Science Questions

How does magnetism multiply?

Sun, 13th Feb 2011

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Alex asked:

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


Dave -   They will pick up some more bar magnets.  If you have a single bar magnet, itís 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, itís not going to make very much difference to the strength of the magnetic field right next to the end of the magnet Ė 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, itís going to have hardly any effect at all.


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Alex asked the Naked Scientists: If I stick two bar magnets together do they pick up more paperclips than if i only had one bar magnet? What do you think? Alex, Fri, 11th Feb 2011

that depends on how you put them together.  if you allow them to attract each other together and stick opposite poles have neutralised the magnetic field of each other and the external magnetic field has been reduced.  It therefore follows there is less energy available to pick up paper clips.  On the other hand if you force them together in the direction that they repel each other their fields add up and they should pick ip more.  This shows one important thing that is often forgotten about magnets.  that is they are under stress.  that is if you cut a bar magnet in half along its axis between the poles it would fly apart.  The design of powerful permanent and electromagnets mus include the effects of the stresses cause by the fields on the materials of the magnet. Soul Surfer, Fri, 11th Feb 2011

Sorry doc I thing that you have got it wrong there.  The inverse square law applies to the field from the individual poles consider one bar magnet say 10cm  long  the inverse square law applies only when you are close to one of the poles these can be considered as being localised at a point inside the magnet near the end of the bar.  when you get further away the neutralising effect of the other pole comes into play and at distances significantly greater than the length of the magnet the result is that the field falls off as the inverse cube of the distance. however if you "stick" two similar bar magnets together they will be opposing poles that stick and the two poles become much closer together and the inverse cube law comes in quicker.  although I agree that in the vicinity of the individual poles themselves the forces will be similar.  However if you clamp two bar magnets together  north to north the field will be stronger.    The disruptive forces on strong magnets can be very powerful and have to be included in big magnet designs. Soul Surfer, Wed, 16th Feb 2011

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