D.C.Motor
Q.1

1: A rectangular coil PQRS is placed between the pole pieces of a horse- shoe magnet as shown in figure?

coil PQRS
(i) What is the direction of the force on each arm?,
(ii) What is the effect of the forces on the coil ?
(iii) How does the effect of forces on the coil change if the battery' sterminals are interchanged?

Ans:

(i) In figure, the current in the coil is in the direction of SRQP.
By Fleming's left-hand rule, on the arm PQ, the force is outward at right angles to the coil plane.
On the arm, BC no force acts. On the arm RS, the force is inwards perpendicular to the plane of the coil.

(ii) The force on the arms PQ and RS are equal in magnitude, but opposite in direction.They form a clockwise couple.
So the coil will rotate clockwise with the arm PQ coming out and the arm RS going in.

(iii) On interchanging the battery's terminals, the direction of current in coil is reversed so that the coil will rotate anticlockwise.

Q.2

Figure shows a rectangular coil PQRS placed between the pole pieces of a horse - shoe magnet with its plane perpendicular to the magnetic field.

rectangular coil PQRS
(i) What is the direction of the force on each arm of the coil ?,
(ii)Will the coil rotate due to the forces on its arms ?

Ans:

(i) In figure, the current in the coil is in the direction of SRQP.
On the arm PQ, the force is upward in the plane of the coil i.e, away from the arm RS.
On the arm QR, the force is outward in the plane of coil i.e., away from the arm PS.
On the arm RS, the force is downward in the plane of the coil i.e, away from the arm PQ.
On arm SP, the force is outward in the plane of the coil i.e., away from the arm QR.

(ii) The coil will not rotate (because all the arms experience an outward pull).