OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Recall that a change in the magnetic field around a conductor can give…
Full specification wording
Uses of magnetism: recall that a change in the magnetic field around a conductor can give rise to an induced potential difference across its ends, which could drive a current, generating a magnetic field that would oppose the original change (content found only in the physics separate science qualification) (Higher tier only)
When the magnetic field around a conductor changes, a potential difference is induced across its ends; this is electromagnetic induction.
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 6 of 8
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
When the magnetic field around a conductor changes, a potential difference is induced across its ends; this is electromagnetic induction. The change can come from moving a magnet near a coil, moving the coil in a field, or changing the current in a nearby circuit. If the conductor forms part of a complete circuit, the induced potential difference drives a current. That induced current generates its own magnetic field, and the direction of this field opposes the original change, in line with Lenz's law. So moving a magnet towards a coil produces a current whose field repels the approaching magnet, resisting the motion. A faster change or a stronger magnet induces a larger potential difference. This content appears only in the physics separate science qualification.
What earns the marks
- States that a change in the magnetic field around a conductor induces a potential difference across its ends.
- Explains that the induced potential difference can drive a current when the conductor is part of a complete circuit.
- States that the induced current generates a magnetic field that opposes the original change.
- Identifies that a faster change in magnetic field or a stronger magnet increases the induced potential difference.
Common mistakes and how to put them right
Claims that a steady magnetic field induces a potential difference.
Put it right by stating that the field around the conductor must change.
Says the induced current's field reinforces the original change.
Put it right by stating that the induced field opposes the original change.
Assumes an induced potential difference always drives a current.
Put it right by noting that a complete circuit is needed for a current to flow.
Exam tips
- Link the induced potential difference to a change in the magnetic field, not to a steady field.
- State that a complete circuit is needed for the induced potential difference to drive a current.
- Use the idea that the induced field opposes the original change to predict the direction of the induced current.
- Remember that a faster change or stronger magnet gives a larger induced potential difference.
Try the lesson
The start of the Uses of magnetism: recall that a change in the magnetic field around a conductor can give rise to an induced potential difference across its ends, which could drive a current, generating a magnetic field that would oppose the original change (content found only in the physics separate science qualification) (Higher tier only) lesson, made from checked teaching notes for this specification point.
Have a go first
A straight wire is held still inside a uniform magnetic field. The wire is then moved so that the magnetic field through it changes. What happens across the ends of the wire?
Listen
When the magnetic field around a conductor changes, a potential difference is induced across its ends; this is electromagnetic induction. The change can come from moving a magnet near a coil, moving the coil in a field, or changing the current in a nearby circuit. If the conductor forms part of a complete circuit, the induced potential difference drives a current. That induced current generates its own magnetic field, and the direction of this field opposes the original change, in line with Lenz's law. So moving a magnet towards a coil produces a current whose field repels the approaching magnet, resisting the motion. A faster change or a stronger magnet induces a larger potential difference. This content appears only in the physics separate science qualification. A strong answer earns marks for these points. States that a change in the magnetic field around a conductor induces a potential difference across its ends. Explains that the induced potential difference can drive a current when the conductor is part of a complete circuit. States that the induced current generates a magnetic field that opposes the original change. Identifies that a faster change in magnetic field or a stronger magnet increases the induced potential difference. A common slip: Claims that a steady magnetic field induces a potential difference. Instead, stating that the field around the conductor must change. Exam tip: Link the induced potential difference to a change in the magnetic field, not to a steady field.
See it
Key words
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Electromagnetic induction
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Induced potential difference from a changing magnetic field
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Induced current and its magnetic field
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Opposing the original change
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See every definition
- Electromagnetic induction
- When the magnetic field around a conductor changes, a potential difference is induced across its ends.
- Induced potential difference from a changing magnetic field
- A change in the magnetic field around a conductor gives rise to a potential difference across its ends.
- Induced current and its magnetic field
- If the conductor forms part of a complete circuit, the induced potential difference drives a current that generates its own magnetic field.
- Opposing the original change
- The direction of the induced current's magnetic field opposes the original change, in line with Lenz's law.
Label the diagram
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