Simple magnetism and magnetic fields

O Level / IGCSE Physics · Electricity and magnetism. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

A magnet has two poles, a north pole and a south pole. Like poles repel and unlike poles attract. A magnet also attracts unmagnetised magnetic materials such as iron and steel, because it turns them into magnets while they are close. This is induced magnetism.

A magnetic field is a region where a magnetic pole feels a force. Field lines show it: they leave the N pole and enter the S pole, and they are closest together where the field is strongest. A plotting compass points along the field line.

Soft iron is easy to magnetise but loses its magnetism quickly, so it makes temporary magnets. Steel keeps its magnetism, so it makes permanent magnets.

Rules to remember

Common mistake

Thinking that attraction proves a bar is a magnet. A magnet attracts unmagnetised iron as well as an unlike pole, so only repulsion proves that both objects are magnets.

Worked example

A soft-iron rod and a steel rod are each placed inside a coil that carries a current. Both rods pick up pins. The current is then switched off. What happens to the pins on each rod?

  1. The current in the coil gives a magnetic field, so both rods are magnetised and both hold pins.
  2. Soft iron is a temporary magnet: when the field is removed it loses nearly all its magnetism.
  3. Steel is a permanent magnet material: it keeps its magnetism after the field is removed.

Answer: The pins fall off the soft-iron rod but stay on the steel rod.

Practice questions

Try each one, then open the answer.

1. An iron nail hangs from a permanent magnet and a second nail hangs from the first. What has happened to the first nail?

  1. A
    it has become a permanent magnet
  2. B
    it has gained an electric charge
  3. C
    it has become an induced (temporary) magnet
  4. D
    nothing; the second nail is attracted to the magnet directly
Show answer

Answer: C. The magnet induces magnetism in the first nail, giving it poles so that it attracts the second nail. Remove the magnet and the induced magnetism mostly disappears.

2. A crane uses an electromagnet with a soft-iron core to lift scrap iron. An engineer replaces the core with a steel one. What problem does this cause?

  1. A
    the scrap stays stuck when the current is switched off
  2. B
    the magnet cannot pick up any of the scrap at all
  3. C
    the magnet grows weaker each time it is switched on
  4. D
    the current in the coil keeps reversing direction
Show answer

Answer: A. Steel keeps its magnetism after the current stops, so it acts as a permanent magnet, while soft iron loses its magnetism. With a steel core the scrap stays stuck to the crane.

3. For which task is an electromagnet more suitable than a permanent magnet?

  1. A
    holding a fridge door closed
  2. B
    making a compass needle
  3. C
    a crane that lifts scrap steel and then releases it
  4. D
    holding notes on a metal board
Show answer

Answer: C. An electromagnet can be switched off, so the crane can drop the steel where it is needed. Permanent magnets suit jobs where the magnetism must stay on all the time.

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