A magnetic field is the region around a magnet where a magnetic force acts on another magnet or a magnetic material.
Representing magnetic fields
Magnetic field lines show the direction and shape of the field.
Rules for field lines:
- Field lines go from north to south outside the magnet.
- Inside the magnet, they go from south to north (continuous loops).
- Lines never cross.
- Closer lines mean a stronger field. The field is strongest at the poles.
- The direction of a field line at any point is the direction a north pole would move if placed there.
Field patterns
Single bar magnet: lines emerge from the north pole, curve around, and enter the south pole. Closest together at the poles (strongest field).
Two north poles facing: lines repel, creating a neutral point (zero field) between them.
North facing south: lines run from one magnet to the other, showing attraction. The field is strongest in the gap between the poles.
Plotting field lines with a compass
- Place a bar magnet on paper.
- Place a small plotting compass near one pole.
- Mark the positions of both ends of the compass needle.
- Move the compass so its tail is at the previous head position.
- Repeat to trace a complete field line from north to south.
The Earth’s magnetic field
The Earth behaves as if it has a bar magnet inside it. The geographic north pole is near the magnetic south pole (because the north end of a compass needle points north, attracted to a magnetic south).
Common errors and how to correct them
Drawing field lines that cross each other. Field lines never cross. At any point there is only one field direction.
Saying “the north pole of a compass points to the Earth’s magnetic north pole.” The compass north pole is attracted to the Earth’s magnetic south pole, which is near the geographic north pole. The naming is a historical convention.
How to apply this in an exam
Draw field lines with arrows from north to south. Make lines closer together where the field is stronger (near the poles). For two magnets, show the interaction pattern (attraction or repulsion) and mark any neutral points.
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