The Motor Effect and Fleming's Left Hand Rule

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The Motor Effect and Fleming's Left Hand Rule

If two magnetic fields combine, a force is exerted. One magnet exerts a force (attractive or repulsive) on the other.

Look at these two fields:

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The strong field behind the wire pushes it upwards as shown.

This effect allows us to turn electrical energy into kinetic energy. We call it the motor effect.

The size of the force can be increased by:

  • Increasing the magnetic field strength;
  • Increasing the length of the wire in the field;
  • Increasing the current in the wire.

F = B I L


F = Force on the wire (N)

I = current (A)

B = magnetic field strength (T)

L = length of the wire (m)

Note: The angle between the current and the magnetic field must be greater than zero for a force to be produced. The greatest effect is when the angle is 90º. So if the wire and the conductor are not perpendicular to each other we must use:

F = B I L sin θ


Note: The angle is measured between the field direction and the conductor.

To work out the direction of force experienced we use Fleming's Left Hand Rule.

  • Your first finger points in the direction of the magnetic field (North to South).
  • Your second finger points in the direction of conventional current (positive to negative).
  • Your thumb points in the direction of the thrust or force on the conductor.
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Remember direction is vital!!!

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