AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Students should be able to show that Fleming's left-hand rule represents…
Full specification wording
Fleming's left-hand rule (HT only): Students should be able to show that Fleming's left-hand rule represents the relative orientation of the force, the current in the conductor and the magnetic field.
Fleming's left-hand rule is a memory aid for the motor effect.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 2 of 10
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Fleming's left-hand rule is a memory aid for the motor effect. Hold the left hand with the thumb, first finger and second finger mutually at right angles. Point the first finger along the external magnetic field, from north to south. Point the second finger in the direction of conventional current, from positive to negative. The thumb then points in the direction of the force on the conductor. For example, if the field points left to right and the current points vertically upwards, the thumb reveals the force direction is into the page. The rule shows relative orientation only. Reversing either the current or the field reverses the force, while reversing both leaves it unchanged.
What earns the marks
- States that the first finger represents the direction of the external magnetic field, conventionally from north to south.
- States that the second finger represents the direction of conventional current in the conductor.
- States that the thumb represents the direction of the force acting on the conductor.
- Explains that the three directions are mutually perpendicular, so the rule gives relative orientation rather than magnitude.
- Applies the rule to a described arrangement, for example identifying the force direction when field and current directions are given.
- Recognises that reversing the current or the magnetic field reverses the force, whereas reversing both leaves the force direction unchanged.
Common mistakes and how to put them right
Using the right hand instead of the left hand.
Put it right: Is to reserve the left hand for motor-effect force questions and the right hand for generator-effect questions.
Confusing which finger represents current and which represents field.
Put it right: Is to remember first finger field, second finger current, thumb thrust or force.
Treating the rule as giving the size of the force.
Put it right: Is to use the rule only for direction and use F = B I l for magnitude when the conductor is perpendicular to the field.
Exam tips
- Sketch a simple hand diagram and label first finger field, second finger current and thumb force before substituting values.
- Check the current direction carefully: conventional current flows from positive to negative, opposite to electron flow.
- When a question reverses a direction, state the effect on the force explicitly rather than redrawing the whole arrangement.
Try the lesson
The start of the Fleming's left-hand rule (HT only): Students should be able to show that Fleming's left-hand rule represents the relative orientation of the force, the current in the conductor and the magnetic field. lesson, made from checked teaching notes for this specification point.
Have a go first
In Fleming's left-hand rule, which part of the hand points in the direction of the force on the conductor?
Listen
Fleming's left-hand rule is a memory aid for the motor effect. Hold the left hand with the thumb, first finger and second finger mutually at right angles. Point the first finger along the external magnetic field, from north to south. Point the second finger in the direction of conventional current, from positive to negative. The thumb then points in the direction of the force on the conductor. For example, if the field points left to right and the current points vertically upwards, the thumb reveals the force direction is into the page. The rule shows relative orientation only. Reversing either the current or the field reverses the force, while reversing both leaves it unchanged. A strong answer earns marks for these points. States that the first finger represents the direction of the external magnetic field, conventionally from north to south. States that the second finger represents the direction of conventional current in the conductor. States that the thumb represents the direction of the force acting on the conductor. Explains that the three directions are mutually perpendicular, so the rule gives relative orientation rather than magnitude. A common slip: Using the right hand instead of the left hand: the error gives a reversed force direction. Instead, is to reserve the left hand for motor-effect force questions and the right hand for generator-effect questions. Exam tip: Sketch a simple hand diagram and label first finger field, second finger current and thumb force before substituting values.
See it
Key words
Say what each word means before you turn the card.
Motor effect
Try answering aloud or in your head before turning the card.
Magnetic field direction
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Conventional current
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Vector direction and relative orientation
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See every definition
- Motor effect
- The effect for which Fleming's left-hand rule is a memory aid, giving the direction of the force on a conductor.
- Magnetic field direction
- The direction the first finger points, along the external magnetic field, from north to south.
- Conventional current
- The direction the second finger points, from positive to negative.
- Vector direction and relative orientation
- The rule shows relative orientation only, as the three directions are mutually perpendicular, rather than magnitude.
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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