DC Motor
Key idea: O Level electromagnetism: how a d.c. motor works using the turning effect on a current-carrying coil in a magnetic field and a split-ring commutator.
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The core idea
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Learning objectives
- State the properties of magnets
- Describe induced magnetism
- Distinguish temporary and permanent magnets
- Determine magnetic-field direction with a compass or bar magnet
- Interpret bar-magnet field patterns
- Draw the magnetic field pattern around a bar magnet and between the poles of two bar magnets
- Interpret the field pattern around a straight current-carrying wire
- Draw the magnetic field pattern around a straight current-carrying wire
- Interpret the field pattern around a current-carrying solenoid
- Draw the magnetic field pattern around a current-carrying solenoid
- Relate current magnitude and direction to magnetic field
- Describe electromagnet applications
- Describe experiments showing the force on a current-carrying conductor in a magnetic field
- Describe magnetic force on a charged-particle beam
- Predict force reversal when current or field reverses
- Use Fleming’s left-hand rule
- Explain the turning effect on a current-carrying coil
- Explain how current and turns increase the turning effect
- Describe split-ring commutator action
- Describe the effect of winding a motor coil on a soft-iron cylinder
- Deduce that a changing magnetic field can induce an e.m.f.
- Deduce that induced e.m.f. opposes the change producing it
- Deduce factors affecting induced e.m.f. magnitude
- Describe a simple a.c. generator and slip rings
- Sketch a simple a.c. generator voltage–time graph
- Describe a simple iron-cored transformer
- Apply ideal-transformer equations
- Explain cable loss and high-voltage transmission
1. Definition
A d.c. motor converts electrical energy into mechanical energy of rotation.
It works because a current-carrying coil in a magnetic field experiences forces on opposite sides that produce a turning effect.
Use the Motor, Generator & Transformer Lab to change the current and magnetic field, then predict the turning effect before checking the model.
2. Key Ideas
- Opposite sides of the coil carry currents in opposite directions → forces act in opposite directions → turning effect.
- Turning effect increases when you increase:
- number of turns on the coil
- current in the coil
- magnetic field strength
- The split-ring commutator reverses the current every half turn so the coil keeps turning in the same direction.
- A soft iron core/cylinder concentrates the magnetic field and increases the turning effect.
3. Detailed Explanations
A. Turning effect on a coil
When the coil is in a magnetic field, the two opposite sides of the coil experience forces in opposite directions (use Fleming’s left-hand rule). The force pair produces a turning effect, so the coil rotates.
B. Why a split-ring commutator is needed
If the current direction in the coil never changed, the turning effect would reverse after half a turn and the coil would not keep rotating in one direction.
The split-ring commutator swaps the coil connections every half-turn, so the current in each side reverses at the right time. This keeps the turning effect in the same rotation direction.
C. Soft iron cylinder (core)
Winding the coil on a soft iron cylinder concentrates the magnetic field in the coil region, giving a stronger turning effect. This increases the motor’s torque; the final speed also depends on the load, friction, and supply.
- Split-ring commutator: d.c. motor (keeps rotation one-way)
- Slip rings: a.c. generator (outputs alternating voltage)
4. Common Mistakes
- Confusing slip rings with a split-ring commutator.
- Forgetting to mention that the commutator reverses the current every half turn.
- Saying “soft iron is used because it is a permanent magnet” (it is used because it is easily magnetised).
- Claiming that a larger current must produce a particular increase in speed. It directly increases the magnetic force and turning effect; speed depends on the whole motor and its load.
5. Exam Tips
- In explanations, include: forces on opposite sides → turning effect → rotation.
- State what increases turning effect: turns, current, field strength (and soft iron core).
- For commutator questions: “reverses current every half turn so torque stays same direction”.
6. Worked Examples
Modelled example 1
Commutator function
Problem
Study the worked solution
Change the connections
Method
State that the split ring swaps the coil’s supply contacts every half turn.Reason
Each half of the commutator meets the opposite brush after half a rotation.Working
Half turn → coil connections swap.Reverse coil current
Method
State that current in each side of the coil reverses.Reason
The swapped connections reverse conventional current through the coil.Working
Connection swap → I_coil reverses.Maintain rotation
Method
State that the turning effect remains in the same rotational sense.Reason
Reversing current at the new coil orientation prevents torque from reversing the rotation.Working
Current reversal each half turn → continuing one-way rotation.
Guided practice 2
Increasing the turning effect
Problem
Cover current, field and repeated force pairs
Hints
Hint 1: force factors
Hint 2: coil design
View solution step by step
Increase magnetic force
Method
Increase current and use a stronger field.Reason
Both changes increase force on each active conductor side.Working
I↑ or B↑ → F_magnetic↑.Increase the number of force pairs
Method
Increase coil turns.Reason
More turns contribute more force pairs to the total turning effect.Working
Turns ↑ → total turning effect ↑.Concentrate the field
Method
Use a soft-iron core as another valid improvement.Reason
It strengthens the magnetic field through the coil region.Working
Soft iron → B in coil region increases.
Common misconception 3
Slip rings vs commutator
Learner response
Match each contact system to its current role
View solution step by step
D.c. motor
Method
Assign the split-ring commutator to the d.c. motor.Reason
It reverses coil current every half turn to maintain one rotation direction.Working
D.c. motor → split ring.A.c. generator
Method
Assign continuous slip rings to the a.c. generator.Reason
They connect each coil end continuously while the induced output reverses naturally.Working
A.c. generator → slip rings.
Examiner practice 4
If the commutator is removed
Examination question
Trace current and torque through the next half turn
View solution step by step
Track current
2 marksMethod
State that slip rings do not swap the coil connections every half turn.Reason
Each coil end stays connected to its own continuous ring.Working
After half turn: coil current has not been commutated.Track torque
1 markMethod
State that the turning effect reverses after half a rotation.Reason
The coil sides exchange positions in the field without their currents reversing.Working
Same coil current + reversed side positions → reversed torque.Predict motion
1 markMethod
State that the coil will not maintain one-way rotation and may stop or oscillate.Reason
The torque no longer consistently drives the original rotation direction.Working
No continuous unidirectional motor action.
Self-mark with the mark scheme
Compare your response with each mark point. Select a point only when your response contains that evidence.
Self-mark contacts, current, torque and motion.
Challenge 5
Reversing rotation direction
Control transfer
Reverse one left-hand-rule input at a time
Hints
Hint 1: one reversal
Hint 2: two reversals
View solution step by step
Reverse one input
Method
Reverse coil current or reverse the magnetic field.Reason
Either single reversal reverses the forces on both coil sides and hence the torque.Working
I reversed alone or B reversed alone → anticlockwise.Reverse both inputs
Method
State that rotation remains clockwise.Reason
Reversing both current and field leaves each force direction unchanged.Working
I and B both reversed → original torque direction.
7. Mind Stretchers
Mind stretcher 1: Passing the “dead spot”Extension
Why does the motor still rotate through the position where the turning effect is small?
Show Answer
The coil has momentum, so it continues moving past the position where the turning effect is small. The split-ring commutator then reverses the current at the right time so the turning effect becomes in the same rotation direction again.
Mind stretcher 2: Torque versus speedExtension
Two changes are made separately: (i) increase the current in the coil, (ii) use a stronger magnet. What can you conclude directly, and what needs more information?
Show Answer
In both cases the magnetic forces and turning effect increase. The coil has a larger angular acceleration for the same opposing load, but its eventual speed also depends on the load, friction, and electrical behaviour of the motor. Do not state a fixed speed increase from these facts alone.
8. Practice and next step
Track coil current through each half-turn in the Motor–Generator–Transformer Lab, then continue to Electromagnetic Induction.
Continue with the next resource in this course.
Course and syllabus information
- Course
- SEC G3 Physics
- Edition
- SEC G3 Physics 2027