiments can be carried out to investigate the other factors that can affect the strength of an electromagnet:

Investigating an electromagnet

1. Investigate how the number of coils affects the number of paper clips attracted to an electromagnet – keep the current the same in this experiment.

2. Investigate how the size of the current affects the number of paper clips attracted to an electromagnet – keep the number of coils the same in this experiment.


Number of coils Number of paper

clips attracted

0

20

40

60

80

Remember – keep the current the same throughout this experiment!

Investigating an electromagnet – results 1

0

8

18

31

46


Current (A) Number of paper

clips attracted

0

1

2

3

4

5

Remember – keep the number of coils the same throughout this experiment!

Investigating an electromagnet – results 2

0

12

23

38

49

60


Graph to show how the number of coils affects the strength of an electromagnet

0

10

20

30

40

50

0 20 40 60 80 100number of coils

num

ber

of c

lips

attr

acte

d

Investigating an electromagnet – graph 1


Graph to show how the current affects the strength of an electromagnet

0

10

20

30

40

50

60

70

0 1 2 3 4 5current (A)

nu

mb

er

of

clip

s a

ttra

cte

d

Investigating an electromagnet – graph 2


What advantages does an electromagnet have over a permanent magnet?

What metals would the electromagnet attract?

A large electromagnet is used in a scrap yard to pick up and move heavy pieces of scrap metal.

Using electromagnets – scrap yards


The circuit for a door bells includes an electromagnet.

Using electromagnets – door bells


Lifts, cars and other large electrical machines use high currents.

Using electromagnets – relay

A relay, which includes an electromagnet, is used to allow a small current in one circuit to control a large current in another circuit.


1. When electricity is passed through a coil of wire, the coil behaves like a magnet and has a magnetic field around it – this is an electromagnet.

Electromagnets – summary

wrap the coil of wire around an iron core;

increase the number of coils;

increase the size of the current.

3. An electromagnet can be easily turned on and off. This is why electromagnets can be used in scrapyards and as switches in electrical devices.

2. There are three ways to make an electromagnet stronger:



Contents

Magnetic materials

Magnetic fields

Electromagnets

Summary activities


Glossary

attraction – The force that pulls things together, e.g. opposite poles of two magnets.

electromagnet – A magnet made by passing electricity through a coil of wire, which often has a core inside.

magnet – An object that has a magnetic field and can attract magnetic materials.

magnetic field – The area around a magnet where its magnetic force can be felt.

magnetic materials – Materials that are attracted to a magnet, e.g. iron, cobalt and nickel.

magnetism – The non-contact force of a magnetic field.poles – The parts of a magnet where its magnetic field is

strongest.repulsion – The force that pushes things away from each

other, e.g. similar poles of two magnets.


Anagrams


Multiple-choice quiz