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10.2.3Magnetism and the Motor Effect

The Motor Effect — Edexcel GCSE Combined Science 1SC0 - Physics Revision Notes & Topical Questions

5 min read 9 parts Lesson 8 of 10

📚 Edexcel GCSE Combined Science 1SC0 - Physics Hub / Topical Revision
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Part 1 of 7A force on a current in a field

When a wire carrying a current is placed in a magnetic field, the two fields interact and the wire experiences a force. This is the motor effect. It is the basis of electric motors, loudspeakers and many measuring instruments.

Why it happens

  • the field of the wire, circles around it, adds to the external field on one side of the wire and subtracts on the other.
  • The field is strengthened on one side and weakened on the other, and the wire is pushed from the strong-field side towards the weak-field side.

Part 2 of 7Conditions for the force

The wire must be carrying a current (moving charge).
Right angles
The wire must not be parallel to the field.
  • The force is greatest when the wire is at right angles to the field and zero when it is parallel to the field lines.
The force is perpendicular to both the field and the current, so it acts at right angles to the plane that contains them.
  • Fleming's left-hand rule (6.1.7) gives its direction.

Part 3 of 7Size of the force: F = BIl

For a straight wire at right angles to a uniform field:

F = B I l

What the symbols mean: F = force on the wire (N) · B = magnetic flux density (T) · I = current (A) · L = length of wire in the field (m) · θ = angle between wire and field

where F is force in newtons (N), B is the magnetic flux density (field strength) in tesla (T), I is the current in amperes (A) and l is the length of wire in the field in metres (m).

The force increases if the current is increased, if the field is stronger (stronger magnets), or if a longer length of wire lies in the field. A coil with N turns experiences N times the force.

Part 4 of 7Reversing the force

The direction of the force reverses if either the current or the field is reversed. Reversing both leaves the force unchanged. This idea allows a motor to keep spinning in one direction (see the commutator below) and a loudspeaker cone to move back and forth when alternating current flows in its coil.

Part 5 of 7Demonstration: the catapult

A rigid copper rod rests across two horizontal rails between the poles of a strong horseshoe magnet. When a current flows through the rod it rolls along the rails. Reversing the current rolls it the other way; turning the magnet upside down also reverses the motion. A bigger current or a stronger magnet gives a greater acceleration.

Part 6 of 7Applications and the DC motor

In a simple motor, a rectangular coil sits between the poles of a magnet.

The two sides of the coil that are perpendicular to the field carry currents in opposite directions,
  • so the forces on them are opposite,
  • producing a turning effect (a moment) on the coil.
Commutator
A split-ring commutator reverses the current every half turn so the turning effect always acts in the same sense,
  • and carbon brushes make contact with the rotating ring.
The speed of the motor can be increased by increasing the current, using stronger magnets or adding more turns.

Explaining the force with fields

Imagine a wire carrying current into the page placed in a uniform field pointing to the right.

1
The wire's own field circles clockwise.
2
Above the wire its field points to the right (same as the external field),
  • so the total field is stronger; below the wire its field points to the left (opposite),
  • so the total is weaker.
3
The wire is pushed from the strong side to the weak side, that is, downwards.
4
Fleming's left-hand rule gives the same result: field right, current into the page, force down.
5
This picture shows that the force is really an interaction between two fields.

Worked reasoning about the direction

Think of a wire in a horizontal field from left to right with the current flowing away from you.

1
Apply the left-hand rule: first finger right, second finger away from you, then the thumb points upwards, so the wire is pushed upwards.
2
If the current flows towards you the force is downwards.
3
If you swap the magnet poles the force is also reversed.
4
In a loudspeaker the coil is attached to the cone and sits in a radial field of a magnet.
5
The alternating audio current causes the coil to move forwards and backwards at the frequency of the signal, producing sound waves.

Part 7 of 7Worked examples

  1. A wire of length 0.15 m, carrying 4.0 A, is at right angles to a uniform field of 0.30 T. Calculate the force.

  2. A wire in a field of 0.050 T has a force of 0.020 N on a 0.20 m length when the current is I. Find I.

  3. The current in a wire is doubled and the field halved. What happens to the force?

Key terms

TermMeaning
Motor effectForce on a current-carrying conductor in a magnetic field.
Flux density BMeasure of magnetic field strength, in tesla (T).
CommutatorSplit ring that reverses current in a motor coil every half turn.
BrushesCarbon contacts that touch the commutator.

Avoid these mistakes

1
Thinking the force is along the wire or along the field. It is at right angles to both.
2
Forgetting the wire must cut across the field; a parallel wire feels no force.
3
Using the wrong units of Btesla.
4
Forgetting the commutator reverses the current every half turn.

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