Revision summary

Poles and evidence

Like poles repel and unlike poles attract. A freely suspended magnet turns approximately north-south when nearby disturbances are small. Its north-seeking end points roughly towards geographic north.

A magnetic material can become magnetised; it need not already be a permanent magnet. An end that repels a known N pole is N in the simple end-to-end test. Attraction alone can also involve an initially unmagnetised magnetic material. Dividing an ordinary bar magnet produces smaller magnets with both poles.

Both ways to induce magnetism

Near a strong magnet

The nearer end of a magnetic rod develops the opposite pole to the inducing pole. In the example with N beside end A, A becomes S and the far end B becomes N. No contact is required.

Inside a current-carrying solenoid

The coil's field magnetises its core. In the supplied N-left/S-right coil orientation, the core is also N-left/S-right. A soft-iron core strengthens the temporary magnetic effect and loses much of its magnetism when current stops.

Soft iron and suitable steel

Soft iron is readily magnetised and has low retention, useful when a switched lifting magnet must release its load. Suitable steel is harder to magnetise but retains magnetism, useful for a compass needle or permanent magnet.

Compare retention after the inducing field has been removed, using comparable samples and a consistent test. The number of objects lifted also depends on their shape, weight and contact.

Compass direction and mapping

The marked N end of a settled compass gives the local field direction. It does not always point towards geographic north or the centre of the nearby magnet.

1. Keep the bar magnet and paper fixed and label the pole positions.
2. Let a small compass settle, then mark its needle ends and N-end direction.
3. Move its S end to the previous N-end mark, let it settle and mark the next position.
4. Join the marks smoothly with arrows in that direction; repeat from other starting positions.

Avoid nearby magnetic disturbances and stuck or moving needles. The compass responds to the resultant field, including Earth's. Iron filings show a rough shape; the compass supplies the arrow direction.

Draw shape and direction together

• Single bar: outside it, lines leave N and enter S. Their return through the magnet is S to N.
• Unlike facing poles: lines connect across the gap from N to S.
• Like facing poles: lines bend away from the central gap region and do not connect N to N or S to S. Reverse the arrows when changing a corresponding N/N pattern to S/S.
• Representation: lines do not cross. Closer spacing gives a qualitative strength comparison within one consistent drawing.
• Meaning: a field arrow follows the local compass N end. It is distinct from a force arrow on a whole magnet or a moving object's path.