The Motor Effect

The motor effect — a current-carrying wire placed in a magnetic field experiences a force, provided the wire is not parallel to the field.

The reason is that the wire's own circular field interacts with the field it sits in. The two fields reinforce on one side and cancel on the other, and the wire is pushed away from the stronger side.

The size of the force:

F = B I L

where F is the force in newtons, B the magnetic flux density in tesla (T), I the current in amperes and L the length of wire in the field in metres.

This equation applies when the wire is at right angles to the field. Three ways to increase the force therefore follow directly: a stronger field, a larger current, or a greater length of wire in the field.

Fleming's left-hand rule gives the direction. Hold the thumb and first two fingers of the left hand mutually at right angles:

• First finger → Field (N to S)
• SeCond finger → Current (conventional, positive to negative)
• ThuMb → Motion (the force)

Using the right hand gives exactly the wrong answer, which is why the rules are worth keeping straight: left for motors, right for generators.

Angle matters:

• The force is greatest when the wire is at 90° to the field.
• The force is zero when the wire is parallel to the field, because then the wire's field and the external field do not interact in a way that produces a sideways push.

Reversing the force. Reversing either the current or the field reverses the direction of the force. Reversing both leaves the force unchanged — a point worth remembering, because it is why a motor connected to a.c. without a commutator simply vibrates rather than turning.

Example: a wire of length 0.050 m carrying 3.0 A sits at right angles to a field of 0.20 T. The force is F = BIL = 0.20 × 3.0 × 0.050 = 0.030 N. Turn the wire to lie along the field and the force falls to zero.