7.1 Permanent and induced magnetism, magnetic forces and fields — Practise

Work through these on your own. Keep your working neat, and keep the units with your numbers.

Both the questions and the answers below come from a set that is checked by computer every time the book is built.

Name two magnetic materials and two materials that are not magnetic.
Describe the difference between a permanent magnet and an induced magnet.
Two bar magnets are placed with their north poles facing each other. Describe the force between them, and give a reason.
A steel paper clip is attracted to a magnet, but a copper coin is not. Explain why.
Explain how the behaviour of a compass gives evidence that the core of the Earth must be magnetic.
Describe how to plot the magnetic field pattern of a bar magnet with a plotting compass.
State where the magnetic field of a bar magnet is strongest and where it is weakest, and give a reason.
A compass sits 12 cm from a magnet. Write the distance in metres.
A magnetic force of 0.60 N acts on a steel ball of mass 0.20 kg. Calculate the acceleration of the ball.
A magnet holds up a steel block of mass 2.5 kg. Take g = 9.8 N/kg. Calculate the weight of the block.
A steel object is brought near a magnet and then taken away. Will the object stay magnetic? Give a reason.
Explain why an induced magnet is always attracted to a magnet, and never repelled.

Answers and concise solutions

1. Magnetic and non-magnetic materials

Iron and steel are magnetic materials. Cobalt and nickel are also magnetic.
Copper and aluminium are not magnetic materials, and neither are plastic or wood.

2. Permanent and induced magnets

A permanent magnet produces its own magnetic field all the time.
An induced magnet becomes a magnet only when it is placed in a magnetic field. Induced magnetism always causes attraction.
When the material is taken out of the field, an induced magnet loses most or all of its magnetism quickly.

3. The force between the two north poles

The two magnets repel each other.
The two north poles are like poles, and two like poles repel. The force acts without contact.

4. Why the clip is attracted and the coin is not

Steel is a magnetic material, so the steel clip becomes an induced magnet and is attracted to the magnet.
Copper is not a magnetic material, so the magnet does not attract the copper coin.

5. The evidence from a compass

A compass needle is a small bar magnet.
The needle lines up only when it is in a magnetic field, and it points north wherever it is used on the Earth.
The Earth must therefore have a magnetic field, which must come from magnetic material deep inside it. This is evidence that the core is magnetic.

6. Plotting the field of a bar magnet

Put the magnet on paper and put the compass near one pole. Mark the two ends of the needle.
Move the compass so that its south end sits where the north end was, and mark the new position of the north end. Repeat towards the other pole.
Join the marks with a smooth curve to show one field line, then start again from a different point.

7. Where the field is strongest and weakest

The field is strongest at the poles, where the field lines are closest together.
The field is weakest far from the magnet, because the field gets weaker as the distance increases.

8. The distance in metres

Answer: The prefix centi means one hundredth, so $12\,\text{cm} = 0.12\,\text{m}$ .

9. Acceleration of the ball

Answer: $a = \dfrac{F}{m} = \dfrac{0.60}{0.20} = 3.0\,\text{m/s}^2$ .

10. Weight of the block

Answer: $W = mg = 2.5 \times 9.8 = 24.5\,\text{N}$ , so $25\,\text{N}$ to 2 significant figures. This is the force the magnet must balance.

11. Whether the object stays magnetic

The object will lose most or all of its magnetism.
The object became an induced magnet only while it was in the magnetic field. Once it is removed from the field, the induced magnetism is lost quickly.

12. Why induced magnetism always attracts

The field lines up the magnetic parts inside the material, so the material becomes an induced magnet with its nearest pole facing the magnet.
The nearest pole is an unlike pole, and unlike poles attract. So the force is always one of attraction.