Turning effect on a current-carrying coil

A coil carrying an electric current can turn when it is placed in a magnetic field. The current flows in opposite directions along opposite sides of the coil, so the magnetic forces on those sides act in opposite directions.

These forces act on different sides of the coil. They can therefore form a turning effect, or torque, which rotates the coil. The torque depends on the coil’s orientation in the field and can be zero at particular orientations. This turning effect is the basis of a d.c. motor.

Increasing the turning effect

The turning effect increases if any of the following is increased, provided the other relevant conditions remain unchanged:

• Number of turns: each additional turn contributes to the total turning effect.
• Current: a larger current produces larger magnetic forces on the sides of the coil.
• Magnetic field strength: a stronger field produces larger forces on the sides of the coil.

D.c. motor operation

A d.c. motor uses a current-carrying coil in a magnetic field to produce rotation. The magnetic field exerts forces on the two opposite sides of the coil. These forces act in opposite directions, creating a turning effect that makes the coil rotate.

The split-ring commutator

The ends of the coil connect to the two halves of a split-ring commutator, which rotates with the coil. Two stationary brushes maintain electrical contact between the d.c. supply and the rotating commutator, allowing current to flow through the coil.

As the coil turns through half a revolution, its opposite sides exchange positions. At the same time, the commutator reverses the current through the coil. This reverses the force on each side, so the forces continue to produce a turning effect in the same rotational direction. The reversal happens every half-turn, allowing the motor to keep rotating in that direction.