15.3 TURNING EFFECT ON A CURRENT- CARRYING COIL IN A MAGNETIC FIELD
If instead of a straight conductor, we place a current- carrying loop inside the magnetic field, the loop will rotate due to the torque acting on the coil. This is also the working principle of electric motors. Consider a rectangular coil of wire with sides PQ and RS, lying perpendicular to the field, placed between the two poles of a permanent magnet (Fig. 15.8). Now if the ends of the coil are connected with the positive and negative terminals of a battery, a current would start flowing through the coil. The current passing through the loop enters from one end of the loop and leaves from the other end.
Now apply Fleming's left hand rule to each side of the coil (Fig. 15.8). We can see that on PQ side of the loop force acts upward, while on the RS side of the loop force acts downward. It is because the direction of the current through the two sides of the loop facing the two poles is at right angles to the field but opposite to each other. The two forces which are equal in magnitude but opposite in direction form a couple. The resulting torque due to this couple rotates the loop, and the magnitude of the torque acting on the loop is proportional to the magnitude of the current passing through the loop. If we increase the number of loops, the turning effect is also increased. This is the working principle of electric motors.
15.4 D.C. MOTOR
We can see from Fig. 15.9 that the simple coil placed in a magnet cannot rotate more than . The forces push the PQ side of the coil up and the RS side of the loop down until the loop reaches the vertical position. In this situation, plane of the loop is perpendicular to the magnetic field and the net force on the coil is zero. So the loop will not continue to turn because the forces are still up and down and hence balanced.