AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Fleming's left-hand rule (HT only): F = B I l
This Higher Tier (HT) only equation calculates the force on a straight current-carrying conductor at right angles to a uniform magnetic field.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 6 of 10
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
This Higher Tier (HT) only equation calculates the force on a straight current-carrying conductor at right angles to a uniform magnetic field. F is force in newtons, B is magnetic flux density in tesla, I is current in amperes and l is the length of conductor inside the field in metres. The force is greatest when perpendicular to the field and zero when parallel.
For example: A 0.20 m wire carrying 3.0 A at 90° to a 0.50 T field experiences F = 0.50 × 3.0 × 0.20 = 0.30 N. The direction of this force is found using Fleming's left-hand rule: first finger along the field, second finger along the current, and thumb showing the force.
What earns the marks
- State that F is the force on the conductor in newtons (N) and that B is the magnetic flux density in tesla (T).
- Identify I as the current in amperes (A) and l as the length of the conductor within the magnetic field in metres (m).
- Substitute values into F = B I l and calculate correctly, including the unit N with the answer.
- Recognise that the equation applies when the conductor is at right angles to the magnetic field, and that the force is zero when the conductor is parallel to the field.
- Use Fleming's left-hand rule to determine the direction of the force once the magnitude has been calculated.
Common mistakes and how to put them right
Using the total length of the wire instead of only the length inside the magnetic field.
Put it right by identifying the portion of conductor between the magnetic poles.
Mixing up B and I in the equation.
Put it right by checking units, since B is in tesla and I is in amperes.
Forgetting to convert centimetres to metres before substituting.
Put it right by dividing the length in cm by 100 to obtain metres.
Exam tips
- Write the equation, then substitute values with units before calculating to reduce arithmetic slips.
- Check that the conductor is perpendicular to the field; if it is parallel, the force is zero and the equation does not apply directly.
- As this is Higher Tier only content, be prepared to combine this calculation with Fleming's left-hand rule to find both magnitude and direction.
Try the lesson
The start of the Fleming's left-hand rule (HT only): F = B I l lesson, made from checked teaching notes for this specification point.
Have a go first
A straight wire of length 0.80 m carries a current of 1.5 A at right angles to a uniform magnetic field of flux density 0.40 T. Calculate the force on the wire.
Listen
This Higher Tier (HT) only equation calculates the force on a straight current-carrying conductor at right angles to a uniform magnetic field. F is force in newtons, B is magnetic flux density in tesla, I is current in amperes and l is the length of conductor inside the field in metres. The force is greatest when perpendicular to the field and zero when parallel. For example, a 0.20 m wire carrying 3.0 A at 90° to a 0.50 T field experiences F = 0.50 × 3.0 × 0.20 = 0.30 N. The direction of this force is found using Fleming's left-hand rule: first finger along the field, second finger along the current, and thumb showing the force. A strong answer earns marks for these points. State that F is the force on the conductor in newtons (N) and that B is the magnetic flux density in tesla (T). Identify I as the current in amperes (A) and l as the length of the conductor within the magnetic field in metres (m). Substitute values into F = B I l and calculate correctly, including the unit N with the answer. Recognise that the equation applies when the conductor is at right angles to the magnetic field, and that the force is zero when the conductor is parallel to the field. A common slip: Using the total length of the wire instead of only the length inside the magnetic field. Instead, identifying the portion of conductor between the magnetic poles. Exam tip: Write the equation, then substitute values with units before calculating to reduce arithmetic slips.
See it
Key words
Say what each word means before you turn the card.
Magnetic flux density
Try answering aloud or in your head before turning the card.
Force on a current-carrying conductor
Try answering aloud or in your head before turning the card.
Current
Try answering aloud or in your head before turning the card.
Length of conductor
Try answering aloud or in your head before turning the card.
See every definition
- Magnetic flux density
- B in the equation, measured in tesla (T).
- Force on a current-carrying conductor
- F in the equation, measured in newtons (N), greatest when perpendicular to the field and zero when parallel.
- Current
- I in the equation, measured in amperes (A).
- Length of conductor
- l in the equation, the length of conductor inside the field in metres (m).
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 Fleming's left-hand rule (HT only): F = B I l
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