How does magnetism multiply?

13 February 2011

Question

If I stick two bar magnets together do they pick up more paperclips than if i only had one bar magnet?

Answer

Dave - They will pick up some more bar magnets. If you have a single bar magnet, it's essentially a north pole and a south pole, and the force applied by those reduces with an inverse square - so if you double the distance, the force goes down to a quarter. However, because the north pole is the opposite of the south pole, if they're very, very close together, then the north pole is effectively reducing the strength of the south pole and vice versa. And so, if you get very, very close, it will be weaker. Conversely, if you make the bar magnet longer, then this actually moves the north pole away from the south pole, so they cancel each other out less. So, especially if you start off with a reasonably long bar magnet, it's not going to make very much difference to the strength of the magnetic field right next to the end of the magnet - a little bit but not very much. But it will drop away much more slowly with distance. And so, especially with the paper clips which are dangling quite a long way away, that might have a significant effect. For the ones which are close to the end of the magnet, it's going to have hardly any effect at all.

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