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When we move a magnet towards a coil, current is induced in it. Magnetic field too develops around it. Since there is interaction of both magnetic fields(that of magnet and coil), there is magnetic potential energy in field(gained from the work we do). This energy is going to do work on magnet (either in repulsion or attraction). In this case Lenz's law tells us that it will be repulsive force.

If it is attractive or repulsive , the stored energy is to be spent on magnet and it does work on magnet. Also whole of energy we supplied is not used in this work as there is heat loss too. Then why we say it (in case of attractive force by coil on magnet) breaks conservation of energy?

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  • $\begingroup$ why do you think it breaks conservation of energy ? Can you elaborate it ? I am not aware of this phenomenon . $\endgroup$
    – Ankit
    Aug 31 '20 at 17:07
  • $\begingroup$ It states that if we bring north pole of magnet towards a coil the induced current is such that its polarity is also north, as to oppose the cause. It says that if induced currents polarity is south , north-south attraction is there so just a little energy in moving magnet produces infinite energy(saying that attraction itself induces current and it wont stop). So they say to conserve energy polarity must be north. Won't the attraction be due to our energy? $\endgroup$ Sep 1 '20 at 3:58
  • $\begingroup$ Can you help me? $\endgroup$ Sep 1 '20 at 11:21
  • $\begingroup$ I have tried to answer your question. Hope it helps . If still have some doubts then comment below. $\endgroup$
    – Ankit
    Sep 1 '20 at 12:08
  • $\begingroup$ was the answer helpful ? $\endgroup$
    – Ankit
    Sep 1 '20 at 15:20
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See if the other case was possible then just a little push (towards the coil) would give it a greater amount of kinetic energy .

Suppose we applied a force of 2N in moving the magnet towards the coil by a few centimetres ( let's say) 10cm then our work or energy transferred equals 0.2 J and this will give the magnet some kinetic energy and some part of this much energy is initially transferred to the electrons from which little energy is released due to the resistance of the coil . So what this means that the kinetic energy of the magnet should be less than earlier because it is moving towards the coil and it's energy is being transformed into heat and magnetic energy and it should stop after some time.

But if the magnet was attracted then it would have got some more kinetic energy . And the cause of that kinetic energy must be some work .

Since we left the magnet under attraction this means that the cause of that extra work was not us but the coil itself. But for inducing current in the coil very little amount of energy was supplied to the electrons , so how could this little energy amplify to such an extent without any external supply . Hence energy is created by the coil or say by induced current.

This means that energy is not conserved . Hence this scenario breaks energy conservation . And so it can't be true .

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  • $\begingroup$ The source of kinetic energy is due to magnetic potential energy whose source is us right? $\endgroup$ Sep 1 '20 at 15:34
  • $\begingroup$ @GRAVITON PI see the source of the induced magnetic field was our energy but it was a fixed amount because we just left the magnet under attraction. So the extra kinetic energy was not due to us but due to attraction. $\endgroup$
    – Ankit
    Sep 1 '20 at 15:38
  • $\begingroup$ Work done on magnet depends on energy stored in field like attraction of huge magnet by a small one $\endgroup$ Sep 1 '20 at 15:38
  • $\begingroup$ @GRAVITON PI yes you are correct but how the induced magnetic field got extra energy to transfer it to the magnet ? $\endgroup$
    – Ankit
    Sep 1 '20 at 15:39
  • $\begingroup$ @GRAVITON PI as in the example I used there was just 0.2 J . So it's Maximum kinetic energy should be less than that because of resistance in the coil but its speed goes on increasing which means extra energy supply. $\endgroup$
    – Ankit
    Sep 1 '20 at 15:43
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if the force is attractive, we only have to start the motion, then the attraction would accelerate it, the current would increase, and we get energy for nothing or a perpetuum mobile.

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