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A DC generator work by converting the mechanical energy of the rotating coil to electrical energy and hence current is generated.

What if this generated current is supplied to a motor which in turn helps of the rotation of the coil in the same generator, wouldn't this lead to perpetual motion?

I know I made a mistake, but I'm not able to find out where

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    $\begingroup$ Friction losses would soon stop it. Not all the energy keeps going around. $\endgroup$
    – Peter
    Commented Aug 8, 2021 at 6:24

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Off the top of my head, I can think of two instances where energy will be lost here. Because wires have some resistance, no matter how small, there will be energy loss in the form of heat as the charges do work to go against the resistance. Also, energy will be lost in the form of contact heat when one motor drives coil.

So there will be no perpetual motion, as energy is continuously lost per cycle. Given enough time, the motion will slowly die out.

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The mistake is that the generator will not be perfectly efficient, and the motor will not be either.

Getting it started will take either an initial charge on the battery, or outside motion. Either of those is outside energy.

Then after being started from outside energy, it will get slower and slower until it stops - due to the imperfect efficiencies of the two devices.

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