A bar magnet has two poles; the north-seeking pole (labelled N) and the south-seeking pole (labelled S). The magnetic force is concentrated at the poles. This can be shown by placing the magnet beneath a thin sheet of card and scattering iron filings over the top. The darker patches of iron filings correspond to stronger magnetic forces. The centre of the bar magnet is where the magnetic force is weakest.
It is best to think of the ends of a bar magnet as north seeking or south seeking – the Earth has a North Pole and a South Pole. Bar magnets and compasses have north-seeking poles and south-seeking poles. If you keep that distinction in your head then questions about magnets will be much easier to answer.
If you do a similar experiment but use little plotting compasses instead of iron filings then you can give the field lines direction too. The pointers on the plotting compass are repelled by the North seeking poles of the magnet but attracted to the South seeking poles.
Combining the two sets of data, you can draw the full field lines around a bar magnet. Note the direction of the arrows, from North to South, and that the lines are closer together at the poles where the forces are stronger.
Questions…
- Why are iron filings used in the demonstration above not the filings of another metal like aluminium or copper?
- If two magnets are brought towards one another and they repel, what can you say about the poles of the magnet being brought together?
- How can you see from the diagram above that the magnetic force is strongest at the poles of the bar magnet?
- What does the N on the bar magnet stand for?
- How could you demonstrate the fields lines around a bar magnet?
Comments