Developing Torque

If a coil in which current is flowing is placed in a magnetic field, a force is produced which will cause the coil to rotate. In the coil shown in the following figure, conventional current flows inward on side A and outward on side B.

FIGURE

The magnetic field about B is anticlockwise and that about A, clockwise. As previously explained, a force will develop which pushes side B downward. At the same time, the field of the magnets and the field about A, in which the current is inward will add at the bottom and subtract at the top. Therefore, A will move upward. The coil will thus rotate until its plane is perpendicular to the magnetic lines between the north and south poles of the magnet, as indicated in the above figure by the white coil at right angles to the black coil.

The tendency of a force to produce rotation is called torque. When the steering wheel of a car is turned, torque is applied. The engine of an airplane gives torque to the propeller. Torque is also developed by the reacting magnetic fields about the current-carrying coil just described. This is the torque which turns the coil.

The left-hand motor rule can be used to determine the direction a current carrying wire, which will move in a magnetic field. As illustrated in the following figure, if the First finger of the left band is pointed in the direction of the magnetic Field and the seCond finger in the direction of Conventional Current flow, the thuMb will indicate the direction the current carrying wire will Move.

FIGURE

The amount of torque developed in a coil depends upon several factors: the strength of the magnetic field, the number of turns in the coil and the position of the coil in the field. Magnets are made of special steel which produces a strong field. Since there is a torque acting on each turn, the greater the number of turns on the coil, the greater the torque. In a coil carrying a steady current located in a uniform magnetic field, the torque will vary at successive positions of rotation. When the plane of the coil is parallel to the lines of force, the torque is zero. When its plane cuts the lines of force at right angles, the torque is 100 percent. At intermediate positions, the torque ranges between zero and 100 percent.