AQA · GCSE · Physics
Course version: 8463 · Version from Sept 2016
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.
Current in the two long sides of the coil runs in opposite directions, and both sides sit in the same magnetic field, so the motor effect force F = B I l pushes one side up and the other side down.
Checked against the AQA 8463 specification (opens in a new tab) on .
- Point 1 of 3
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Current in the two long sides of the coil runs in opposite directions, and both sides sit in the same magnetic field, so the motor effect force F = B I l pushes one side up and the other side down. Two equal forces acting in opposite directions on either side of the axle form a couple, and a couple is a turning effect, so the coil rotates. Fleming's left hand rule fixes which way round: first finger field, second finger current, thumb motion. The force on a side is largest when that wire lies at right angles to the field lines. As the coil turns the forces stay the same size, but the perpendicular distance from the axle shrinks, so the turning effect is greatest when the plane of the coil lies along the field and falls to zero when the coil reaches the position at right angles to it.
What earns the marks
- Stating that the current in the two sides of the coil is in opposite directions.
- A force acting on each side, in opposite directions, because of the motor effect.
- The two forces forming a couple that turns the coil about its axle.
- The force on a conductor being a maximum when it is at right angles to the magnetic field.
Common mistakes and how to put them right
Saying the whole coil is pushed sideways, because both sides are treated as carrying current the same way.
Using Fleming's right hand rule, which belongs to the generator effect, and getting the rotation backwards.
Pointing the first finger from south to north, when field lines run north to south outside the magnet.
Claiming the turning effect is the same in every position, when it falls to zero once the coil is at right angles to the field.
Exam tips
- Draw current arrows on both sides of the coil before using the left hand rule; they point opposite ways, and that is where the turning comes from.
- In an explain answer, name the rule you used and state the field direction you assumed, north to south.
- A turning effect needs two forces, so an answer that mentions only one force rarely gets past the first mark.
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. lesson, made from checked teaching notes for this specification point.
Have a go first
A horizontal coil of wire carries a direct current and is placed in a uniform magnetic field. What effect does the magnetic field have on the coil?
Listen
Current in the two long sides of the coil runs in opposite directions, and both sides sit in the same magnetic field, so the motor effect force F = B I l pushes one side up and the other side down. Two equal forces acting in opposite directions on either side of the axle form a couple, and a couple is a turning effect, so the coil rotates. Fleming's left hand rule fixes which way round: first finger field, second finger current, thumb motion. The force on a side is largest when that wire lies at right angles to the field lines. As the coil turns the forces stay the same size, but the perpendicular distance from the axle shrinks, so the turning effect is greatest when the plane of the coil lies along the field and falls to zero when the coil reaches the position at right angles to it. A strong answer earns marks for these points. stating that the current in the two sides of the coil is in opposite directions. a force acting on each side, in opposite directions, because of the motor effect. the two forces forming a couple that turns the coil about its axle. the force on a conductor being a maximum when it is at right angles to the magnetic field. Exam tip: Draw current arrows on both sides of the coil before using the left hand rule; they point opposite ways, and that is where the turning comes from.
Key words
Say what each word means before you turn the card.
motor effect
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couple on a coil
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Fleming's left hand rule
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turning effect
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See every definition
- motor effect
- The effect that gives a force on each side of a coil carrying a current in a magnetic field, where the force is F = B I l.
- couple on a coil
- Two equal forces acting in opposite directions on either side of the axle, which turns the coil about its axle.
- Fleming's left hand rule
- The rule that fixes which way round the rotation goes: first finger field, second finger current, thumb motion.
- turning effect
- What a couple produces, and it is greatest when the plane of the coil lies along the field and falls to zero when the coil is at right angles to it.
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