OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Show that Fleming’s left-hand rule represents the relative orientations of…
Full specification wording
Uses of magnetism: show that Fleming’s left-hand rule represents the relative orientations of the force, the current and the magnetic field (Higher tier only)
Fleming's left-hand rule is a memory aid for the motor effect: a current-carrying conductor in a magnetic field can experience a force.
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 4 of 8
- 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: a current-carrying conductor in a magnetic field can experience a force. Hold the left hand with thumb, first finger and second finger mutually at right angles. The first finger points along the magnetic field (from north to south pole), the second finger points along the conventional current, and the thumb then points along the force on the conductor. Because the three directions are perpendicular, rotating the hand rotates all three together, so the rule shows their relative orientations rather than a fixed compass direction. To show the rule works, take a known field direction and a known current direction, set the fingers accordingly, and read off the force direction; then check that reversing the current reverses the force while reversing the field also reverses it.
What earns the marks
- States that the first finger represents the magnetic field direction, from north pole to south pole.
- States that the second finger represents the conventional current direction in the conductor.
- States that the thumb represents the direction of the force on the conductor.
- Explains that the three fingers are held mutually at right angles so the rule shows relative orientations of force, current and field.
- Applies the rule to a stated field and current direction to deduce the force direction, and notes that reversing the current reverses the force.
Common mistakes and how to put them right
Uses the right hand instead of the left hand.
Put it right: Keep to the left hand for the motor effect.
Points the first finger along the current and the second along the field.
Put it right: Assign the first finger to the field and the second to the current.
Treats the thumb as the current direction.
Put it right: Reserve the thumb for the force on the conductor.
Assumes the rule gives an absolute compass bearing.
Put it right: Recognise that it gives relative orientations that rotate with the hand.
Exam tips
- Label a hand diagram with field, current and force before answering, so each finger's role is unambiguous.
- When a question reverses the current or the field, state explicitly that the force direction reverses.
- Check that the three directions you give are mutually perpendicular, since the rule depends on right angles.
Try the lesson
The start of the Uses of magnetism: show that Fleming’s left-hand rule represents the relative orientations of the force, the current and the magnetic field (Higher tier only) lesson, made from checked teaching notes for this specification point.
Have a go first
In Fleming's left-hand rule, the thumb, first finger and second finger are held mutually at right angles. Which finger points in the direction of the magnetic field?
Listen
Fleming's left-hand rule is a memory aid for the motor effect: a current-carrying conductor in a magnetic field can experience a force. Hold the left hand with thumb, first finger and second finger mutually at right angles. The first finger points along the magnetic field (from north to south pole), the second finger points along the conventional current, and the thumb then points along the force on the conductor. Because the three directions are perpendicular, rotating the hand rotates all three together, so the rule shows their relative orientations rather than a fixed compass direction. To show the rule works, take a known field direction and a known current direction, set the fingers accordingly, and read off the force direction; then check that reversing the current reverses the force while reversing the field also reverses it. A strong answer earns marks for these points. States that the first finger represents the magnetic field direction, from north pole to south pole. States that the second finger represents the conventional current direction in the conductor. States that the thumb represents the direction of the force on the conductor. Explains that the three fingers are held mutually at right angles so the rule shows relative orientations of force, current and field. Exam tip: Label a hand diagram with field, current and force before answering, so each finger's role is unambiguous.
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 flux density and field direction
Try answering aloud or in your head before turning the card.
Conventional current direction
Try answering aloud or in your head before turning the card.
Vector directions and reversals
Try answering aloud or in your head before turning the card.
See every definition
- Motor effect
- A current-carrying conductor in a magnetic field can experience a force.
- Magnetic flux density and field direction
- The first finger points along the magnetic field, from north to south pole.
- Conventional current direction
- The second finger points along the conventional current in the conductor.
- Vector directions and reversals
- Reversing the current reverses the force, and reversing the field also reverses it.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
There is more to this lesson
The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Test your understanding of Uses of magnetism: show that Fleming’s left-hand rule represents the relative orientations of the force, the current and the magnetic field (Higher tier only)
Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge