The Motor Effect

When a current-carrying conductor is placed in a magnetic field, it experiences a force. This is the motor effect.

Why it happens

The magnetic field around the current-carrying wire interacts with the external magnetic field. On one side of the wire, the fields reinforce (stronger); on the other side, they cancel (weaker). The wire is pushed from the strong side to the weak side.

Fleming’s left-hand rule

Use your LEFT hand:

• First finger (index) = Field direction (N to S)
• Second finger (middle) = Current direction (conventional: positive to negative)
• Thumb = Thrust (force/movement direction)

All three fingers must be at right angles to each other.

Factors affecting the force

The force is larger when:

• The current is larger ($F \propto I$).
• The magnetic field is stronger ($F \propto B$).
• The length of conductor in the field is longer ($F \propto l$).

$F = BIl$

where $B$ is the magnetic flux density (T), $I$ is the current (A), and $l$ is the length (m).

No force when

• The current is parallel to the field (the wire runs along the field lines).
• The current is zero.

Common errors and how to correct them

• Using the right hand instead of the left hand (the right-hand rule is for generators, not motors).
• Using electron flow direction instead of conventional current direction.

How to apply this in an exam

Identify the field direction and current direction. Apply Fleming’s left-hand rule. State the direction of the force. If asked for a stronger force, increase current, field strength or conductor length.