Electric motors (HT only) — AQA GCSE Combined Science
Test yourself on Electric motors (HT only) with AQA GCSE practice questions.
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Electric motors (HT only) explained
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.
Read the full explanation
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.
Students should be able to explain how the force on a conductor in a magnetic field causes the rotation of the coil in an electric motor.
A current-carrying wire in a magnetic field experiences a force; the direction follows Fleming's left-hand rule. In a motor, the coil sits between opposite magnetic poles, so one side carries current in one direction and the opposite side carries it in the reverse direction. Each side therefore feels a force of equal size but opposite direction, and this pair of forces produces a turning effect about the axis. Because the two forces act on opposite sides of the pivot, the coil rotates continuously. A split-ring commutator reverses the current every half turn, so the forces swap direction and rotation continues. This is Higher Tier only content.
Your focus
- Describe how forces on a current-carrying coil in a magnetic field produce rotation.
- Explain the function of the split-ring commutator in an electric motor.
- Relate changes in current, number of turns and magnetic flux density to the turning effect on the coil.
Show all 6 objectives
- Describe the force on a current-carrying conductor in a magnetic field and state its direction using Fleming's left-hand rule.
- Explain how opposite forces on the two sides of a coil produce rotation about its axis.
- Explain the role of the split-ring commutator in maintaining continuous rotation in the same direction.
Electric motors (HT only) exam tips
Marking 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.
- Explain how increasing current, turns or magnetic flux density increases the turning effect on the coil.
- A current-carrying conductor placed in a magnetic field experiences a force; this is the motor effect.
- The direction of the force is given by Fleming's left-hand rule: first finger field, second finger current, thumb force (motion).
- In a coil, the two sides carrying current in opposite directions experience forces in opposite directions.
- The two opposite forces act at a distance from the axis, producing a turning effect (moment) that rotates the coil.
- The split-ring commutator reverses the current in the coil every half rotation so the coil continues turning in the same direction.
- Increasing current, magnetic flux density or number of turns in the coil increases the size of the force and so the turning effect.
Examiner 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.
- 💡Sketch the coil between N and S poles, label the current direction on each side and draw the force arrows in opposite directions.
- 💡Use the phrase 'forces act on opposite sides of the axis, producing a turning effect' to link force to rotation.
- 💡As this is Higher Tier only, expect questions to require a full cause-and-effect explanation of the rotation.
Common Mistakes
- Saying the coil rotates because it is attracted to one magnetic pole; correct this by explaining the forces on opposite sides act in opposite directions to create a turning effect.
- Forgetting the commutator reverses the current; correct this by stating it swaps the current direction every half turn so rotation continues the same way.
- Confusing the motor effect with electromagnetic induction; correct this by noting that a current in a field causes motion, whereas motion in a field can induce a current.
- Thinking both sides of the coil feel force in the same direction: correct this by noting the current directions are opposite on the two sides, so the forces are opposite.
- Believing the commutator increases the current: correct this by explaining it reverses the current direction every half turn to maintain rotation.
- Confusing Fleming's left-hand rule with the right-hand rule for generators: correct this by linking the left hand to the motor effect (force) and the right hand to induction.