The Motor Effect: GCSE Physics Guide

What causes the motor effect?

A current-carrying conductor produces its own magnetic field. When this conductor is placed inside an external magnetic field, the two fields interact. On one side of the conductor the fields reinforce each other (add up); on the other side they cancel out. This unequal field creates an unequal force — the conductor is pushed towards the weaker side. The result is a net force on the conductor perpendicular to both the current direction and the field direction.

This is not the same as electrostatic attraction. The force arises purely from the interaction of two magnetic fields — the external field and the field created by the current.

How do you predict the direction of force? (Fleming's left-hand rule)

Fleming's left-hand rule predicts the direction of the force on a conductor in a magnetic field:

Hold your left hand with:

• First finger (index finger) pointing in the direction of the magnetic Field (north to south)
• Second finger (middle finger) pointing in the direction of conventional Current (positive to negative)
• Thumb pointing in the direction of the Motion (force) on the conductor

A common memory aid: First finger = Field; seCond finger = Current; thuMb = Motion.

What factors affect the size of the force?

The force (F) on a current-carrying conductor in a magnetic field depends on three variables:

F = B × I × L

where:

• F = force in newtons (N)
• B = magnetic flux density in tesla (T) — how strong the magnetic field is
• I = current in amperes (A)
• L = length of conductor in the magnetic field in metres (m)

Worked example

A wire carrying a current of 3 A is placed at right angles to a magnetic field of 0.05 T. The length of the wire in the field is 0.2 m.

F = B × I × L = 0.05 × 3 × 0.2 = 0.03 N

How does an electric motor work?

A simple electric motor uses the motor effect to produce continuous rotation:

1. A rectangular coil of wire is placed between the poles of a permanent magnet.
2. Current flows through the coil from a power supply.
3. The motor effect produces an upward force on one side of the coil and a downward force on the other side. These forces form a turning effect (couple) that rotates the coil.
4. A split-ring commutator reverses the direction of current every half-turn, ensuring the forces always act in the same rotational direction, so the coil keeps spinning rather than oscillating.
5. Brushes (carbon contacts) maintain an electrical connection with the rotating commutator.

Without the split-ring commutator, the coil would rotate half a turn and then be pushed back — the commutator is what converts alternating push into continuous rotation.

What are the applications of the motor effect?

The motor effect is the operating principle behind:

• Electric motors in fans, trains, electric vehicles, washing machines and every other device requiring rotational force
• Loudspeakers: current through a coil attached to a speaker cone causes it to vibrate back and forth, producing sound waves
• Galvanometers and ammeters: the deflection of a pointer is proportional to the current through a coil in a magnetic field

Frequently asked questions

Why does Fleming's left-hand rule use the LEFT hand and not the right?

By convention, Fleming's left-hand rule applies to conventional current direction (the direction positive charges would flow, from + to −). The right hand is used for the generator effect (electromagnetic induction — current induced by a moving conductor in a field). Using different hands avoids confusion between the two situations. If you accidentally use the right hand for the motor effect, you will predict the force direction in the opposite direction.

Why is the force on the conductor perpendicular to both the current and the field?

This follows from the physics of magnetic force on a moving charge. When charges moving in one direction encounter a magnetic field in another direction, the force they experience is described by the cross product of the velocity and field vectors — always perpendicular to both. This is a mathematical consequence of how magnetic fields interact with moving charges, not an empirical rule. At GCSE you need to know the result (perpendicular force) and apply it with Fleming's rule, without needing the vector cross product.

What would happen to an electric motor if there were no split-ring commutator?

Without a commutator, the current through the coil would not change direction as the coil rotates. After rotating 90° to the vertical position, the forces on both sides of the coil would point towards the same axis (upward or downward) — no net turning effect. By 180°, the forces would try to push the coil back. The coil would oscillate through 90° and stop — a useless design. The split-ring commutator solves this by swapping the current direction every half-turn, so the coil always experiences a net turning force in the same direction.

How does increasing the current improve a motor's performance?

Greater current means a greater force on each side of the coil (F = BIL, so increasing I increases F). A greater force produces a greater turning effect (moment), which means the motor can exert more torque — useful for lifting heavy loads or accelerating quickly. Electric motors in vehicles and industrial machines use very high currents (and strong permanent magnets or electromagnets) to achieve the large forces needed. Power losses in the resistance of the coil wire limit how much current can flow before overheating becomes a problem.

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