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(law of conservation of energy ) apply to electricity & magnetism

  1. Sep 20, 2005 #1
    (law of conservation of energy!!!) apply to electricity & magnetism....

    ok guys... first look at the PICTURE. (now imagine)

    suppose i put SUCH CYLINDRICAL magnet in zero-gravity, perfect vaccum. Then say, i rotate the cylindrical magnet about its axis.

    a). what should happen?
    ->well, there is no any other external force acting on the magnet so that it should rotate about its axis for infinite amount of time.....

    OK, now i introduce a copper coil (or say... any solenoid) inside the rotating cylingrical magnet.... (WITHOUT DISRUPTING ITS MOTION OR ANYTHING)

    what should happen?
    -> mmm... electricity should be induced in the COIL & can be tapped out from its terminals....

    my question?
    HERE; it seems like IN THIS situation there is no external force acting to disrupt the motion fo the cylindrical magnet.... SO will the magnet rotate about its axis for infinite amount of time? IF SO.... electricity will be induced in the coil for infinite amount of time..... :surprised

    <theoritically, it seems impossible> IF this is possible.... then the electrical energy that can be used for useful workwhich is taped out from 'this mechanism' (after some time) will be far far far more than the THE INITIAL work done used to rotate the magnet about its centre!

    hence law of conservation of energy fails!
    & output work will be >>> than input work.

    EXPLAIN my doubt!

    offtrack: wait! don't tell me the rotation will degrate to a stand still due to the opposing force between electrons & the rotating magnet cauz.... SAY! the rotating cylindrical magnet can be MADE up of (say) tens of tons of material so that ITS moment of inertia will be >>> than that opposing force. so it can be compared to..... (g=G m1xm2/R^2) so if we jump in the earth the moment of inertia of earth is >>> than of US so earth dont fall on us... we fall on earth! like situation.....

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    Last edited by a moderator: Sep 20, 2005
  2. jcsd
  3. Sep 20, 2005 #2

    Doc Al

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    Staff: Mentor

    Any energy you get out will come at the expense of the rotational energy that you supplied. Yes, the rotation will eventually stop. Making the magnet tens of tons will just require that much more energy to get it moving.

    Sorry... no free energy.
  4. Sep 20, 2005 #3
    the idea for making a bigger magnet was just to justify moment of inertia.......


    Yap, other wise it will voilate those basic laws.... I KNOW THIS!~

    BUT WHY? why should the rotation eventually stop??? (i need a proper/convincing scientific explaination, please )
    Last edited: Sep 20, 2005
  5. Sep 20, 2005 #4


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    Try this in a clearer fashion...

    You are in space, no gravity, etc. etc. A bar magnet is floating by at constant velocity and it drifts into a coil of wire. The ends of the coil are connected to a light bulb. What would happen?

    Two things:

    1. The bar magnet induces a current in the coil. The current then causes light in the bulb. Energy creation? Nope. Because...

    2. The INDUCED CURRENT itself has a "resisting effect" onto the bar magnet. When the bar magnet is moving in, the induced magnetic field due to the induce current will be in such a way as to produce a force RESISTING the intruding magnet. This is Lenz's Law. The bar magnet will slow down! The change in KE of the bar magnet will be equal to the energy induced in the coil (assuming perfect energy transfer).

    This is an elementary example of a "self-energy" process, where the object that is causing the effect, in turn gets affected by what it causes.

  6. May 20, 2009 #5
    Re: (law of conservation of energy!!!) apply to electricity & magnetism....

    According to lenz's law, the same flux that induced current in the solenoid, will induce mechanical force on the cylnder to stop it.
  7. May 20, 2009 #6


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    Re: (law of conservation of energy!!!) apply to electricity & magnetism....

    Just so you know, this is a thread from 2005!

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