AQA · GCSE · Physics
Course version: 8463 · Version from Sept 2016
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.
Examiners want a chain here, not a list.
Checked against the AQA 8463 specification (opens in a new tab) on .
- Point 3 of 3
Study notes
From the checked teaching notes for this specification point.
The idea
Examiners want a chain here, not a list. Start with the current in the coil, then say each long side is a conductor lying at right angles to the field, so each one feels a motor effect force. Use Fleming's left hand rule to say those forces are in opposite directions, one side up and the other down. Two opposite forces either side of the axle give a turning effect, so the coil rotates. Then deal with what happens as the coil passes the vertical: the split-ring commutator swaps which brush each end of the coil touches, so the current through each side reverses at the moment that side crosses over. The force on the side now rising still points upwards, so the coil carries on turning the same way instead of rocking back. Every link in that chain is usually worth a mark.
What earns the marks
- A force acting on each side of the coil because it carries a current across a magnetic field.
- The two forces acting in opposite directions, one up and one down.
- The pair of forces producing a turning effect about the axle.
- The commutator reversing the current in the coil every half revolution.
- Explaining that the reversal keeps the rotation going in the same direction.
Common mistakes and how to put them right
Writing a list of parts instead of a sequence of causes, which caps a levels answer at the lower band.
Saying the magnets attract and repel the coil, which is the electromagnet idea rather than the motor effect.
Putting the commutator reversal at the wrong moment, so that it cancels the rotation instead of maintaining it.
Using Fleming's right hand rule, which belongs to the generator effect.
Exam tips
- Label a sketch with the field direction, the current direction and the two force arrows, then write the answer from the sketch.
- Use the words force, opposite directions, turning effect and commutator; each of them tends to carry a mark.
- Check the mark allocation: a five mark explanation needs five separate linked statements.
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
A rectangular coil of wire sits in a uniform magnetic field. The coil is connected to a direct current supply via a split-ring commutator. When the current flows, the coil begins to rotate continuously. Which statement best explains why the coil rotates?
Listen
Examiners want a chain here, not a list. Start with the current in the coil, then say each long side is a conductor lying at right angles to the field, so each one feels a motor effect force. Use Fleming's left hand rule to say those forces are in opposite directions, one side up and the other down. Two opposite forces either side of the axle give a turning effect, so the coil rotates. Then deal with what happens as the coil passes the vertical: the split-ring commutator swaps which brush each end of the coil touches, so the current through each side reverses at the moment that side crosses over. The force on the side now rising still points upwards, so the coil carries on turning the same way instead of rocking back. Every link in that chain is usually worth a mark. A strong answer earns marks for these points. a force acting on each side of the coil because it carries a current across a magnetic field. the two forces acting in opposite directions, one up and one down. the pair of forces producing a turning effect about the axle. the commutator reversing the current in the coil every half revolution. Exam tip: Label a sketch with the field direction, the current direction and the two force arrows, then write the answer from the sketch.
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