GCSE Physics revision

Electric motors (HT only)

The motor effect

AQA 4.7.2.3

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AQA student objectives

Learning pathway

ALL 🎯

Explain how the force on a conductor in a magnetic field causes the rotation of the coil in an electric motor.

MOST 🎯🎯

Apply the scientific explanation of electric motors to a relevant example.

SOME 🎯🎯🎯

Analyse a new situation involving electric motors and explain the scientific reasoning.

Revision summary

Read on screen, then print for Cornell-style active revision.

When a current-carrying wire is placed in a magnetic field, it experiences a force — this is known as the motor effect.
Fleming's Left-Hand Rule is used to determine the direction of the force based on the directions of the current and the magnetic field.
If the current in a wire is reversed, the direction of the force acting on that wire is also reversed.

1. The Motor EffectAQA

When a current-carrying wire is placed in a magnetic field, it experiences a force — this is known as the motor effect.
Fleming's Left-Hand Rule is used to determine the direction of the force based on the directions of the current and the magnetic field.
If the current in a wire is reversed, the direction of the force acting on that wire is also reversed.

When a rectangular coil carries current in a magnetic field, opposite sides of the coil experience forces in opposite directions.
If the left side of the coil experiences an upward force and the right side a downward force, the coil will begin to rotate clockwise.
After rotating 180°, the current direction in each side of the coil reverses, causing the forces to swap and the coil to rotate back — meaning it just flips back and forth without completing full rotations.

2. Forces on a Current-Carrying CoilAQA

When a rectangular coil carries current in a magnetic field, opposite sides of the coil experience forces in opposite directions.
If the left side of the coil experiences an upward force and the right side a downward force, the coil will begin to rotate clockwise.
After rotating 180°, the current direction in each side of the coil reverses, causing the forces to swap and the coil to rotate back — meaning it just flips back and forth without completing full rotations.

Without any modification, the coil would oscillate back and forth rather than continuously rotating in one direction.
To make a useful motor, the current direction must be swapped every half turn so that the forces always act in the same rotational direction.

A split-ring commutator is a device that swaps the positive and negative connections to the coil every half turn.
By swapping the connections every half turn, the direction of the current through the coil also reverses every half turn.
This ensures the forces on the coil always act in the same rotational direction, allowing the coil to spin continuously in one direction.

4. The Split-Ring CommutatorAQA

A split-ring commutator is a device that swaps the positive and negative connections to the coil every half turn.
By swapping the connections every half turn, the direction of the current through the coil also reverses every half turn.
This ensures the forces on the coil always act in the same rotational direction, allowing the coil to spin continuously in one direction.

The positive and negative terminals remain stationary while the split-ring commutator rotates with the coil, maintaining a consistent current flow direction around the coil.
The coil experiences a continuous rotational force (clockwise or anticlockwise depending on the setup), converting electrical energy into kinetic energy.
This simple but effective mechanism is used in everyday devices such as fans, vehicles, and hard drives.

5. How the Full Motor WorksAQA

The positive and negative terminals remain stationary while the split-ring commutator rotates with the coil, maintaining a consistent current flow direction around the coil.
The coil experiences a continuous rotational force (clockwise or anticlockwise depending on the setup), converting electrical energy into kinetic energy.
This simple but effective mechanism is used in everyday devices such as fans, vehicles, and hard drives.

The speed of rotation of an electric motor can be increased by increasing the current flowing through the coil.
Adding more turns to the coil increases the overall force, making the motor spin faster.
Using stronger magnets increases the magnetic flux density, which also increases the force on the coil and the speed of rotation.

6. Increasing Motor SpeedAQA

The speed of rotation of an electric motor can be increased by increasing the current flowing through the coil.
Adding more turns to the coil increases the overall force, making the motor spin faster.
Using stronger magnets increases the magnetic flux density, which also increases the force on the coil and the speed of rotation.