Question

A straight wire carrying an electric current is placed along the axis of a uniformly charged ring. Will there be a magnetic force on the wire if the ring starts rotating about the wire? If yes, in which direction?

Answer 1

There will not be any magnetic force on the wire.  

Explanation:

• The wire carrying an electric current i has the magnetic force, F→=i.(l→×B→)F→=i.(l→×B→), where B is the magnetic field of the wire and l is the wire’s length.
• When the ring that is uniformly charged starts revolving everywhere a straight wire, then as per the right-hand thumb rule, due to the current of the ring, the magnetic field on the straight wire positioned at its axis will be equivalent to it. So, the cross product will be  (l→×B→) = 0

        ⇒F→=0(l→×B→)=0⇒F→=0

• So, there will be no magnetic force on the wire.