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From where do permanent magnets get their energy or power? I can put a fridge magnet on a fridge, and it seems as if it will stay there forever with no sign of any power source. Also, if I try to push the like poles of two bar magnets together, my arms will grow tired long before the magnets grow weak, yet again there is no power or energy source. Can we not harness this invisible and seemingly endless source of energy? Brian Starkey |
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Alastair Rae, University of Birmingham At this time of year, many of us decorate our 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?’
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. |
December 2007 |
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| Every electron in the metal acts as a magnetic dipole. Dipoles are the simplest sources of magnetic fields. In order to generate a permanent magnet form a metal, you force all these dipoles to line up, so the tiny fields they each generate all add together to make a big field. If you stick this magnet to a fridge, I think the fields from the magnet should cause the electrons in the field to line up in an opposite direction. This means if you have the north pole of a magnet near a fridge, the fridge will start to act like a south pole, and viola--your magnet will stick to your fridge. If you try to push two like poles of magnets together, they repel because the electrons in each magnet are already aligned to repel each other and won't change direction easily. Finally, in terms of energy. In physics, change in energy is defined as a force applied over a distance. If you hold two magnets near each other until your arms get tired, you're not actually putting any energy into the magnets. Similarly, by holding the magnets near each other, you're not getting energy out. To get energy out, you'd probably have to let the magnets fly apart and use that motion to generate electricity or move something. The simple reason why that isn't practical is that you have to put the same amount of energy into the system in the first place in order to put the magnets near each other! ... - jpetruccelli - 7th Dec 07
I think this question can be addressed two ways: are we asking about the nature of magnetism itself, or about the "energy" source? You could ask almost the same question but replace "magnet" with "spring". e.g. "If I squash the spring between my fingers it keeps pushing back; where does the power come from?" Or "If I pull the spring it keeps pulling back, seemingly forever without getting any weaker." As already mentioned, "work" (ie energy) is only done when something actually moves against (or as a result of) the force. You cannot extract any energy without there being motion, and since the force weakens as the distance increases there is only a finite and rather small amount of energy available. Forcing two repelling magnets together, or pulling an attracting magnet away from something merely stores energy (temporarily) in the field. You can only get back the energy which you put in previously.... - techmind - 7th Dec 07
About "their energy" it depends on what you mean: every magnetic field has an energy associated with itself: U = 1/2 μ H2 where U is the energy density (energy per unit volume), so, to create a magnet, which in this case is equivalent to align those magnetic moments of which jpetruccelli wrote, you certainly have to give some energy to that system. Let's make an example: you magnetize a piece of iron with a magnet; you approach the two bodies, and you don't have to make work for this, then (let's say the magnet magnetizes slowly the iron) at the end, they attract each other; now, if you want to take them apart, you have to make work against that attractive force, and that work goes to the system of the two bodies.... - lightarrow - 7th Dec 07
You have beaten me by 20 seconds!... - lightarrow - 7th Dec 07
We do harness this energy, its called a generator (or dynamo). Check electromagnetic induction, Lenz's law and Faraday laws. How does the magnet get its magnetism? In the case of permanent magnets, there are magnetic domains where various numbers of molecules of the material "line up" in a particular direction. (These are not molecular magnets.) These domains exist in many materials in random directions but in certain materials, eg. nickel cobalt and iron they can be made to line up forming a permanent magnet.... - Fridgemagnet - 13th Dec 07
I notice that nobody has actually answered the question yet. It comes from electricity. Magnets get their energy from the factory that they are made. Making a magnet involves immersing the magnet in a strong magnetic field, usually generated by an electromagnet. After this is done, the permanent magnet has stored some of the magnetic energy from the electromagnet in the form of its own magnetic field. The energy in the field of the electromagnet, came from the electricity flowing through the coils. Once it is stored in the magnet, it's fairly hard to make the magnet lose the energy, although a strong repetitive shock will do so, as will a sufficiently strong alternating field, or heating the magnet up to its 'Curie point'. However it is lost, the energy usually ends up as tiny increase in the temperature of the magnet. The amount of energy in a typical bar magnet you might have on your fridge is not usually very much, just a few joules. This would be theoretically enough to run a 100W lightbulb for maybe a tenth or a hundredth of a second or so; and it would be possible with the right equipment to do this, by using the magnet to move itself towards a piece of iron, through a coil connected to the lightbulb for example. Because magnetism is what is called a 'conservative force' the energy you need to remove the iron from the magnet would be as much as it took to light the lightbulb, so the magnetic energy is not used up by doing this, and you can get it back again and the magnet can be reused. Other conservative forces include gravity- gravity never needs recharging either!... - wolfekeeper - 17th Dec 07
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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.
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