P7.1 Permanent and induced magnetism

A permanent magnet creates a magnetic field in the space around it without needing any external power. A piece of magnetic material placed in this field becomes an induced magnet — it is magnetised temporarily by the field.

Poles and forces

Every magnet has a north pole and a south pole. The rule: like poles repel; unlike poles attract. This is not the same as electrostatics — you cannot isolate a single magnetic pole; every magnet always has both N and S poles together.

Magnetic materials — iron, steel, nickel and cobalt — can be attracted to a magnet and can become induced magnets. Non-magnetic materials (aluminium, copper, plastic, wood) are not attracted and cannot be magnetised.

• Permanent magnets (e.g. made of steel): once magnetised, they retain their magnetism even when removed from an external field.
• Induced magnets (e.g. iron paper-clips): gain magnetism in the presence of a field but lose most of it when the field is removed.

Magnetic field lines

Field lines show:

• Direction: lines run from N-pole to S-pole externally (this is the direction a free N-pole would move, or equivalently the direction a compass N-needle would point).
• Strength: the closer/denser the field lines, the stronger the field.

Around a bar magnet: field lines emerge from the N-pole, curve around, and enter at the S-pole. The field is strongest near the poles (lines closest together).

Plotting a field with a compass

Place the bar magnet on paper. Position the compass at different points around the magnet. Mark where the N end points. Move the compass so its S end is at the last dot; mark again. Join the dots to trace a field line.

Exam technique

• The question "which pole?" usually requires you to apply like-repels-unlike-attracts.
• If asked to sketch a field pattern, draw closed loops (inside the magnet too) and arrows pointing N→S outside.