AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
A coil of wire carrying a current in a magnetic field tends to rotate.…
Full specification wording
Electric motors (HT only): A coil of wire carrying a current in a magnetic field tends to rotate. This is the basis of an electric motor.
When a current flows through a coil placed in a magnetic field, each side of the coil experiences a force because the current is at an angle to the field.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 1 of 2
Study notes
From the checked teaching notes for this specification point.
The idea
When a current flows through a coil placed in a magnetic field, each side of the coil experiences a force because the current is at an angle to the field. The forces on opposite sides act in opposite directions, creating a turning effect that makes the coil rotate. This is the motor effect in action and is the basis of an electric motor. In a simple motor, a split-ring commutator reverses the current every half turn so the coil keeps rotating in the same direction. Increasing the current, using more turns on the coil, or increasing the magnetic flux density increases the turning effect. This topic is assessed at Higher Tier only.
What earns the marks
- Describe that a current-carrying coil in a magnetic field experiences forces on its sides.
- Explain that the forces on opposite sides act in opposite directions, producing a turning effect.
- State that the coil rotates because of this turning effect, which is the basis of an electric motor.
- Describe the role of the split-ring commutator in reversing the current every half turn to maintain rotation in one direction.
- Explain how increasing current, turns or magnetic flux density increases the turning effect on the coil.
Common mistakes and how to put them right
Saying the coil rotates because it is attracted to one magnetic pole.
Put it right by explaining the forces on opposite sides act in opposite directions to create a turning effect.
Forgetting the commutator reverses the current.
Put it right by stating it swaps the current direction every half turn so rotation continues the same way.
Confusing the motor effect with electromagnetic induction.
Put it right by noting that a current in a field causes motion, whereas motion in a field can induce a current.
Exam tips
- Use the terms force, turning effect and rotation when describing how the coil moves.
- Link each feature of the motor, such as the commutator, to its function in keeping the coil turning.
- Remember that the detailed explanation of electric motors is assessed at Higher Tier only.
Try the lesson
The start of the Electric motors (HT only): A coil of wire carrying a current in a magnetic field tends to rotate. This is the basis of an electric motor. lesson, made from checked teaching notes for this specification point.
Have a go first
What happens to a coil of wire that carries a current when it is placed in a magnetic field?
Listen
When a current flows through a coil placed in a magnetic field, each side of the coil experiences a force because the current is at an angle to the field. The forces on opposite sides act in opposite directions, creating a turning effect that makes the coil rotate. This is the motor effect in action and is the basis of an electric motor. In a simple motor, a split-ring commutator reverses the current every half turn so the coil keeps rotating in the same direction. Increasing the current, using more turns on the coil, or increasing the magnetic flux density increases the turning effect. This topic is assessed at Higher Tier only. A strong answer earns marks for these points. Describe that a current-carrying coil in a magnetic field experiences forces on its sides. Explain that the forces on opposite sides act in opposite directions, producing a turning effect. State that the coil rotates because of this turning effect, which is the basis of an electric motor. Describe the role of the split-ring commutator in reversing the current every half turn to maintain rotation in one direction. A common slip: Saying the coil rotates because it is attracted to one magnetic pole. Instead, explaining the forces on opposite sides act in opposite directions to create a turning effect. Exam tip: Use the terms force, turning effect and rotation when describing how the coil moves.
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