Electric Motors

An electric motor converts electrical energy into kinetic energy (rotational motion). It works by using the motor effect — a current-carrying coil in a magnetic field experiences a force that makes it rotate. This lesson covers the DC motor and its components. It maps to AQA GCSE Combined Science Trilogy (8464) specification section 6.7.2.

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The Simple DC Motor

A simple DC (direct current) motor consists of:

graph TD
    subgraph "DC Motor Components"
        A["Rectangular coil of wire"] --- B["Permanent magnets (N and S poles)"]
        B --- C["Split-ring commutator"]
        C --- D["Carbon brushes"]
        D --- E["DC power supply"]
    end

Component	Function
Rectangular coil	Carries the current in the magnetic field; experiences the motor effect force
Permanent magnets	Provide the external magnetic field
Split-ring commutator	Two half-rings that reverse the current direction every half turn
Carbon brushes	Maintain electrical contact between the external circuit and the spinning commutator
DC power supply	Provides the current

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How the DC Motor Works

Step-by-Step

1. Current flows through the coil via the brushes and commutator.
2. The two sides of the coil carry current in opposite directions (one side up, one side down).
3. By the motor effect, each side experiences a force — but the forces are in opposite directions (because the current direction is opposite on each side).
4. These opposite forces create a couple (turning effect), causing the coil to rotate.
5. When the coil reaches the vertical position (perpendicular to the field), the two halves of the split-ring commutator swap contacts with the brushes.
6. This reverses the current direction in the coil, so the forces reverse and the coil continues to spin in the same direction.
7. The coil's momentum (inertia) carries it past the vertical position where the force is momentarily zero.

Why Is the Commutator Essential?

Without the split-ring commutator, the coil would rotate half a turn and then stop (or oscillate back and forth). The commutator reverses the current every half turn, ensuring the coil always spins in the same direction.

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Increasing the Speed of Rotation

Change	Effect
Increase the current	Greater force → faster rotation
Use a stronger magnet	Greater force → faster rotation
Increase the number of turns on the coil	Greater total force → faster rotation
Reverse the current	Coil spins in the opposite direction
Reverse the magnetic field	Coil spins in the opposite direction
Reverse BOTH current and field	Coil spins in the same direction (two reversals cancel)

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When Is the Force Greatest?

Coil Position	Force on Coil	Explanation
Horizontal (parallel to field)	Maximum	The sides of the coil are at right angles to the field
Vertical (perpendicular to field)	Zero	The sides of the coil are parallel to the field lines

Exam Tip: The coil continues past the zero-force vertical position because of its momentum/inertia. This is a common AQA exam question — always mention inertia or momentum.

graph LR
    subgraph "Motor Coil Rotation"
        H["Horizontal coil — maximum force"] --> V["Vertical coil — zero force (commutator swaps)"]
        V --> H2["Horizontal again — maximum force (current reversed)"]
        H2 --> V2["Vertical again — zero force (commutator swaps)"]
        V2 --> H
    end

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Applications of Electric Motors

Electric motors are used in a huge range of everyday devices:

• Washing machines
• Electric fans
• Electric vehicles
• Vacuum cleaners
• Hard disk drives
• Power tools (drills, saws)
• Lifts (elevators)

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Worked Example

Q: A student builds a simple DC motor using a coil of 20 turns in a magnetic field. The motor is slow. Suggest two ways to make the motor spin faster.

A:

1. Increase the current through the coil — this increases the force on each side of the coil (since $F = BIl$F=BIl), producing a greater turning effect.
2. Use stronger magnets — this increases the magnetic flux density ($B$B), which increases the force on the coil, making it spin faster.

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Common Mistakes

Mistake	Correction
"The commutator increases the speed"	The commutator reverses the current — it doesn't increase speed
"The coil stops at the vertical position"	Momentum carries it past the vertical position
"Both sides of the coil experience force in the same direction"	The forces are in opposite directions — this creates the turning effect
Confusing the commutator with slip rings	Split-ring commutator (DC motor) reverses current; slip rings (AC generator) maintain continuous connection

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Motor vs Generator — Preview

Feature	DC Motor	AC Generator
Energy conversion	Electrical → kinetic	Kinetic → electrical
Input	Current (electricity)	Rotational motion
Output	Rotation (movement)	Alternating current
Contact mechanism	Split-ring commutator	Slip rings
Principle	Motor effect	Electromagnetic induction

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Summary

• A DC motor converts electrical energy into kinetic energy using the motor effect.
• The coil experiences opposite forces on each side, creating a turning effect (couple).
• The split-ring commutator reverses the current every half turn so the coil keeps spinning.
• Carbon brushes maintain electrical contact with the spinning commutator.
• Speed increases with more current, stronger magnets, or more turns.
• The force is maximum when the coil is horizontal and zero when vertical — momentum carries the coil past the zero-force position.

Exam Tip (AQA 8464): Be able to label a diagram of a DC motor, explain the function of each component, and explain why the commutator is essential. AQA commonly asks: "What would happen without the commutator?" Answer: the coil would oscillate back and forth instead of rotating continuously.

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The Split-Ring Commutator — A Detailed Walk-Through

The split-ring commutator is the single most important engineering component in a DC motor. Without it, a DC motor simply would not work. Here is exactly what it does:

1. The two halves of the commutator are mounted on the same axle as the coil and rotate with the coil.
2. The halves are electrically insulated from each other (a tiny gap separates them).
3. Each half is permanently connected to one end of the coil.
4. The carbon brushes — stationary contacts — press against whichever half-ring happens to be facing them.
5. Every half turn, the brushes swap which half-ring they touch, effectively swapping the connections to the coil.
6. This swap reverses the current in the coil.
7. The force on each side therefore also reverses, but because the coil has also rotated 180°, the sides have swapped positions — so the force continues to push the coil in the same direction of rotation.

Why Can't a DC Motor Use Slip Rings?

If you used continuous slip rings (as in an AC generator) with a DC supply: