Magnetism

Permanent vs induced magnets

• A permanent magnet produces its own magnetic field (e.g. iron bar magnet, neodymium).
• An induced magnet is a magnetic material that becomes a magnet only when placed in another magnet's field. The induced magnet is always attracted; once the external field is removed, it loses most or all of its magnetism.

Magnetic materials: iron, steel, nickel, cobalt.

Forces between magnets

• Like poles repel.
• Unlike poles attract.

Forces between magnets act at a distance through the magnetic field.

Magnetic field

A magnetic field is the region around a magnet where another magnet or magnetic material experiences a force.

• Field lines run from north to south outside the magnet.
• The closer the lines, the stronger the field.
• Field is strongest at the poles.

The field can be shown using iron filings or a plotting compass (the compass needle aligns with the field at each point).

The Earth's magnetic field

The Earth itself acts as a giant magnet, with the magnetic south pole near the geographic North Pole. A compass needle's "north" end therefore points towards geographic north. The field is generated by convection currents of molten iron in the outer core.

Plotting a field with a compass

1. Place the bar magnet on paper and draw around it.
2. Place a small compass next to the magnet's north pole.
3. Mark a dot at each end of the compass needle.
4. Move the compass so its tail sits where the head was; mark the new head position.
5. Repeat to trace one complete field line, then start again at a different point near the magnet to plot more lines.

WJEC exam tip

If a question shows a bar magnet with a piece of iron nearby, remember the iron becomes an induced magnet — the pole nearest the bar is always opposite to the bar's pole, so they attract. State both halves: "induced" + "always attracted".