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"A bar magnet is moved towards a freely hanging coil. Determine if the coil stays stationary or not. If it moves, determine if it moves away or towards the magnet."

My hypothesis: From Lenz's law an e.m.f will be induced in the coil to oppose the change in magnetic flux due to the relative approach of the bar magnet. The resultant current in the coil produces a force on the coil equal to the Lorentz force due to the approach of the bar magnet, and in the opposite direction. As such, the forces should cancel out, causing the magnet to remain stationary.

I suspect that there are some flaws in my hypothesis. Are my deductions correct, and is there a better explanation? (note: I'm not sure if the coil will move or not)

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Obviously, the coil will move. When we are moving the bar magnet toward the coil a current will flow through the coil in a direction to reduce the rate of change of flux through it. Now you can think the coil as a tiny magnetic dipole of dipole moment m=I a(or a small bar magnet for your convenience ). And this will move in the presence of other magnet if it is freely hanging.

You may find this Wikipedia article useful.

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  • $\begingroup$ Would it move towards or away from the magnet? $\endgroup$ – Yiyuan Lee Jul 2 '14 at 14:39
  • $\begingroup$ say the bar magnet has its north pole nearer to the coil. then to oppose the change in flux the current in the coil will be such that if you see the coil along the direction of the motion of the magnet it will be anticlockwise. so you can think it as,if you wish, the coil has its north pole nearer to the bar magnet's north pole. so it will be away from the magnet. When the bar magnet has its south pole nearer to the coil, you find it yourself. $\endgroup$ – user22180 Jul 2 '14 at 14:44
  • $\begingroup$ You can think in terms of Newton's third law of motion. You know from the lenz's law the current will be such so as to reduce the change in flux through the area enclosed by it. You know what should be the direction of the magnetic field produced by this current to reduce the change in flux (actually by knowing this you find the direction of current). This magnetic field will act a force on the bar magnet. And from the Newton's third law of motion you know the force on the coil will be opposite to that. $\endgroup$ – user22180 Jul 2 '14 at 15:00
  • $\begingroup$ I find the second explanation with Newton in the picture clearer. Thanks a lot! $\endgroup$ – Yiyuan Lee Jul 2 '14 at 15:02

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