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15.1 Magnetic Fields

GCSE Physics · AQA · Chapter 15: Electromagnetism

A free GCSE Physics lesson for AQA (8463) from OBA School: magnetic poles, permanent and induced magnets, field lines, plotting compasses and the Earth’s magnetic field. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • State the force rule for two magnetic poles near each other
  • Describe and draw the magnetic field pattern of a bar magnet, showing where the field is strongest and its direction
  • Describe how to plot a field line using a plotting compass
  • Explain the difference between a permanent magnet and an induced magnet, and why steel (not iron) is used for permanent magnets
  • Explain how the behaviour of a compass is evidence that the Earth has a magnetic core

Key words

  • Magnetic pole: An end of a magnet where its magnetic force is strongest. Every magnet has a north (N) pole and a south (S) pole.
  • Magnetic field: The region around a magnet where a magnetic material or another magnet feels a force.
  • Field line: A line showing the direction of the force on a north pole placed at that point. Also called a line of force.
  • Permanent magnet: A magnet that produces its own magnetic field all the time, for example a steel bar magnet.
  • Induced magnet: A material that becomes a magnet only while it is in a magnetic field. It loses most or all of its magnetism when taken out.
  • Magnetic material: A material attracted by a magnet: iron, steel, cobalt and nickel.
  • Plotting compass: A small magnetised needle on a pivot. Its N end points along the field line it is in.

Poles and the force rule

A freely hanging magnet turns until one end points north. That end is the north-seeking pole, called the N-pole. The other end is the S-pole. If you bring two magnets close together you find one simple rule:

Key fact: Like poles repel. Unlike poles attract. (N and N repel, S and S repel, N and S attract.)

The forces between magnets are non-contact forces: they act across a gap, and they get weaker very quickly as the gap gets bigger.

Exam tip: The only sure test for a magnet is repulsion. A magnet attracts an unmagnetised piece of iron at either end, so attraction does not prove that the iron is a magnet.

Magnetic fields and field lines

The space around a magnet where it can exert a force is its magnetic field. We show the field with field lines:

  • Field lines go from the N-pole to the S-pole outside the magnet. The arrow shows the direction of the force on a north pole placed there.
  • The field is strongest at the poles, where the lines are closest together.
  • The field gets weaker with distance from the magnet, so the lines spread out.
  • Field lines never cross or touch each other.

Sprinkling iron filings on paper over a magnet quickly shows the shape of the pattern, but it does not show the direction. A plotting compass shows the direction too.

Plotting a field line with a compass

Place the magnet in the middle of a sheet of paper and draw round it.

1. Put a small dot next to the N-pole of the magnet.

2. Place the plotting compass so that the tail of its needle is over the dot.

3. Mark a new dot exactly at the tip of the needle.

4. Move the compass so that its tail is over the new dot, and mark the tip again. Repeat until you reach the S-pole.

5. Join the dots with a smooth curve and add an arrow pointing from N to S.

6. Start from a different point near the N-pole to draw more lines.

Exam tip: Keep other magnets, steel rulers and iron objects away from the paper, because they would change the direction of the compass needle.

Permanent and induced magnets

A permanent magnet produces its own field. An induced magnet is a piece of magnetic material that becomes a magnet only when it is placed in a magnetic field.

  • The end of the material nearest the magnet always gets the opposite pole, so induced magnetism always causes attraction, whichever pole of the magnet is used.
  • When the material is taken out of the field, it loses most or all of its magnetism quickly.
  • Iron is magnetically “soft”: it magnetises and demagnetises easily, so it is used for the cores of electromagnets and transformers.
  • Steel is magnetically “hard”: once magnetised it does not lose its magnetism easily, so permanent magnets are made of steel.

Worked example: testing two rods

Rod X and rod Y look the same. The S-pole of a magnet attracts one end of X and repels one end of Y.
Y must be a magnet (only a magnet can be repelled). X could be an unmagnetised piece of iron or steel.

Answer: Y is a permanent magnet; X is a magnetic material that may or may not be magnetised

The Earth’s magnetic field

A compass needle is a tiny bar magnet. Wherever you are, a compass settles pointing north–south. This shows that the Earth has a magnetic field. The pattern is like the field of a huge bar magnet inside the Earth, which is evidence that the Earth’s core is magnetic (the core is mostly iron and nickel).

Exam tip: Because unlike poles attract, the pole of the Earth’s “magnet” near the geographic North Pole is really a magnetic S-pole.

Common mistakes

  • Drawing field lines from S to N. Outside a magnet they always go from N to S.
  • Saying attraction proves an object is a magnet. Only repulsion proves it.
  • Thinking induced magnetism can cause repulsion. It always causes attraction.
  • Saying permanent magnets are made of iron. Iron loses its magnetism easily; steel keeps it.
  • Drawing field lines that cross, or lines that start and stop in empty space.

Revision questions

Try these, then check your answers below each one.

Question 1. The N-pole of one bar magnet is brought close to the N-pole of another bar magnet. What happens? (Choose from: They repel, They attract, Nothing happens, They attract, then repel.)

Answer: They repel

Working: Like poles repel, so two N-poles push each other apart.

Question 2. True or false? A bar magnet can repel an unmagnetised iron nail if the nail is held near the correct pole.

Answer: False

Working: False. The magnet induces the opposite pole in the near end of the nail, so induced magnetism always causes attraction. Only another magnet can be repelled.

Question 3. Which material is the best choice for making a permanent magnet? (Choose from: Steel, Iron, Copper, Aluminium.)

Answer: Steel

Working: Steel is magnetically hard: it does not lose its magnetism easily. Iron loses it easily; copper and aluminium are not magnetic.

Related lessons

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