GCSE Physics Electromagnetism & the Motor Effect Notes
When a current-carrying wire sits in a magnetic field it feels a force — the motor effect. Fleming's left-hand rule gives the direction, and it's how electric motors turn.
Key concepts
- Motor effect
- A wire carrying a current in a magnetic field experiences a force, provided the current is not parallel to the field.
- Fleming's left-hand rule
- Thumb = force (motion), first finger = field (N to S), second finger = current. Use your left hand to find the direction of the force.
- Electric motor
- A coil in a magnetic field turns because the forces on its two sides act in opposite directions; a split-ring commutator reverses the current every half-turn to keep it spinning.
- The force is greatest when the wire is at 90° to the magnetic field.
- Reversing either the current or the field reverses the direction of the force.
- Increasing the current or the magnetic flux density increases the force.
Key formulas
Force on a conductor
F = B × I × L
force (N) = flux density (T) × current (A) × length (m).
Worked example
A 0.5 m wire carries a current of 3 A at right angles to a field of flux density 0.4 T. Find the force on it.
- 1Write the equation: F = B × I × L.
- 2Substitute: F = 0.4 × 3 × 0.5.
- 3Multiply: F = 0.6.
- 4Add the unit: F = 0.6 N.
Answer: F = 0.6 N
Common mistakes
✗ Using the right hand for Fleming's left-hand rule.
✓ The motor effect uses the LEFT hand — thumb force, first finger field, second finger current.
✗ Forgetting the wire must be at an angle to the field.
✓ No force acts if the current is parallel to the field; it is maximum at 90°.
✗ Leaving out a variable in F = BIL.
✓ Include flux density, current and length, all in SI units.
Quick check
Try these, then reveal the answers.
Which hand is used for the motor effect?Reveal answer
The left hand (Fleming's left-hand rule).
What does the second finger represent in Fleming's rule?Reveal answer
The direction of the conventional current.
Name two ways to increase the force on the wire.Reveal answer
Increase the current or increase the magnetic flux density.
Exam tips
- Practise setting up Fleming's left-hand rule quickly and neatly in the exam.
- State that the force is maximum when the wire is perpendicular to the field.
- Keep all quantities in SI units before using F = BIL.
Related practice
Practise this topic
Exam-style questions with instant Gradion feedback.
Flashcards
Quick-fire recall to lock in the key facts.
Worksheets
Printable practice you can work through offline.
Past papers
Full papers with examiner-grade marking.
Gradion
Ask for a step-by-step explanation any time.
All Physics
Every Physics topic across every exam board.
Related topics
Frequently asked questions
What is the motor effect?+
It is the force experienced by a current-carrying conductor placed in a magnetic field, as long as the current is not parallel to the field.
How does a split-ring commutator keep a motor turning?+
It reverses the direction of the current in the coil every half-turn, so the force on each side keeps pushing the coil round in the same direction.
Create a free account to practise electromagnetism & the motor effect with Gradion feedback
Try 5 questions now, get instant examiner-grade marking and ask the Gradion to explain anything. Free during our early launch period — no credit card required.