The Motor Effect and the D.C. Motor
A wire carrying a current in a magnetic field experiences a force. That single fact turns electrical energy into movement, and it powers every motor.
1. The motor effect
When a current-carrying conductor is placed in a magnetic field, it experiences a force.
Why it happens:
The current creates its own magnetic field around the wire. This interacts with the permanent magnetic field — the two fields reinforce on one side and cancel on the other, pushing the wire towards the weaker side.
Mention the interaction between the wire’s own magnetic field and the permanent field. Tutors flagged this specifically as the point that earns full marks — “the wire experiences a force” alone is a description, not an explanation.
No force occurs when the current is parallel to the magnetic field. The force is greatest when the current is at 90° to the field.
2. Fleming’s left-hand rule
Use the LEFT hand for MOTORS (finding the force from a current).
Hold the left hand with thumb, first finger and second finger mutually perpendicular:
| Digit | Represents |
|---|---|
| First finger | Field (N → S) |
| seCond finger | Current (conventional: + → −) |
| thuMb | Motion / force |
Left hand = motor; right hand = generator. Confusing the two was recorded repeatedly, and it reverses every answer. The mnemonic that works: L for Left and for the motor’s eLectrical input.
Use CONVENTIONAL current — positive to negative. A recorded error used the direction of electron flow, which is the opposite way and gives the wrong force direction.
Mark the known field and current directions on the diagram before applying the rule. Tutors recommended this, and it prevents most sign errors.
To reverse the force: reverse the current, or reverse the magnetic field. Reversing both leaves the force unchanged.
To increase the force: increase the current, use a stronger magnetic field, or use a longer length of wire in the field.
3. The d.c. motor
Construction: a coil of wire in a magnetic field, connected to a d.c. supply through a split-ring commutator and carbon brushes.
How it turns:
- Current flows through the coil, which sits in the magnetic field
- The two sides of the coil carry current in opposite directions, so (by Fleming’s left-hand rule) they experience forces in opposite directions — one up, one down
- This pair of forces creates a turning effect (moment), rotating the coil
- After half a turn, the split-ring commutator reverses the current in the coil
- So the forces on each side stay in the same rotational sense, and the coil keeps turning continuously in one direction
The commutator’s job is to REVERSE THE CURRENT every half turn, which keeps the rotation going the same way. Without it, the coil would turn half a turn and then be pushed back. Misunderstanding the commutator’s role was recorded.
Split-ring commutator → d.c. motor. Slip rings → a.c. generator. Keep these apart.
The brushes are carbon and press against the commutator, maintaining electrical contact while allowing rotation.
When the coil is vertical (parallel to the field), the turning effect is momentarily zero — but momentum carries it past this position, and the commutator has by then swapped the connections.
The coil doesn’t stop when parallel to the magnets. A recorded error described the motion as stopping; in practice momentum carries it through the dead point.
To make the motor turn faster: increase the current, use a stronger magnet, add more turns to the coil, or use a larger coil area.
Write “more turns on the coil” and “a stronger magnet” — the precise wording, as with induction.
To reverse the direction of rotation: reverse the current (swap the supply connections) or reverse the magnetic field (swap the magnets).
4. Why the current falls as a motor speeds up
A subtle point that came up in lessons:
As the motor spins faster, the coil cuts field lines more rapidly, inducing a back e.m.f. that opposes the supply voltage (Lenz’s law). This reduces the net voltage across the coil, so the current falls.
A recorded confusion expected the current to increase with speed. It does the opposite — which is also why motors draw a large current at start-up, when there is no back e.m.f. yet.
5. Motor effect vs generator effect
| Motor effect | Generator effect | |
|---|---|---|
| Input | current | movement |
| Output | movement | current / e.m.f. |
| Hand rule | LEFT | RIGHT |
| Rings | split-ring commutator | slip rings |
| Energy | electrical → kinetic | kinetic → electrical |
Confusing these two was one of the most-recorded errors in the electromagnetism topics. Ask what the question supplies: a battery means motor; a rotation means generator.
6. Other applications
- Loudspeaker — a varying current in a coil, in a magnetic field, makes the coil and cone vibrate, producing sound
- Relay — a small current operates an electromagnet that switches a larger circuit
7. Mistakes that cost marks
Using the right hand for a motor.
Using electron flow instead of conventional current.
Not mentioning the interaction of the two magnetic fields.
Misunderstanding the commutator’s role.
Confusing the commutator with slip rings.
Saying the coil stops when parallel to the field.
Saying current increases as the motor speeds up.
Vague wording like “more coils” or “bigger magnet”.
Confusing the motor effect with the generator effect.
Frequently asked questions
What is the motor effect? A current-carrying conductor in a magnetic field experiences a force.
Why does the force occur? The wire’s own magnetic field interacts with the permanent field, reinforcing on one side and cancelling on the other.
Which hand rule do I use? Fleming’s LEFT-hand rule for motors.
What do the digits represent? First finger = Field, seCond = Current, thuMb = Motion.
Which current direction do I use? Conventional current — positive to negative.
How do I reverse the force? Reverse the current or the magnetic field — but not both.
What does the split-ring commutator do? Reverses the current in the coil every half turn, keeping rotation in one direction.
What’s the difference between a commutator and slip rings? A commutator gives d.c. rotation in a motor; slip rings give a.c. output in a generator.
How can I make a motor turn faster? More current, a stronger magnet, more turns, or a larger coil.
Why does the current fall as a motor speeds up? A back e.m.f. is induced that opposes the supply voltage.
Quick revision checklist
- I can state the motor effect
- I can explain it via interacting magnetic fields
- I know the force is zero when current is parallel to the field
- I use Fleming’s LEFT-hand rule for motors
- I know what each digit represents
- I use conventional current
- I know how to reverse and how to increase the force
- I can describe the d.c. motor in full
- I know the commutator reverses the current each half turn
- I know the brushes are carbon
- I can list four ways to speed up a motor, in the right words
- I can explain the back e.m.f. effect
- I can distinguish the motor and generator effects
These notes cover the motor effect and the d.c. motor in the Cambridge IGCSE Physics (0625) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Physics lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus and formula list for your own exam series.
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