 # The Motor Effect and Fleming's Left Hand Rule

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## The Motor Effect and Fleming's Left Hand Rule

#### The Motor Effect

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:

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.

#### Factors Affecting the Size of the Force

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

Where:

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.

#### Fleming's Left Hand Rule

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. Remember direction is vital!!!

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