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A coil of radius R carries a current I. The plane of another concentric coil of radius r (r << R) carrying current I is perpendicular to the first coil. Both are isolated from rest of the universe and are free to rotate about their common diameter (but not free to translate). Find the maximum KE of the smaller coil. (Assume that there is no friction anywhere).

My problem here is about angular momentum. Since angular momentum is conserved when the net torque acting on the system is zero. Can anyone justify as how angular momentum is conserved, if at all?

I was thinking that since the coils are only rotating, so its only torque that can do work. Since the system is isolated from rest of the universe, no external agent will do work in the system. So, the total energy of the system shall remain constant. (Assuming no heat loss as the coils are assumed to be resistanceless).So, the net torque acting on the system must remain zero.

But at the same time i also felt that the net WD by the torque may be zero, but stillthe net torque may not be zero.

Thus, from these arguments, what shall we conclude about angular momentum. Or is there any other arguments to justify the conservation of angular momentum or otherwise.

I was also wondering that since the coils rotate, emf will be induced in the coils, and so it will be tough to calculate the torque acting on both the coils during the process of its motion.

Please help!!

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# Homework Help: Angular momentum conservation?

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