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a mass of metal is near a current carrying wire, say a DC current. the metal feels a magnetic force of attraction and moves toward the wire overcoming some kinetic friction. the work done by the field is force X distance the mass moved. does this draw power from the circuit, can this power be measured and will it equal fXd to move the mass in time t?

for a permanent magnetic will it lose some magnetism moving the mass as it has expended energy to do so?

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  • $\begingroup$ What you're talking about is basically an electric machine (which is a machine that converts electromagnetic energy to mechanical and vice versa). $\endgroup$
    – TZDZ
    Nov 27, 2014 at 10:16

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Theoretically, yes: the kinetic energy of the metal is due to the work of the electrical source that maintains the current. This is what happens: in order for the metal to feel a magnetic force, it needs to have some magnetic moment (which can change over time). When this magnetic moment moves, it creates an induced e.m.f. on the wire that carries the current. In order to overcome this e.m.f. to maintain the current, the source needs to perform some work. However, not all of the work comes into the kinetic energy of the metal. Some part of the work is transferred to the magnetisation energy of the metal, Foucault currents, etc.

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  • $\begingroup$ think I am getting it, so a power change should be measurable in the circuit or from another perspective if the circuit was run from a battery the battery should go flatter quicker than if there was no mass to attract. would it make sense to say that if you moved the mass of metal away with your hand the power should be given back to the circuit? $\endgroup$
    – Peter
    Nov 27, 2014 at 9:24
  • $\begingroup$ @Peter In principle "yes", if you move the metal object slowly enough. Note that now your hand does the work, which is transferred to the electrical source. When the metal object is moved away to its initial place, the system (the metal object + the current + the source, not consider electrical resistance) is unchanged and the work of your hand will be equaled to the heat that has dissipated due to the friction which halted the object when it moved to the current. $\endgroup$
    – ttuethao
    Nov 27, 2014 at 10:28
  • $\begingroup$ cheers answers, I get some down time in a few weeks I am going to see if I can measure it, the current induced fields are so weak though I am expecting it to be a difficult experiment to set up in an amateur home sci lab. any suggestions on how to get big currents and sensitive current meter resolution for small changes of big current. $\endgroup$
    – Peter
    Nov 27, 2014 at 10:40
  • $\begingroup$ I am a theorist and have no idea how experiments work :-) But is it worth testing such a phenomenon anyway? $\endgroup$
    – ttuethao
    Nov 27, 2014 at 15:32

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