The motor effect
When a current-carrying conductor is placed in a magnetic field, the magnetic field of the current interacts with the external field. The two fields combine into a stronger 'catapult' field on one side and a weaker field on the other, producing a force that pushes the conductor. This is called the motor effect.
Direction and size of the force
The direction of the force is found using Fleming's left-hand rule: hold the thumb, first finger and second finger of the left hand at right angles. The First finger points along the magnetic Field (N to S), the seCond finger along the Current, and the thuMb shows the Motion (force).
Key formula
F = B I L
F = force (N), B = magnetic flux density (T), I = current (A), L = length of conductor in the field (m). This applies when the conductor is at right angles to the field.
The force is greatest when the conductor is perpendicular to the field, and zero when it lies parallel to the field. This effect is the basis of the direct current (d.c.) motor, in which a current-carrying coil in a magnetic field turns.
Worked example
A straight wire of length 0.20 m carries a current of 3.0 A at right angles to a magnetic field of flux density 0.50 T. Find the force on the wire.
F = B I L = 0.50 × 3.0 × 0.20 = 0.30 N.
If the current is doubled to 6.0 A, the force doubles to 0.60 N.
Remember
- Fleming's LEFT-hand rule is for the motor (force) effect.
- Reversing either the current OR the field reverses the force.
- Force is maximum at 90° to the field, zero when parallel.