OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Describe how a magnet and a current- carrying conductor exert a force on…
Full specification wording
Uses of magnetism: describe how a magnet and a current- carrying conductor exert a force on one another (Higher tier only)
A current-carrying conductor creates its own magnetic field around it.
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 3 of 8
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A current-carrying conductor creates its own magnetic field around it. When this field interacts with the field of a nearby magnet, the two fields combine and the conductor experiences a force; by Newton's third law the magnet experiences an equal and opposite force. The force is greatest when the conductor is at right angles to the magnetic field and is zero when it is parallel to the field. Reversing the current or reversing the magnet's poles reverses the direction of the force. This is the motor effect, exploited in loudspeakers and electric motors. This content is Higher tier only.
What earns the marks
- States that a current-carrying conductor produces a magnetic field around itself.
- Explains that the conductor's field interacts with the magnet's field to produce a force on the conductor.
- Applies Newton's third law to state that the magnet experiences an equal and opposite force.
- Describes the force as greatest when the conductor is perpendicular to the field and zero when parallel.
- Explains that reversing the current or the magnetic poles reverses the direction of the force.
Common mistakes and how to put them right
Claims the force acts only on the conductor.
Put it right: State that the magnet also feels an equal and opposite force.
Says the force is largest when the conductor is parallel to the field.
Put it right: The force is greatest at right angles.
Treats the force as independent of current direction.
Put it right: Reversing the current reverses the force.
Exam tips
- Name both fields (the conductor's field and the magnet's field) when explaining the interaction.
- Use Fleming's left-hand rule or the perpendicular/parallel rule to justify the direction and size of the force.
- Link the effect to a real device such as a motor or loudspeaker to show understanding.
Try the lesson
The start of the Uses of magnetism: describe how a magnet and a current- carrying conductor exert a force on one another (Higher tier only) lesson, made from checked teaching notes for this specification point.
Have a go first
A straight wire carrying a current is placed between the poles of a magnet so that the wire is at right angles to the magnetic field. Which statement describes the force that arises?
Listen
A current-carrying conductor creates its own magnetic field around it. When this field interacts with the field of a nearby magnet, the two fields combine and the conductor experiences a force; by Newton's third law the magnet experiences an equal and opposite force. The force is greatest when the conductor is at right angles to the magnetic field and is zero when it is parallel to the field. Reversing the current or reversing the magnet's poles reverses the direction of the force. This is the motor effect, exploited in loudspeakers and electric motors. This content is Higher tier only. A strong answer earns marks for these points. States that a current-carrying conductor produces a magnetic field around itself. Explains that the conductor's field interacts with the magnet's field to produce a force on the conductor. Applies Newton's third law to state that the magnet experiences an equal and opposite force. Describes the force as greatest when the conductor is perpendicular to the field and zero when parallel. Exam tip: Name both fields (the conductor's field and the magnet's field) when explaining the interaction.
Key words
Say what each word means before you turn the card.
Magnetic field around a current
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Motor effect
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Newton's third law
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Force direction and orientation
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See every definition
- Magnetic field around a current
- A current-carrying conductor produces a magnetic field around itself, and this field interacts with the field of a nearby magnet.
- Motor effect
- The conductor's field interacts with the magnet's field to produce a force on the conductor, and this is exploited in loudspeakers and electric motors.
- Newton's third law
- The magnet experiences an equal and opposite force to the force the conductor experiences.
- Force direction and orientation
- The force is greatest when the conductor is at right angles to the magnetic field and is zero when it is parallel to the field.
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