Well, this is a paradox from Feynman lectures on Physics, so if it is right to just discuss it here, then I would like to do so.
It says that we have a device in which there is a circular plastic disc in whose center, there is an axis. Around the axis, a coil of wire is attached which is further attached to a battery. Also, on the perimeter of the disc, there are some equally charged metal spheres. Everything is at rest. Suppose now that by accident, the current in the solenoid is interrupted. So long as the current continued, there was a magnetic flux more or less parallel to the axis. When the current is interrupted, the magnetic flux must go to 0. There will therefore be an E-field induced which will circulate around in the circles centered at the axis. The charged spheres on the perimeter will all experience an E-field tangential to the perimeter. There will be a net torque since field is same in each one.
Now, there can be 2 arguments as to if the disc will rotate when the current is stopped.
From above argument only, when the current is stopped, there is a change in magnetic flux and there should be an induced emf, and the disc must rotate.
But, using the principle of conservation of angular momentum, the angular momentum of the system previously was 0 and it should be so when current disappears. There should be no rotation.
Now, what's the answer to this paradox?