Electric Motors

This lesson explains how the motor effect is used to make a DC electric motor spin, including the role of the split-ring commutator, as required by the Edexcel GCSE Combined Science specification (1SC0).

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Structure of a Simple DC Motor

A simple DC motor has the following components:

Component	Role
Rectangular coil of wire	Carries the current; experiences a force in the magnetic field
Permanent magnets	Provide the external magnetic field (N and S poles)
Split-ring commutator	Two half-rings that reverse the current direction every half turn
Brushes (usually carbon)	Stationary contacts that press against the commutator and connect to the power supply
Axle	The coil (and commutator) rotate around this central shaft

graph LR
    subgraph "DC Motor — Side View"
    N["N pole"] --- Coil["Rectangular coil on axle"]
    Coil --- S["S pole"]
    Coil --- Comm["Split-ring commutator"]
    Comm --- Brush["Carbon brushes → to power supply"]
    end

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How the Motor Works

Step-by-Step

1. Current flows through the coil from the power supply, via the brushes and commutator.
2. One side of the coil has current flowing up; the other side has current flowing down (because it is a loop).
3. By Fleming's left-hand rule, each side experiences a force — but the forces act in opposite directions (one up, one down).
4. These two opposite forces create a turning effect (couple), making the coil rotate.
5. When the coil reaches the vertical position, the two halves of the split-ring commutator swap contacts with the brushes.
6. This reversal of current direction ensures the forces continue to push the coil in the same rotational direction.
7. The coil completes a full rotation and the cycle repeats — continuous rotation.

Why the Commutator Is Essential

Without the commutator, the current direction in the coil would not reverse at the vertical position. The forces would then reverse every half turn, and the coil would simply oscillate (rock back and forth) instead of rotating continuously.

Exam Tip: A very common 4–6 mark question asks you to explain how a DC motor works. You must mention: the motor effect (force on a current-carrying conductor in a magnetic field), Fleming's left-hand rule, the opposite forces creating a turning effect, and the role of the split-ring commutator in reversing the current to maintain rotation.

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Increasing the Speed or Force of the Motor

Change	Effect
Increase the current	Greater force on the coil → faster rotation
Use a stronger magnet (increase B)	Greater force → faster rotation
Add more turns to the coil	Each turn experiences a force → greater total turning effect

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Increasing the Motor's Practical Performance

In real motors, several design improvements are used:

Improvement	Benefit
Curved pole pieces (concave magnets)	Ensure the magnetic field is always radial (perpendicular to the coil), giving a constant force throughout rotation
Laminated iron core inside the coil	Concentrates the field and increases the turning effect
Multiple coils at different angles	Provide a smooth, continuous turning force (no dead spots)

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Worked Example

A student builds a simple DC motor but the coil only oscillates back and forth. Suggest why, and how to fix it.

The commutator is likely not working correctly — either it is not a split-ring type or the brushes are not making proper contact. The purpose of the split-ring commutator is to reverse the current direction every half turn so that the forces always act in the same rotational direction. Replacing or adjusting the commutator should allow continuous rotation.

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DC Motor vs AC Motor (Overview)

Feature	DC Motor	AC Motor
Power supply	Direct current	Alternating current
Commutator	Split-ring	Not needed (AC reverses automatically)
Common use	Small devices, toys	Industrial machines, fans

Exam Tip: For Combined Science you mainly need to know the DC motor. AC motors are mentioned for awareness but detailed AC motor operation is not required.

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Common Misconceptions

Misconception	Correction
The whole coil experiences a single force	The two sides of the coil experience forces in opposite directions, creating a couple (turning effect)
The motor works without a commutator	Without a commutator the coil would oscillate, not rotate continuously
The brushes rotate with the coil	The brushes are stationary; the commutator rotates

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Summary

• A DC motor uses the motor effect to convert electrical energy into rotational kinetic energy.
• The coil is in a magnetic field; current through the coil causes opposite forces on each side → a turning effect.
• The split-ring commutator reverses the current every half turn, ensuring continuous rotation.
• Speed/force is increased by increasing the current, using a stronger magnet or adding more turns.
• Carbon brushes are stationary contacts that connect the rotating commutator to the external circuit.

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Deeper Dive: The Turning Effect

Consider a rectangular coil with sides labelled AB and CD, carrying a current. Side AB has current flowing (say) towards you; side CD has current flowing away from you. Both sides sit in the same magnetic field B (from N to S across the coil). Using Fleming's left-hand rule on each side:

Side	Current Direction	Force Direction
AB	Towards viewer	Upwards
CD	Away from viewer	Downwards

The two forces are equal in magnitude ($F = BIL$F=BIL) but act in opposite directions on opposite sides of the axle. This is a couple, which produces a turning effect (torque) about the axle. The coil rotates.

When the coil has turned through 90°, sides AB and CD briefly move parallel to the field — no more cutting of field lines, the force on each side does no useful torque about the axle. Without a commutator, the coil would simply swing back as the forces reverse direction. The split-ring commutator swaps the current direction in the coil at this critical moment, so the forces remain in the correct sense to keep pushing the coil round in the same direction.

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Worked Example: Designing a Faster Motor

A student has built a DC motor that turns at 200 revolutions per minute using a 2.0 V battery, a single-turn coil and a weak bar magnet. Suggest three distinct changes and explain how each would increase the rotational speed.