Turning Effect On A Current Carrying Coil In A Magnetic Field 17.3: A rectangular coil ABCD has been placed in between the pole pieces of a magnet in such a way that it can freely rotate about its axis in the space between the poles. The initial position of the coil is shown in fig. 17.10. In this position, the sides AB and CD of the coil are perpendicular to the field, where as the side BD and DA are parallel. Now if the ends of the coil are connected to the positive and negative terminals of a battery, a current would start flowing through the coil as shown in fig. 17. 10.

The direction of the current in the sides BC and DA is parallel to the magnetic field, so no force would act on these sides. In sides AB and CE, the direction of current is at right angles to the field, so force would act on these sides. According to Fleming’s left hand rule, the forces acting on these perpendicular sides would act horizontally in the opposite directions (fig. 17. 10). The magnitude of these forces would be equal. Under the action of these two equal but opposite forces, a couple would act on the coil due to which it would start rotating.

D.C. Motor 17.4

D.C. Motor consists of a rectangular coil ABCD mounted on a spindle so that it can rotate between the poles of a permanent magnet. A copper ring is fixed on the spindle of the coil. This ring is split into two halves, S1 and S2. One end of the coil is soldered with the split ring S1 and the other to split ring S2. Two carbon brushes X, Y are caused to press lightly against the rings by means of springs (Fig. 17. 11). When these brushes are connected to a battery, the coil starts rotating suppose when the current is switched on; the coil is in horizontal position. In this position, the split ring S1 is in contact with brush X and split ring S2 si in contact with brush Y. the current flows through different sides of the coil in the direction as shown in fig. 17. 11. According to Fleming’s left hand rule, the force acting on side AB is directed upward and that on side CD is directed downwards. Under the action of these two equal but opposite forces, a couple acts upon the coil, which causes it to rotate in the clockwise direction. When the coil reaches the vertical position.

The brushes reach to the space between the split rings and their connection with the coil is cut off. In spite of the fact that there is no current flow, the coil instead of becoming stationary, due to its momentum it continues to move beyond this vertical position and the connection of split ring with the branches is again established. But this time brush X is in contact with the split ring S2 and brush Y is in contact with the split ring S1. In this way, the direction of current flow in the coil is reversed. Now the side AB is on the right hand side with a downward force acting on it and side CD is on left hand side with an upward force acting on it. Thus the coil continues to rotate in clockwise direction as long as the battery is supplying the current to the coil. In this way the D.C. motor converts the electrical energy into mechanical energy, which is utilized for different types of work.