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Why exactly can energy not be gained from magnets?

  1. Oct 15, 2009 #1
    Since magnets cannot be "switched off" without input energy, energy cannot be gained from them(right?).

    But how do you explain and prove this? If I wanted to tell an inventor attempting to find free energy, how would I explain this to him/her?
     
  2. jcsd
  3. Oct 15, 2009 #2

    vanesch

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    In fact, you *can* gain - in principle - energy from magnets, but "only once". The reason is that there is a certain amount of energy in the magnetic field, which has been "put there" when the magnet was created, and you could, in principle, get that back.

    The fundamental reason why a magnet cannot be a "permanent energy source" (which is what I guess you are asking) is that the electromagnetic field is conservative: it makes sense to talk about the energy content of an EM field, because it enters in the total energy balance: its energy is equal to what you put in, minus what you take out of it. The relationships are more complicated, but it is the same principle as the fact that gravity is also conservative:
    if you put a stone on a shelf, it has more (potential gravitational) energy, because you did work to put it there. If you drop the stone from the shelf, you can get that energy back, it has now less potential gravitational energy.

    Or, in other words, the energy of the stone, or the energy of the EM field, is like a bank account (assuming normal banking!). You can get money out of it, if you first put money in. You can't use a bank account as a permanent source of money.
     
  4. Oct 15, 2009 #3
    Thanks, that was exactly what I was looking for.

    So am I right in saying that if a metal object entered and left a magnetic field, by the time it leaves the magnetic field, there is no net energy gained?
     
  5. Oct 15, 2009 #4

    vanesch

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    IF:
    - the field is static
    - the field is the same "after" and "before"
    - the source didn't bring in any energy,

    then, yes.

    But to state that in all generality is too strong, because you can find counter examples. Electric motors are such counter examples, for instance. A part of the rotor can enter a magnetic field (generated by the coils), leave it again, but it did gain energy (which it gave, mechanically, to the axis of the motor). However, the magnetic field was not static, and the source (the windings) did do work (or better, the electric power source that powered the coils did do some work).

    What counts is that the *total energy balance* is ok.

    Even just a "static field" wouldn't be sufficient: you could think for instance that by bringing in some magnetizable object, the object gets magnetized, and actually takes some energy from the original magnet (in the form of some electrical power if it is an electromagnet, or in the form of a partial demagnetization if it is a permanent magnet).
     
  6. Oct 15, 2009 #5
    Thx for that explanation Vanesch
     
  7. Oct 29, 2009 #6

    MacLaddy

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    Reading this thread brought on another question for me.

    With the explanations above, how would you describe an object in gravitational orbit around a planet?
    Is it constantly acquiring, and releasing, the same amount of energy?

    Have not taken any physics classes yet... Still working on getting "there."
     
  8. Oct 29, 2009 #7

    vanesch

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    Yes. That's exactly it.

    You will learn that you can express the gravitational interaction by "potential energy". And what happens is that sometimes, objects go faster due to gravity, which comes down for them to "loose potential energy, and win kinetic energy", and sometimes they slow down, and then they "win potential energy, and loose kinetic energy".

    In fact, it is somewhat more involved, because although you can assign kinetic energy to individual objects (it's given by their velocity and rotation), you cannot always assign potential energy to a single object, but rather to the whole of interacting objects.

    But the sum of all the kinetic energies, and all potential energy, remains constant. So there is continuously exchange between kinetic energies, and potential energy, but the total budget remains constant.
     
  9. Oct 29, 2009 #8
    Ultimately the bookkeeping works out because of Noether's theorem. Because the laws of magnetism and gravity don't change over time, the total amount of energy remains constant.

    Here is an attempt to come up with an inituitive explain of why that is. Note that this may be totally wrong.

    Take a set of magnets at rest. Stare at them for a day. No energy is coming in or out. Ho hum.....

    Now look at the magnets. Leave to get a cup of coffee for a day, and then come back. And they look at them again. They are exactly the same as before.

    Now the way that magnetism works is that the rules of magnetism say that everything is time invariant and conservative, which means that what happens to a set of magnets can be determined by looking at what look like at a given moment. So if the first case and the second case, the universe should look exactly the same, and if they aren't then the rules aren't time invariant (and if you change the rules then you tell me what happens).

    The interesting thing is that in the day you weren't looking at the magnets, a lot of stuff could have happened. There could have been a total explosion that sent one of the magnets to the moon and back, but because the rules of magnetism say that what happens in between doesn't matter, the second situation and the first situation will lead to exactly the same situation, and in the first situation, you didn't have any energy gained or lost.
     
  10. Oct 29, 2009 #9

    MacLaddy

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    Didn't understand all of that, but I believe I grasp it as well as I can without the forthcoming classes.

    As geeky as it may sound, Physics is Cool...
    Oh well, I'm old enough to sound geeky. Can't wait to start learning the fundamentals.
     
  11. Oct 30, 2009 #10

    vanesch

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    Some very serious advice: if you can't wait to start learning... well, then don't wait ! Start immediately, on your own, and the best thing I could probably tell you, is: get your hands on the first volume of "The Feynman lectures of Physics". It reads easily, it is not too formal, and it's really, really cool.
     
  12. Oct 30, 2009 #11

    MacLaddy

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    Good advice, and I have saved all three volumes on Amazon, but unfortunately I am not academically ready for that. I need just a wee-bit of math touch-up. (as a complete understatement.)

    I've managed to live 31 years without hardly any math at all, so I have some major catching up to do.
     
  13. Oct 30, 2009 #12
    Ditto, I really a copy of what one author calls his "lost lectures" on physics... its awesome!! ^^
     
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