A ring with a clockwise current as seen from above the ring

A ring with a clockwise current (as seen from above the ring) is situated with its center directly above another ring, which has a counter-clockwise current, as shown in Figure 20.7. In what direction is the net magnetic force exerted on the top ring? Figure 20.7 (near side) A) downward B) upward D) Impossible to determine C) The net force is zero.

Solution

The answer is (B) upwards ; both the north poles of the magnetic fields created by the two rings face each other.

Reason : The magnetic field created by the lower ring point upwards , which mean that the south pole is downward .

This can be imagined by right hand thub rule. if you close your right hand 4 fingers and open just the thumb ; then the curling fingers point current direction and thumb points north of magnetic field produced.

simillarly the north pole of the magnetic field created by upper ring points downwards, so the north of the upper ring faces the north pole created by the lower ring , there by upper ring experiencing a repulsive upwards force.