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Q & A: magnets and energy

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Most recent answer: 01/07/2017
Q:
My questions have to do with �Magnets vs Gravity� and the �Conservation of Energy�.I have had a magnet my grandfather gave me, stuck on various refrigerators for the last 40 years or so. I currently use it to hold my sharpest knife in the kitchen, in a safe place.If I were to make an electromagnet and had to calculate how much electricity I would need, for it to be able to defy gravity and support its weight and the weight of the knife it holds for the next 40 years... I would imagine I would go through cases and cases of batteries or good deal of AC power.The more I think about this, the more it seems like one or more of the fundamental laws of physics is being broken. You shouldn't be able to counteract the force of gravity without expending energy and energy cannot be created or destroyed, it can only be transformed or transferred.With those things in mind, if my "permanent" magnet holds my kitchen knife up �perpetually� (or at least until the refrigerator disintegrates); You see what I'm getting at...? Conservation of Energy. you shouldn't be able to get more energy out of a system than was put into it. So... where is the magnet's energy coming from? None of my other refrigerator magnets have spontaneously fallen to the floor so I assume this is not an isolated incident...?Questions:* Is a permanent magnet exchanging energy with anything in anyway, in order to counteract the force of gravity that is pulling on the knife?* Is an excessive amount of energy required to produce a permanent magnet? If not, where did all the energy come from that was required to counteract the force gravity has been exerting on the knife and on the magnet itself all this time?
- Mike (age 49)
PA, USA
A:

We've addressed this question numerous times before: http://van.physics.illinois.edu/qa/listing.php?id=339. The magnet doesn't have to supply energy any more than your car engine has to supply energy to keep the car parked on a sloping street. 

But what about the electromagnet? Inside the electromagnet, the magnetism comes from electrical currents. If it's not a superconductor, those currents of moving electrons run into electrical resistance, friction with the non-moving atomic nuclei. So their energy is turned into heat, just like electrical energy in a light bulb or an electric blanket turns to heat. 

Inside the permanent magnet the magnetism comes from the permanent internal spin property of each electron, not from the sort of large-scale currents that can run down. The way a "permanent" magnet can run down is if the spins in different electrons get out of line with each other. That will happen if it gets too hot.

Mike W.

 


(published on 01/07/2017)

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