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Why does a moving (at constant velocity) magnet release more energy in the form of magnetic fields when the number of charges placed near it are increased?

Say two current carrying coils $C1$ and $C2$ are in a system in which $C1$ is moving at constant velocity. The magnet fields from $C1$ will reach $C2$ after an interval of time. So at the moment $C1$ releases energy (M.F) it doesn't know about charges surrounding and it can't make predictions of how much energy is to be released at the instant. So it can only give out a fixed amount of energy in M.F. Let's say it is equal to $M'$. When we add another coil say $C3$ in system, the amount of energy released by C1 is still $M'$. Therefore the total output can't exceed $M'$. This statement says that you can't get more wireless energy by constant velocity gaining magnet in output even if you use many sets of coils. But I think that it is false and we can get larger amount of energy as output by using extra coils. C2, C3, ... Cn

Explain what is right

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This statement says that you can't get more wireless energy by constant velocity gaining magnet in output even if you use many sets of coils. But i think that it is false and we can get larger amount of energy as output by using extra coil. C2,C3....Cn Explain what is right

The statement is correct, it is not false as you think.

Conservation of energy in electromagnetism is given by Poynting's theorem. Poynting's theorem says that the electromagnetic field has an energy density which is given by the square of the E field plus the square of the B field (in natural units). The theorem says that the energy density can only change one of two ways: EM energy can flow from other places in the EM field according to the Poynting vector, or the electromagnetic field can exchange energy with matter (work).

If you have an accelerating or rotating magnet (notice that an inertial magnet does not radiate) then it will produce an electromagnetic field which will carry energy. Per Poynting's theorem, this energy can move from place to place as the EM field moves outward from the magnet.

If there is a coil in the path of this EM wave, then that coil can receive some of that energy, and work can be done on the coil. When that happens, the energy in the field will be reduced and less of that energy will be available for doing work on additional coils. There is thus a limit to how much energy can be extracted from the EM wave, and in this scenario that limit is $M'$. Once that much energy is extracted the energy density is 0 and so per Poynting's theorem the fields are 0 and so also is the energy flux.

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  • $\begingroup$ If the statement were right then it says that the heat energy lost in a changing current carrying coil $C1$ with a given rate of change in current will have a fixed amount of heat energy lost in heating its conductor material and that energy loss rate dosen't get affected even if we put other coils in the system( we can take any number from 0 to infinity) i haven't expected that really! $\endgroup$ Jun 24, 2021 at 21:29

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