AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
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 called the motor effect.
When a current-carrying conductor is placed in a magnetic field, the field of the magnet and the field around the conductor interact.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 1 of 10
- 2 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
When a current-carrying conductor is placed in a magnetic field, the field of the magnet and the field around the conductor interact. The result is a force on the conductor, and by Newton's third law the conductor exerts an equal and opposite force on the magnet. This is the motor effect (applicable to both tiers). The force is greatest when the conductor is at 90° to the magnetic field lines and zero when parallel. For Higher Tier students, the direction of the force is found using Fleming's left-hand rule: the thumb gives the force (motion), the first finger gives the magnetic field (north to south), and the second finger gives conventional current. Reversing current or field reverses the force.
What earns the marks
- A current-carrying conductor in a magnetic field experiences a force; this is the motor effect.
- The force arises because the magnetic field of the current interacts with the magnetic field of the magnet.
- The conductor and the magnet exert equal and opposite forces on each other, consistent with Newton's third law.
- The force is maximum when the conductor is at 90° to the magnetic field and zero when it is parallel to the field.
- (Higher Tier) Fleming's left-hand rule predicts the direction of the force: thumb = force, first finger = magnetic field, second finger = conventional current.
- Reversing the current or reversing the magnetic field reverses the direction of the force.
Common mistakes and how to put them right
Using the right hand for Fleming's left-hand rule.
Put it right: Use the left hand, with the thumb, first finger and second finger mutually perpendicular.
Confusing the direction of conventional current with electron flow.
Put it right: The second finger represents conventional current, from positive to negative.
Thinking the force acts only on the magnet or only on the conductor.
Put it right: The conductor and magnet exert equal and opposite forces on each other.
Exam tips
- State that the conductor must be at an angle to the magnetic field, with maximum force at 90°, to explain why the motor effect occurs.
- When using Fleming's left-hand rule, identify each finger's meaning before applying it to the diagram.
- Remember that reversing either the current or the magnetic field reverses the force, but reversing both leaves the force direction unchanged.
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. This is called the motor effect. lesson, made from checked teaching notes for this specification point.
Have a go first
What name is given to the effect in which a current-carrying conductor placed in a magnetic field experiences a force?
Listen
When a current-carrying conductor is placed in a magnetic field, the field of the magnet and the field around the conductor interact. The result is a force on the conductor, and by Newton's third law the conductor exerts an equal and opposite force on the magnet. This is the motor effect (applicable to both tiers). The force is greatest when the conductor is at 90° to the magnetic field lines and zero when parallel. For Higher Tier students, the direction of the force is found using Fleming's left-hand rule: the thumb gives the force (motion), the first finger gives the magnetic field (north to south), and the second finger gives conventional current. Reversing current or field reverses the force. A strong answer earns marks for these points. A current-carrying conductor in a magnetic field experiences a force; this is the motor effect. The force arises because the magnetic field of the current interacts with the magnetic field of the magnet. The conductor and the magnet exert equal and opposite forces on each other, consistent with Newton's third law. The force is maximum when the conductor is at 90° to the magnetic field and zero when it is parallel to the field. A common slip: Using the right hand for Fleming's left-hand rule. Instead, use the left hand, with the thumb, first finger and second finger mutually perpendicular. Exam tip: State that the conductor must be at an angle to the magnetic field, with maximum force at 90°, to explain why the motor effect occurs.
Key words
Say what each word means before you turn the card.
Motor effect
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Fleming's left-hand rule
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See every definition
- Motor effect
- 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.
- Fleming's left-hand rule
- The thumb gives the force (motion), the first finger gives the magnetic field (north to south), and the second finger gives conventional current.
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