The Electric Motor [H]

The electric motor converts electrical energy into kinetic energy using the motor effect. In this lesson you will learn how a simple DC motor works, the role of the split-ring commutator, and how to increase the speed of a motor. This is Higher tier content from AQA GCSE Physics specification 4.7.2.

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How a Simple DC Motor Works

A simple DC motor consists of:

• A rectangular coil of wire mounted on an axle so it can rotate.
• A permanent magnet providing an external magnetic field.
• A split-ring commutator — two half-rings of copper connected to the ends of the coil.
• Carbon brushes — pressed against the commutator to maintain electrical contact as the coil rotates.
• A DC power supply providing the current.

graph TD
    subgraph "Simple DC Motor"
        PS["DC Power Supply"] --> B1["Carbon Brush (left)"]
        B1 --> SR["Split-Ring Commutator"]
        SR --> COIL["Rectangular Coil (rotates between magnets)"]
        COIL --> SR
        SR --> B2["Carbon Brush (right)"]
        B2 --> PS
        N["N pole (left magnet)"] -.->|"Magnetic field"| S["S pole (right magnet)"]
    end

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Step-by-Step Explanation of Motor Operation

Step 1: Current Flows Through the Coil

The DC power supply sends a current through the carbon brushes, through the split-ring commutator, and into the coil. The current flows in one direction on one side of the coil and in the opposite direction on the other side.

Step 2: The Motor Effect Creates a Force

Each side of the coil is a current-carrying conductor in a magnetic field. By the motor effect, a force acts on each side. Using Fleming's left-hand rule:

• The force on one side of the coil acts upward.
• The force on the other side acts downward.

These two forces create a turning effect (a couple or torque) that makes the coil rotate.

Step 3: The Coil Rotates

The coil rotates on its axle. As it turns, the forces continue to act, keeping it spinning.

Step 4: The Commutator Reverses the Current

When the coil reaches the vertical position (perpendicular to the magnets), it would normally stop because the forces would no longer create a turning effect. However, at this exact moment, the split-ring commutator swaps the connections.

The two halves of the commutator swap contact from one brush to the other. This reverses the direction of the current in the coil, which reverses the direction of the forces, ensuring the coil continues to rotate in the same direction.

Step 5: Continuous Rotation

The process repeats every half turn, and the coil rotates continuously.

Stage	What Happens
Current enters coil	Motor effect produces forces on both sides
Forces create turning effect	Coil begins to rotate
Coil reaches vertical	Commutator swaps connections, reversing current
Current reversal	Forces reverse, maintaining rotation in same direction
Repeat	Continuous rotation

Exam Tip: The split-ring commutator is the most commonly examined part of the DC motor. You must explain that it reverses the current direction every half turn to keep the coil spinning in the same direction. Without it, the coil would oscillate back and forth.

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The Role of the Split-Ring Commutator

The split-ring commutator is critical to the operation of a DC motor:

• It is made of two half-rings of conducting material (usually copper).
• Each half-ring is connected to one end of the coil.
• The carbon brushes press against the commutator and provide the electrical connection to the power supply.
• Every half turn, the contact switches from one half-ring to the other, reversing the current in the coil.

Without the commutator, the coil would rotate to the vertical position and then the forces would push it back — the motor would just oscillate, not rotate continuously.

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The Role of the Carbon Brushes

The carbon brushes serve two purposes:

1. They maintain electrical contact between the stationary power supply and the rotating commutator.
2. They are made of carbon (graphite) because it is:
   • A good conductor of electricity.
   • Self-lubricating — reduces friction as the commutator slides against them.
   • Soft enough not to wear down the commutator too quickly.

Exam Tip: If asked why carbon is used for the brushes, mention that it conducts electricity, is self-lubricating, and reduces wear. These are all valid points that could earn marks.

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Increasing the Speed of a DC Motor

The speed of rotation of a DC motor can be increased by:

Method	Explanation
Increase the current	Greater force on the coil (F = B x I x l)
Use a stronger magnet	Greater magnetic flux density (B), so greater force
Increase the number of turns on the coil	More wire in the field = greater total force
Increase the area of the coil	Longer sides of the coil experience force over a greater length

graph LR
    subgraph "Factors Increasing Motor Speed"
        A["Increase current"] --> S["Faster rotation"]
        B["Stronger magnet"] --> S
        C["More turns on coil"] --> S
        D["Larger coil area"] --> S
    end

Exam Tip: "How would you increase the speed of the motor?" is a very common exam question. Give at least two methods with explanations. The easiest to explain are increasing the current and using a stronger magnet.

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Why the Coil Does Not Stop at the Vertical Position

At the vertical position:

• The plane of the coil is parallel to the magnetic field.
• The forces on the sides of the coil act along the same line (they do not create a turning effect).
• The coil should stop — but it does not because of two reasons:

1. The coil has momentum (inertia) — it keeps moving past the vertical position.
2. The commutator reverses the current at this point, so as soon as the coil passes the vertical, the forces act in the correct direction to continue the rotation.

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Real DC Motors vs. the Simple Motor Model

Real electric motors are more complex than the simple GCSE model:

Feature	Simple GCSE Motor	Real Motor
Coil	Single rectangular loop	Many loops wound on an armature
Magnets	Two flat permanent magnets	Curved magnets or electromagnets
Commutator	Two-segment split ring	Multi-segment commutator
Power	Low	Can be very high
Efficiency	Low	Much higher

Real motors use curved pole pieces to create a radial field — this means the coil is always perpendicular to the field, giving a more constant force and smoother rotation.

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