Learning outcomes
- Define a magnetic field.
- Draw field patterns around bar magnets.
- Use field-line spacing to compare field strength.
- State field direction.
- Describe plotting with a compass and iron filings.
2.1 Meaning of a magnetic field
A magnetic field is a region in which a magnetic pole, a magnetic material or a current-carrying conductor experiences a force. The field is strongest near the poles of a bar magnet and becomes weaker with distance. A field exists even when no test object is present; the test object merely reveals its effect.
Magnetic field strength and direction vary from point to point. A small compass placed at a point aligns its north-seeking end with the local field direction. A sequence of compass directions can therefore be used to construct a field-line diagram.
2.2 Field lines and their direction
Field lines are a model used to represent a magnetic field. Outside a permanent magnet, arrows point from the north pole to the south pole. Inside the magnet they continue from south to north, forming closed loops. Field lines never cross because the field at one point cannot have two directions.
Where field lines are close together, the field is stronger. Lines spread out as the field weakens. The number of lines drawn is a convention, so field strength should be inferred from relative spacing in the same diagram, not by counting lines from unrelated drawings.
2.3 Field between poles
Between opposite flat poles placed close together, the field can be approximately uniform. It is represented by straight, parallel, equally spaced lines directed from north to south. A uniform field has the same strength and direction throughout the central region.
Between like poles, field lines bend away from the central region. A neutral point may occur where fields from different magnets cancel. A compass at a true neutral point has no preferred direction due to those magnets, though Earth’s magnetic field may still influence it.
2.4 Plotting with a compass
Place a bar magnet under or on a sheet of paper and draw its outline. Put a small plotting compass near one pole and mark the position of both ends of the needle. Move the compass so that its tail is at the previous head mark and repeat. Join the marks smoothly and add an arrow from north to south.
Repeat from several starting points to show the full pattern. The compass should be small so it disturbs the field as little as possible. Nearby steel objects, magnets and electric currents should be kept away because they may distort the pattern.
2.5 Using iron filings
Iron filings sprinkled on card above a magnet become temporarily magnetised and align in chains along the field. Gentle tapping reduces friction so the pattern becomes clearer. Filings reveal field shape and relative strength, but they do not show direction because they have no marked north end.
For a complete investigation, filings can show the overall pattern and a compass can provide arrow direction. Students should not draw field lines entering a north pole or leaving a south pole outside the magnet.
Worked examples
Reading strength
At point P, field lines are twice as closely spaced as at Q in the same diagram. The field is stronger at P; no exact numerical ratio can be inferred unless a scale is defined.
Direction near a south pole
Outside a bar magnet, a compass north end points towards the south pole because field lines enter S.
Practical focus
Investigation
Plot at least five field lines around a bar magnet with a plotting compass. Repeat for two unlike poles facing and then two like poles facing. Label poles, add arrows and identify regions where the field is strongest or approximately uniform.
Examination guidance
- Field lines outside a magnet go N to S.
- Lines must not cross.
- Iron filings show pattern but not direction.
- Use a compass north-seeking end to define direction.
Check your understanding
- What does close spacing of field lines indicate?
- Why can magnetic field lines not cross?
- How does a plotting compass show field direction?
Answers
- A stronger magnetic field.
- The field at one point has only one resultant direction.
- Its north-seeking end points in the local field direction.
