Electricity and magnetism · GCSE Physics
The motor effect
Teacher-written GCSE Physics revision on the motor effect: a current at right angles to a magnetic field feels a force, F = BIl on Higher, Fleming’s left-hand rule, and how a d.c. motor keeps turning.
A wire carrying current in a magnetic field feels a force if the current and field are not parallel. Fleming’s left hand: thuMb motion, First finger field, seCond finger current.
The important bits
What you need to know
- 1
The motor effect: a current-carrying conductor in a magnetic field experiences a force when the current is not parallel to the field. Maximum force when they are at 90°.
- 2
On Higher tier, F = BIl when current and field are at right angles. F is in newtons, B (magnetic flux density) in tesla (T), I in amperes, l in metres (length in the field).
- 3
Fleming’s left-hand rule gives the direction: thumb = motion (force), first finger = field (N to S), second finger = conventional current.
- 4
Reversing either the current or the field reverses the force. Reversing both leaves the force direction the same.
- 5
A coil in a magnetic field becomes a motor: forces on opposite sides are opposite, so there is a moment and the coil rotates.
- 6
A split-ring commutator in a d.c. motor reverses the current each half-turn so the coil keeps rotating in the same sense instead of stalling at the vertical.
- 7
Loudspeakers use the motor effect: an alternating current in a coil in a magnetic field makes the cone vibrate and produce sound.
- 8
Increase the force with a larger current, a stronger field (stronger magnets or more flux density), or a longer wire in the field (more turns on a coil).
Quotations worth analysing
Short evidence. Real method.
“F = BIl”
Only when B and I are perpendicular. Length l is the length in the field, in metres, not the whole reel of wire on the bench.
“Fleming’s left-hand rule: motion, field, current.”
Left hand is the motor. Right hand is the generator (Physics). Mixing the hands flips the predicted force.
Go deeper
Motors are two forces and a commutator
Each side of the coil that sits at right angles to the field gets a force. One side is pushed up, the other down, so the coil turns. When the coil is vertical the forces try to tear it apart rather than turn it, and without help it would stop. The split-ring commutator swaps the contacts, reversing the current in the coil, so the forces swap and the coil is pushed onward through the next half-turn. Brushes slide on the split ring. Increase speed or turning effect with more current, more turns, or a stronger magnet. Students describe a motor as “magnetism makes it go” and miss every labelled part. Draw coil, magnets, brushes and split ring, then apply Fleming’s left hand to one side only — the other side follows.
Go deeper
F = BIl is a substitution, not a slogan
Magnetic flux density B in tesla tells you how strong the field is: 1 T is a large laboratory field; the Earth’s field is about 5 × 10⁻⁵ T. A 0.050 m wire carrying 3.0 A at right angles to 0.20 T feels F = 0.20 × 3.0 × 0.050 = 0.030 N. If the wire is parallel to the field, the force is zero — there is no motor effect to use. If the angle is between, Combined usually avoids the sine; Higher may mention that only the perpendicular component counts. Units: N, T, A, m. An answer in newtons that is thousands of times too big usually means centimetres were not converted. Write l = 5.0 cm = 0.050 m on the line before you multiply.
See the idea in action
A wire of length 0.050 m carries 3.0 A at right angles to a 0.20 T field. Force F = BIl = 0.20 × 3.0 × 0.050 = 0.030 N. Direction from Fleming’s left hand: if the field is left to right (first finger) and the current is into the page (second finger), the thumb gives the force direction — mark the diagram rather than guessing. If this wire is one side of a coil, that force provides a moment and the coil rotates until the commutator reverses the current. Doubling B, I or l would double the force to 0.060 N.
Exam technique
Turn knowledge into marks
Label field N to S, current direction, and force direction. Quote F = BIl with l in metres on Higher. For motors, name the split-ring commutator and why the current must reverse each half-turn.
Common mistakes
Do not give these marks away
- 01
Using the right-hand rule for the motor effect (left hand is force on a current in a field).
- 02
Leaving length in centimetres in F = BIl, or using F = BIl when current is parallel to the field.
- 03
Forgetting the commutator, so the coil would rock and stop instead of rotating continuously on d.c.
A 0.10 m wire carries 2.0 A at right angles to a 0.50 T magnetic field. What is the force on the wire?
A0.010 N
B0.10 N
C1.0 N
D10 N
Show the answer
0.10 N. F = BIl = 0.50 × 2.0 × 0.10 = 0.10 N. 1.0 N would be a factor-of-ten error in length; 10 N ignored the 0.10 m entirely.
Quick questions
If this is the bit you searched
What is Fleming’s left-hand rule?
A way to find the direction of the force on a current-carrying conductor in a magnetic field: thumb = motion, first finger = field, second finger = current.
What does a split-ring commutator do?
It reverses the current in the coil of a d.c. motor every half-turn so the coil keeps rotating in the same direction.
How do you increase the force in the motor effect?
Increase the current, increase the magnetic flux density, or increase the length of wire in the field (including more turns).
Do loudspeakers use the motor effect?
Yes. An a.c. signal in a coil in a magnetic field produces a changing force, so the cone vibrates and produces sound waves.