The Motor Effect

The motor effect describes the force experienced by a current-carrying conductor placed in a magnetic field. This is one of the most important principles in the AQA GCSE Physics magnetism topic (specification 4.7.2) and underpins the operation of electric motors, loudspeakers and many other devices.

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What Is the Motor Effect?

When a conductor (wire) carrying an electric current is placed in a magnetic field, the wire experiences a force. This is called the motor effect.

The force arises because the magnetic field around the current-carrying wire interacts with the external magnetic field. Where the two fields are in the same direction, the combined field is strengthened. Where they are in opposite directions, the combined field is weakened. This imbalance creates a net force on the wire.

Exam Tip: The motor effect only occurs when the current-carrying conductor is NOT parallel to the magnetic field. The force is greatest when the wire is at right angles (perpendicular) to the field lines, and zero when the wire is parallel to them.

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Conditions for the Motor Effect

For the motor effect to occur, three things must be present:

1. A current flowing through a conductor.
2. An external magnetic field (e.g., from a permanent magnet).
3. The current must have a component perpendicular to the magnetic field.

Condition	Requirement
Current	Must be non-zero (wire must carry a current)
Magnetic field	Must be present (from a permanent magnet or another electromagnet)
Angle	Current and field must NOT be parallel (force is maximum at 90 degrees)

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Direction of the Force

The direction of the force on the conductor depends on:

• The direction of the current in the conductor.
• The direction of the magnetic field (from N to S).

If either the current direction or the magnetic field direction is reversed, the direction of the force reverses.

If both the current and the field are reversed, the force direction stays the same.

graph LR
    subgraph "Motor Effect Setup"
        M1["N pole of magnet"] -->|"Magnetic field direction (N to S)"| M2["S pole of magnet"]
        W["Wire carrying current INTO page"] -->|"Force acts UPWARD on wire"| F["Force"]
    end

Exam Tip: To score full marks on a motor effect question, always state: (1) the direction of the current, (2) the direction of the magnetic field, and (3) the resulting direction of the force. Use Fleming's left-hand rule if the question is Higher tier.

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Factors Affecting the Size of the Force

The size of the force on a current-carrying conductor in a magnetic field depends on three factors:

Factor	Effect
Magnetic flux density (B)	Stronger magnetic field = greater force
Current (I)	Larger current = greater force
Length of conductor in the field (l)	Longer wire in the field = greater force

These three factors are combined in the equation (Higher tier only):

F = B x I x l

Where:

• F = force on the conductor, in newtons (N)
• B = magnetic flux density, in tesla (T)
• I = current, in amperes (A)
• l = length of conductor in the magnetic field, in metres (m)

Exam Tip: This equation is only on the Higher tier paper. Make sure you can rearrange it: B = F / (I x l) and I = F / (B x l) and l = F / (B x I). Always check your units — length must be in metres, not centimetres.

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Magnetic Flux Density

Magnetic flux density (B) is a measure of the strength of a magnetic field. It is measured in tesla (T).

Magnetic Flux Density	Example
About 0.5 T	A typical fridge magnet
About 1 T	A strong permanent magnet
About 1.5 T	An MRI scanner
About 0.00005 T (50 microtesla)	The Earth's magnetic field

A higher magnetic flux density means a stronger field and a greater force on a current-carrying conductor.

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Demonstrating the Motor Effect

The Catapult Field Experiment

A simple demonstration of the motor effect:

1. Place a flexible wire between the poles of a U-shaped (horseshoe) magnet.
2. Connect the wire to a power supply.
3. Switch on the current.
4. The wire jumps (moves) in a direction perpendicular to both the current and the field.
5. Reverse the current — the wire moves in the opposite direction.
6. Reverse the magnets — the wire also moves in the opposite direction.

This demonstrates that the force depends on both the current direction and the field direction.

graph TD
    subgraph "Catapult Field Experiment"
        A["Horseshoe Magnet (N on left, S on right)"] --- B["Wire carrying current (perpendicular to field)"]
        B -->|"Force pushes wire UP or DOWN"| C["Wire moves"]
    end

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The Force on a Moving Charge

It is not just wires that experience a force in a magnetic field. Any moving charged particle in a magnetic field experiences a force (as long as it is not moving parallel to the field).

This is because a moving charge is essentially a current. The force on the charged particle is perpendicular to both its direction of motion and the magnetic field. This causes the particle to follow a curved path.

Particle	Charge	Effect in Magnetic Field
Electron	Negative	Curves in one direction
Proton	Positive	Curves in the opposite direction
Neutron	None	No force — unaffected

Exam Tip: Moving charged particles curving in a magnetic field is a common exam question. Remember: the force is always perpendicular to the velocity, so the particle moves in a circle (or spiral). Neutral particles are unaffected.

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Real-World Applications of the Motor Effect

Application	How It Uses the Motor Effect
Electric motors	A coil carrying current in a magnetic field experiences a turning force
Loudspeakers	A coil attached to a cone vibrates in a magnetic field when AC flows through it
Moving-coil meters	A coil rotates proportionally to the current, moving a pointer on a scale
Particle accelerators	Magnetic fields steer and focus beams of charged particles

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Worked Example