AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Students should be able to explain how the force on a conductor in a…
Full specification wording
Electric motors (HT 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.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 2 of 2
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
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.
What earns the marks
- 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.
Common mistakes and how to put them right
Thinking both sides of the coil feel force in the same direction.
Put it right by noting the current directions are opposite on the two sides, so the forces are opposite.
Believing the commutator increases the current.
Put it right 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.
Put it right by linking the left hand to the motor effect (force) and the right hand to induction.
Exam tips
- 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.
Try the lesson
The start of the Electric motors (HT 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. lesson, made from checked teaching notes for this specification point.
Have a go first
In a simple electric motor, what causes the coil to rotate?
Listen
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. A strong answer earns marks for these points. 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. A common slip: Thinking both sides of the coil feel force in the same direction. Instead, noting the current directions are opposite on the two sides, so the forces are opposite. Exam tip: Sketch the coil between N and S poles, label the current direction on each side and draw the force arrows in opposite directions.
Key words
Say what each word means before you turn the card.
Motor effect
Try answering aloud or in your head before turning the card.
Fleming's left-hand rule
Try answering aloud or in your head before turning the card.
Turning effect of a force pair
Try answering aloud or in your head before turning the card.
Commutator action
Try answering aloud or in your head before turning the card.
See every definition
- Motor effect
- A current-carrying conductor placed in a magnetic field experiences a force.
- Fleming's left-hand rule
- The direction of the force is given by first finger field, second finger current, thumb force (motion).
- Turning effect of a force pair
- The two opposite forces act at a distance from the axis, producing a turning effect (moment) that rotates the coil.
- Commutator action
- The split-ring commutator reverses the current in the coil every half rotation so the coil continues turning in the same direction.
There is more to this lesson
The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Test your understanding of Electric motors (HT 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.
Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge