Why a light metal disc on the top of an electromagnet is thrown up as the current is switched ON. ?
When the current begins to grow through the electromagnet, the magnetic flux through the disc begins to increase. This sets up eddy current in the disc in the same direction as that of the electromagnetic current.
Thus, if the upper surface of electromagnet acquires N- polarity, the lower surface of the disc also acquires N-polarity. As, same magnetic poles repel each other, the light metallic disc is thrown up.