P7.2 The Motor Effect

Current and magnetic fields

A wire carrying an electric current produces a magnetic field around it:

• The field forms concentric circles around the wire.
• Direction: use the right-hand grip rule — wrap the right hand around the wire with thumb pointing in direction of current; fingers curl in the direction of the field.

The motor effect

When a current-carrying wire is placed in an external magnetic field, the wire experiences a force — the motor effect.

This happens because the wire's own magnetic field interacts with the external field.

Conditions for maximum force: The current must be perpendicular to the external field.

No force: If current is parallel to the field, no force acts.

Fleming's Left-Hand Rule

To find the direction of the force on a current-carrying wire in a magnetic field:

• Thumb = direction of Force (motion/thrust)
• First finger = direction of Field (N→S)
• seCond finger = direction of Current (conventional, + to −)

Hold all three at 90° to each other.

"FBI" alternative: Field (first finger), Current (second finger), Thrust/Force (thumb).

Magnitude of the force

F = BIl
F = force (N)
B = magnetic flux density (T — tesla)
I = current A
l = length of wire in the field (m)

Increasing the force: Increase B (stronger magnet), increase I (more current), increase l (longer wire in field), or ensure maximum perpendicularity.

The electric motor

An electric motor uses the motor effect to convert electrical energy → kinetic energy.

Key components:

• Coil of wire (armature) — current-carrying
• Permanent magnets — provide external field
• Split-ring commutator — reverses the current direction every half-turn so the coil always rotates in the same direction
• Carbon brushes — maintain electrical contact with the rotating commutator

How it works:

1. Current flows through the coil in the magnetic field.
2. Fleming's Left-Hand Rule gives upward force on one side, downward on the other → coil rotates.
3. At 90° to field, commutator reverses current → coil continues rotating in same direction.

Increasing motor speed / force:

• Increase current
• Increase number of coil turns
• Increase magnetic field strength

Loudspeaker

A loudspeaker uses the motor effect:

• Alternating current through a coil in a magnetic field
• Coil (attached to cone) vibrates back and forth
• Produces sound waves

Common exam errors

1. Using the right hand for Fleming's Left-Hand Rule — it's the LEFT hand for motors.
2. Confusing which finger is which — thumb = force, first = field, second = current.
3. Forgetting the split-ring commutator — without it, the coil would oscillate, not rotate.