A flat coil ABCD is freely suspended between the pole pieces of a U-shaped permanent magnet with the plane of coil parallel to the magnetic field.

(a) What happens when a current is passed in the coil?

(b) When will the coil come to rest?

(c) When will the couple acting on the coil be (i) maximum, (ii) minimum?

(d) Name an instrument which makes use of the principle stated above.

(a) When a current is passed in the coil, the coil experiences a torque due to which it rotates.

(b) The coil will come to rest when it's plane becomes normal to the magnetic field.

(c) Maximum — The couple acting on the coil be maximum, when the plane of coil is parallel to the magnetic field.
Minimum — The couple acting on the coil be minimum, when the plane of coil is normal to the magnetic field.

(d) The instrument which makes use of the principle is D.C. Motor.