If a current carrying coil is placed in a magnetic field (As shown in diagram above), a pair of forces will be produced on the coil. This is due to the interaction of the magnetic field of the permanent magnet and the magnetic filed of the current carrying coil. The diagram below shows the catapult field produced.

The direction of the force can be determined by Fleming's left hand rule. Since the current in both sides of the coil flow in opposite direction, the forces produced are also in opposite direction. The 2 forces in opposite direction constitute a couple which produces a turning effect to make the coil rotate.

Examples of electric equipment whose operation is based on this turning effect are
  1. the direct current motor
  2. the moving coil meter.


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