What is magnetism and electromagnetism? - BBC Bitesize (2025)

• Plotting compasses can be used to investigate the shape and direction of the around a bar magnet or produced by the in a coil of wire.

• The strength of the magnetic field produced by the current in a coil of wire depends on the current in the coil, the number of turns in the coil and the material used as the core of the coil.

Bar magnets are permanent .

This means that their magnetism is there all the time and cannot be turned on or off as it can with an .

Bar magnets have two :

• North pole – normally shown as N.

• South pole – normally shown as S.

Opposite (unlike) poles attract, and like poles repel.

A magnetic material is one that is attracted to a magnet.

Not all metals are magnetic, indeed the only three magnetic elements are iron, nickel and cobalt.

Metals like copper and aluminium are non-magnetic, this means they will not be attracted to a magnet.

Magnets are made from magnetic metals.

Iron, nickel and cobalt are the only pure metals that can be turned into a magnet.

Steel is an of iron and so can also be made into a magnet.

How can magnets be tested?

You can only show that an object is a magnet if it repels another magnet.

A magnetic field is the region around a magnet where a force acts on another magnet or on a .

What are magnetic field lines?

Magnetic fields lines are invisible but they fill the space around a magnet where the magnetic forces work on magnetic materials, and where they can attract or repel other magnets.

Although we cannot see magnetic fields, we can detect them using iron filings.

The tiny pieces of iron line up in a magnetic field.

How to use a magnetic compass to detect the direction of a magnetic field

The direction of lines can be detected using a magnetic compass.

A compass contains a small bar magnet on a so that it can rotate.

The compass needle points in the direction of the Earth's magnetic field, or the magnetic field of a magnet.

Magnetic fields can be mapped out using small plotting compasses:

1. Place the plotting compass near the magnet on a piece of paper.

2. Mark the direction the compass needle points.

3. Move the plotting compass to many different positions in the magnetic field, marking the needle direction each time.

The needle of a plotting compass points to the south pole of the magnet.

The diagram shows these key features:

• Magnetic field lines never cross each other.

• The closer the lines are together, the stronger the magnetic field – magnets are strongest at the poles.

• The lines have arrowheads to show the direction of the magnetic field.

• The direction of a magnetic field at a point is the direction of the force on a north pole at that point.

• Magnetic field lines point from the north pole of the magnet to its south pole.

What is the magnetic field pattern for two bar magnets?

The magnetic field pattern when two magnets are used is shown in this diagram.

When magnetic field lines are parallel and the same distance apart from each other, we say that the magnetic field is uniform.

This is shown in the diagram:

When a current flows in a wire, it creates a circular around the wire.

This magnetic field can deflect the needle of a magnetic compass.

If the direction of the current reverses (as shown in the diagram) then the direction of the magnetic field also reverses.

The strength of the magnetic field is greater:

• Closer to the wire.

• If the current is increased.

When an electric current flows, the shape of the is very similar to the field of a bar magnet.

The field inside a is strong and uniform.

The small magnetic field caused by the current in each coil add together to make a stronger overall magnetic field.

What is the right-hand grip rule?

The north pole of the can also be found by using your right hand.

Point the fingers of your right hand in the same direction as the current is flowing in the coil.

Your thumb points to the north pole of the electromagnet.

Key points

The magnetic field produced by the current in a coil of wire:

• Is very similar to the field of a bar magnet.

• Reverses when the direction of the current in the coil is reversed.

An electromagnet uses an electric current to produce a magnetic field.

An iron core increases the strength of the electromagnet.

A simple electromagnet is made by coiling wire around an iron nail.

Key points

Iron is easily magnetised and demagnetised.

Steel is more difficult to magnetise and is not easily demagnetised.

An iron core makes a temporary .

It loses its magnetism as soon as the switch is opened and the current is switched off.

A steel core makes a more permanent magnet.

It does not lose its magnetism quickly when the current is switched off.

Electromagnets with iron cores are used:

• In scrap yards to lift such as iron and steel.

• To separate magnetic materials such as iron and steel from non-magnetic materials.

• In electric bells.

• In relay switches and door locks that can be controlled remotely.

• To hold open fire doors – the electromagnet switches off when a fire alarm is sounded and the doors close.

To investigate, describe and recall how the strength of the depends on the current in the coil, the number of turns in the coil and the material used as the core of the coil.

The main variables in a science experiment are the independent variable, the dependent variable and the control variables.

The independent variable is what we change or control in the experiment.

The dependent variable is what we are testing and will be measured in the experiment.

The control variables are what we keep the same during the experiment to make sure it’s a fair test.

What are the variables in prescribed practical P9?

In this experiment the:

• Independent variable is the current in the coil.

• Dependent variable is the strength of the electromagnet, indicated by the number of paper clips that the electromagnet can hold.

• Control variables are the material of the core, the number of turns of the electromagnet coil, size of paper clips.

Remember - these variables are controlled (or kept the same) because to make it a fair test, only 1 variable can be changed, which in this case is the current.

What is the prediction for this practical?

As the current increases, the electromagnet will get stronger and hold more paper clips.

What is the justification for this prediction?

The magnetic field is caused by the current flowing in the wire.

The bigger the current the stronger the magnetic field and hence the stronger the electromagnet.

What apparatus is needed to carry out prescribed practical P9?

1m length of insulated wire, iron nail, a variable low voltage power pack, an ammeter, connecting leads, a retort stand, a boss and clamp, 2 crocodile clips, paper clips.

1. Wind the insulated wire tightly around the nail to make an electromagnet.

2. Set up the equipment as shown in the diagram.

3. Adjust the power pack so that a current of 0.1 A flows. Record the current in a suitable table.

4. Attach as many paper clips to the electromagnet as it will hold. Count the number of paper clips and record in the table. Switch off the electromagnet.

5. Switch on again. Check that the current is still 0.1 A and repeat the paper clip reading. Record in the table and calculate the average number of paper clips held.

6. Repeat this process increasing the current by 0.1 A up to 0.6 A.

Error

Ensure that the paper clips are the same size and material.

They should not be too big.

How to record the results of this practical

Graph

As the current increases the number of paper clips held also increases.

This means that the strength of the electromagnet increases with increasing current, which agrees with our prediction.

In fact, since the line of best fit is a straight line through the origin, we can be even more precise.

We can say that, the strength of the electromagnet is to the current.

If you double the current, you double the strength of the electromagnet.

A second experiment can be carried out to investigate how the strength of the electromagnet depends on the number of turns in the coil.

Method

1. Set up the equipment as in the previous experiment.

2. Wind the insulated wire tightly around the nail to make an electromagnet of 10 turns. Record the number of turns in a suitable table.

3. Adjust the power pack so that a current of 0.5 A flows.

4. Attach as many paper clips to the electromagnet as it will hold. Count the number of paper clips and record in the table. Switch off the electromagnet.

5. Switch on again. Check that the current is still 0.5 A and repeat the paper clip reading. Record in the table and calculate the average number of paper clips held.

6. Repeat this process increasing the number of turns by 10 up to 60 coils. Keep the current at 0.5 A

As the number of turns increases the number of paper clips held increase.

This means that the strength of the magnet increases with increasing number of turns in the coil.

Once again, since the line of best fit is a straight line through the origin, we can be even more precise.

We can say that, the strength of the is to the number of turns in the coil.

If you double the number of turns, you double the strength of the electromagnet.

How to investigate the material of the core of the electromagnet

Repeat the experiment once more, keeping the current and number of coils the same.

Replace the iron core with similar lengths of different materials such as steel, wood, plastic, aluminium and air.

Count the number of paper clips held by each material.

Record in a table and compare results.

Iron and steel make strong electromagnets.

Key points

The strength of an electromagnet depends on:

1. The size of the current.

2. The number of turns in the coil.

3. The material of the core.

The strength of the magnetic field increases as:

1. The current increases.

2. The number of turns in the coil increases.

3. An iron core makes a strong electromagnet which can be easily magnetised and demagnetised