AQA · GCSE · Physics
Course version: 8463 · Version from Sept 2016
When a conductor carrying a current is placed in a magnetic field the…
Full specification wording
Fleming's left-hand rule (HT only): When a conductor carrying a current is placed in a magnetic field the magnet producing the field and the conductor exert a force on each other.
This is Higher tier only.
Checked against the AQA 8463 specification (opens in a new tab) on .
- Point 1 of 5
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
This is Higher tier only. A current carrying wire produces its own magnetic field, and when it is placed in the field of a magnet the two fields interact: they reinforce on one side of the wire and partly cancel on the other, so the wire is pushed from the strong side towards the weak side. By Newton's third law the magnet is pushed back with an equal and opposite force, which is why a magnet resting on a balance shows a change in reading when the current is switched on. The force is greatest when the wire is at right angles to the field, and falls to zero when the wire lies along the field.
What earns the marks
- Stating that a force acts on a current carrying conductor placed in a magnetic field.
- Stating that the magnet experiences an equal and opposite force.
- Stating that the force is greatest when the conductor is at right angles to the field.
- Stating that no force acts when the current is parallel to the field.
Common mistakes and how to put them right
Saying only the wire experiences a force and the magnet does not.
Expecting a force when the wire is lined up along the field direction.
Explaining the effect by saying the magnet attracts the wire, as though the wire were magnetic material.
Forgetting that there is no force at all until the current is switched on.
Exam tips
- Mention both objects; the mark scheme credits the force on the magnet as well as on the wire.
- State the right angle condition whenever you describe the size of the force.
- This content is Higher tier only, so expect it in explain questions of two or three marks.
Try the lesson
The start of the Fleming's left-hand rule (HT only): When a conductor carrying a current is placed in a magnetic field the magnet producing the field and the conductor exert a force on each other. lesson, made from checked teaching notes for this specification point.
Have a go first
A student places a straight wire carrying an electric current between the north and south poles of a permanent magnet. The magnetic field from the magnet exerts a force on the wire. Which statement describes the force exerted by the wire on the permanent magnet?
Listen
This is Higher tier only. A current carrying wire produces its own magnetic field, and when it is placed in the field of a magnet the two fields interact: they reinforce on one side of the wire and partly cancel on the other, so the wire is pushed from the strong side towards the weak side. By Newton's third law the magnet is pushed back with an equal and opposite force, which is why a magnet resting on a balance shows a change in reading when the current is switched on. The force is greatest when the wire is at right angles to the field, and falls to zero when the wire lies along the field. A strong answer earns marks for these points. stating that a force acts on a current carrying conductor placed in a magnetic field. stating that the magnet experiences an equal and opposite force. stating that the force is greatest when the conductor is at right angles to the field. stating that no force acts when the current is parallel to the field. Exam tip: Mention both objects; the mark scheme credits the force on the magnet as well as on the wire.
Key words
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interaction of two fields
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force on a conductor
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equal and opposite forces
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right angles to the field
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See every definition
- interaction of two fields
- A current carrying wire produces its own magnetic field, and when placed in the field of a magnet the two fields interact, reinforcing on one side of the wire and partly cancelling on the other.
- force on a conductor
- When a conductor carrying a current is placed in a magnetic field, the magnet producing the field and the conductor exert a force on each other.
- equal and opposite forces
- By Newton's third law the magnet is pushed back with an equal and opposite force, which is why a magnet resting on a balance shows a change in reading when the current is switched on.
- right angles to the field
- The force is greatest when the wire is at right angles to the field, and falls to zero when the wire lies along the field.
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