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Answer to Question #69953 in Electricity and Magnetism for Hemalkumar Pansuriya
Question #69953
Suppose we have a magnet and a coil at sufficiently large distances apart such that any movement of magnet can not be known instantly at the coil, as change in magnetic field travels at the speed of light.
Now, if i give magnet some kinetic energy. After moving to some distance, i stop the magnet.
This change in magnetic field reaches to coil and it will generate current opposing motion of magnet. But, magnet is already at rest so when this opposing magnetic field will reach at the magnet, the magnet will gain kinetic energy in opposite direction and this will in turn produce current in coil again.
isn't this violating energy conservation law, as magnet is not going to feel any resistance instantaneously.
Now, if i give magnet some kinetic energy. After moving to some distance, i stop the magnet.
This change in magnetic field reaches to coil and it will generate current opposing motion of magnet. But, magnet is already at rest so when this opposing magnetic field will reach at the magnet, the magnet will gain kinetic energy in opposite direction and this will in turn produce current in coil again.
isn't this violating energy conservation law, as magnet is not going to feel any resistance instantaneously.
Expert's answer
Electromagnetic field is a physical quantity and it contains energy itself, so when one increase kinetic energy of magnet it is also wasted energy to changes in electromagnetic field and this corresponds to propagation of energy in field (and with field) with speed of light. Therefore, this propagation of energy leads to delay in resistance appearance.
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Comments
Dear Hemalkumar Pansuriya, Questions in this section are answered for free. We can't fulfill them all and there is no guarantee of answering certain question but we are doing our best. And if answer is published it means it was attentively checked by experts. You can try it yourself by publishing your question. Although if you have serious assignment that requires large amount of work and hence cannot be done for free you can submit it as assignment and our experts will surely assist you
Further more, for EMF generated in the coil, you can draw any large amount of current in the coil ( by decreasing resistance ) And since, you are not going to feel any resistance in magnet, your input energy won't increase by increasing output energy in coil.
You can have very large coil and very powerful magnet. Distances between them doesn't have to be light year apart, it can be like distance between earth and moon. Even that distance also will have 5 sec delay time. If you imagine a coil as big as earth and a very powerful magnet at distance of moon then you have to consider this effect.
The problem is that you propose to consider very theoretic problem. For a quite large distances influence you consider will be negligibly small and cannot be discussed using only mental experiment.
But due to delay, when magnet is in constant motion at that time it will not feel any resistance force. But in constant motion also it will produce energy in the coil. Moreover, the energy produced in coil is V*I, current can be drawn large ( by decreasing resistance in circuit) .this increase in output is not increasing your input as magnet is not feeling any resistance.
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