Magnets and field lines

A compass needle is a small bar magnet that can turn freely. The end that points north is its north pole; the other end is its south pole. Like poles repel; unlike poles attract.

The region around a magnet where its force can be felt is its magnetic field. It has both a strength and a direction.

Seeing the field. Lay a bar magnet under a sheet of paper, sprinkle iron filings on top and tap the paper gently. The filings arrange themselves in a pattern of curved lines from one pole to the other. Or move a small compass around the magnet, mark where its needle points each time, and join the marks: you trace out the same lines. These are magnetic field lines.

Rules for field lines:

• outside the magnet they run from the north pole to the south pole; inside the magnet, from south back to north. So each line is a closed loop;
• where the lines are crowded, near the poles, the field is strongest;
• two field lines never cross.

"Why can't two field lines cross?" At any point, a compass needle points along the field line there. If two lines crossed, the needle would have to point in two directions at once, which is impossible.