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Nugget of Knowledge: Magnet falling through a copper pipe

This is also the answer to ScrapLabs Challenger – #2

The Challenger: http://blog.scraplabs.in/?p=119

The Winners: http://blog.scraplabs.in/?p=167

This can be best explained in two ways – via electromagnetic induction and conservation of energy. I’ll explain using both.

  1. Electromagnetic induction
    Note that we have two hands which are mirror image of each other. Note that when electricity is generated in a conductor via a magnetic field the Fleming’s right hand rule comes into play. However when a force (due to magnetic repulsion or attraction) is created by a current the Fleming’s left hand rule comes into play. Basically, you can generate electricity from magnetism and magnetism from electricity, but in opposite directions. This is the reason why induced magnetic field opposes the initial magnetic field of the magnet.
  2. Conservation of Energy
    Revisit that a fall of an object is basically a process of turning Potential energy to kinetic energy. Also note that resistance of copper is very low. Therefore, when a magnet induces even a very small EMF in the copper tube, it translates to a huge current because of the low resistance. This also means that a lot of energy is dissipated in form of heat. Where does this heat energy come from? 
    This heat energy is a portion of the kinetic energy that the magnet gains during the fall. The magnet slows down itself because a lot of its kinetic energy gets converted to heat in the pipe.

I would also quote a statement made by a student named Shivam Agarwal while in a similar discussion

I would say that the main reason is that if the magnetic field of the eddy currents would move in the direction of the magnetic field of the magnet, the net field would be increased. This would increase the current even more. If the current would be increased, the magnetic field from the current would also be increased which would in turn increase the current even more. This would give an ever increasing power output which would contradict the law of conservation of energy. Hence, the magnetic field from eddy currents has to oppose the magnetic field of the magnet and slow it down so that the net energy remains the same.

Interesting right? 🙂


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