Power of a Magnet?

Alastair Rae, University of Birmingham

在每年的这个时候,许多我们装修fridges by attaching magnets carrying pictures of Christmas puddings, holly, Father Christmas, snowmen and so on. One advantage of these magnets is that they are easily removed and replaced when Christmas is over. Brian Starkey asks, 'how can they stay on the fridges when there is no obvious power source?'

The first point to note is that we don't need any energy to stand still! A stationary car with its engine turned off doesn't use any petrol! Power is required only when the engine starts turning and the car starts moving. What we have in the case of a fridge magnet is a magnetic force pulling the magnet against the iron door; this then leads to a frictional force that stops the magnet sliding down under gravity, but once the magnet is in place, no energy or power is consumed keeping it there. It's not very different in principle to sticking the magnet onto the fridge using glue.

When Brian pushes the two like poles of a magnet together, he has to apply a force and use energy. If they are then allowed to move apart, this energy is released and converted into motion. However, if he holds them together without letting them move, no more power is needed. It's perhaps easier to understand this if we think of the magnets being supported by a rigid frame instead of by a person. Why then do Brian's arms grow tired if he is not doing any work? This is all to do with biology and the complex way our muscles work: chemical energy has to be burned to keep them stiff and able to exert pressure. But magnets are not like that: they exert a force pushing each other apart and do not consume any power as long as they don't move.