IGCSE Physics / Magnetism and electromagnetism
Magnetic fields and field lines
Edexcel International GCSE Physics (4PH1), chapter 20: Magnetism and electromagnetism, specification point 6.4, 6.6. Free lesson.
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A magnetic field is the region around a magnet where a magnetic material feels a force. Field lines show the direction and strength of the field, and help you describe how magnets interact.
What magnetic field lines show
A field line shows the direction of the force on a north pole at each point. The arrows point from north to south outside the magnet. The closer the lines are, the stronger the field, so lines are closest together near the poles. Field lines are a way of drawing a magnetic field. Lines never cross because a compass can point in only one direction at any point. Line spacing shows field strength. The length of a line does not.
How to plot a field with a compass
A plotting compass turns to line up with the field at its position. By moving it along in small steps and marking its direction each time, you can trace a field line. Repeat from different starting points to map more of the field. Iron filings can show the field’s shape quickly if you tap the paper, but they do not show its direction. A compass shows the direction because its needle points along the field line.
The field around one bar magnet
Outside a bar magnet, the field lines curve out from the north pole and round into the south pole. They are closest together near the poles, where the field is strongest. Use arrows to show the field direction on every line. Draw smooth curved lines that do not cross.
Fields between two magnets
The pattern depends on which poles face each other. Unlike poles attract, so their field lines join across the gap. Like poles repel, so the lines bend away from the gap. With like poles, the fields cancel midway, creating a neutral point where the field is zero. With north facing south, lines go across the gap from north to south. The field there is strong and fairly even. Further out, lines loop round both magnets. With two north poles facing, lines from each pole bend away from the gap and do not cross it. The same repelling pattern applies to two south poles facing. At the neutral…
How to avoid mistakes in field drawings
A field drawing needs both arrows and lines that never cross. These details show the direction of the field and the fact that a compass can point only one way at each spot. Add arrows from north to south. Check that no two field lines cross.