When a current passes through the coil, I am happy that if the metal disc rotates and cuts the flux, then eddy currents will be induced in the metal disc to oppose field of the electromagnet.
My question is, what is the direction of the force on the currents, or how you work it out, as I know that the disc will stop as the force created will oppose that of the motion of the disc, but how can this be if the currents are on the surface of the disc?
Best Answer
You're right that the magnetic force with oppose the rotation disc so the force on the eddy currents would be in the opposite direction of the disc's motion.
A simple way to think of it is to think of an eddy current circulating in the part of the disc that is about to pass between the poles of the magnet. If the near pole is is a north pole then the eddy current would flow counterclockwise producing a north pole on the near side of the disc that would be repelled by the magnet's north pole. At the same time, on the part of the disc that has just passed through the magnet and is moving away from it, there would be an eddy current current flowing clockwise which would produce a south pole on the near side of the disc which would be attracted back to the magnet.