OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Recall that the strength of the field depends on the current and the…
Full specification wording
Magnets and magnetic fields: recall that the strength of the field depends on the current and the distance from the conductor
The magnetic field around a conductor is not the same everywhere.
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 6 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The magnetic field around a conductor is not the same everywhere. Its strength increases as the current through the conductor increases, so a larger current gives a stronger field and a greater deflection of a nearby plotting compass. Its strength decreases as the distance from the conductor increases, so the field is strongest close to the wire and weaker further away. The two factors act independently: doubling the current strengthens the field at a fixed distance, while moving further from the wire weakens it at a fixed current. A compass placed close to a high-current wire therefore deflects more than one placed far from a low-current wire. Field lines drawn closer together represent a stronger field.
What earns the marks
- States that increasing the current increases the strength of the magnetic field.
- States that increasing the distance from the conductor decreases the strength of the magnetic field.
- Explains that the field is strongest close to the conductor and weaker further away.
- Identifies both current and distance as the two factors on which field strength depends.
Common mistakes and how to put them right
Claims that a larger current weakens the field.
Put it right: State that a larger current strengthens the field.
States that field strength is the same at all distances from the wire.
Put it right: State that it decreases as distance increases.
Names only one factor, such as current, and omits distance.
Put it right: Name both current and distance.
Exam tips
- Link each factor to its effect in one clear sentence: more current means stronger field, greater distance means weaker field.
- Use the spacing of field lines to justify a statement about strength.
- Check that both variables, current and distance, appear in your answer when the question asks what field strength depends on.
Try the lesson
The start of the Magnets and magnetic fields: recall that the strength of the field depends on the current and the distance from the conductor lesson, made from checked teaching notes for this specification point.
Have a go first
How does increasing the current in a straight wire affect the strength of the magnetic field around it?
Listen
The magnetic field around a conductor is not the same everywhere. Its strength increases as the current through the conductor increases, so a larger current gives a stronger field and a greater deflection of a nearby plotting compass. Its strength decreases as the distance from the conductor increases, so the field is strongest close to the wire and weaker further away. The two factors act independently: doubling the current strengthens the field at a fixed distance, while moving further from the wire weakens it at a fixed current. A compass placed close to a high-current wire therefore deflects more than one placed far from a low-current wire. Field lines drawn closer together represent a stronger field. A strong answer earns marks for these points. States that increasing the current increases the strength of the magnetic field. States that increasing the distance from the conductor decreases the strength of the magnetic field. Explains that the field is strongest close to the conductor and weaker further away. Identifies both current and distance as the two factors on which field strength depends. Exam tip: Link each factor to its effect in one clear sentence: more current means stronger field, greater distance means weaker field.
See it
Key words
Say what each word means before you turn the card.
Field strength and current
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Field strength and distance from the conductor
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Weaker field further from the wire
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Representing strength by field-line spacing
Try answering aloud or in your head before turning the card.
See every definition
- Field strength and current
- The strength of the field increases as the current through the conductor increases, so a larger current gives a stronger field.
- Field strength and distance from the conductor
- The strength of the field decreases as the distance from the conductor increases, so the field is strongest close to the wire.
- Weaker field further from the wire
- The field is strongest close to the conductor and weaker further away, because strength decreases as distance increases.
- Representing strength by field-line spacing
- Field lines drawn closer together represent a stronger field.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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