All revision notes topics

The Motor EffectOxford AQA IGCSE Physics: Revision notes

Section 1

What is the motor effect?

The motor effect occurs when a current-carrying conductor is placed in a magnetic field: the conductor experiences a force.

  • The force is largest when the conductor is at right angles to (cutting across) the magnetic field lines
  • There is no force if the conductor is parallel to the magnetic field
  • This force is what allows electric motors and loudspeakers to work
Key termsmotor effect

Section 2

How can the size of the force be increased?

The force on a current-carrying conductor in a magnetic field can be increased by:

  • Increasing the strength of the magnetic field
  • Increasing the current flowing through the conductor
  • Increasing the length of the conductor within the field
Example

Doubling the current through a wire in a magnetic field doubles the force it experiences.

Section 3

What determines the direction of the force?

Reversing the direction of the current, or reversing the direction of the magnetic field, reverses the direction of the force.

The direction of the force can be found using Fleming's left-hand rule:

  • The First finger points in the direction of the magnetic Field
  • The seCond finger points in the direction of the Current
  • The thuMb points in the direction of the force (Motion)
Key termsFleming's left-hand rule
Exam tip

Always state which finger/thumb represents which quantity when using Fleming's left-hand rule in an answer — examiners want the reasoning, not just the final direction.

Section 4

How does the motor effect lead to an electric motor?

When a coil of wire carrying a current is placed in a magnetic field, the forces on each side of the coil act in opposite directions (because the current flows in opposite directions along each side of the coil).

This pair of opposite forces creates a turning effect (torque), causing the coil to rotate. This rotation is the basis of how an electric motor works.

Key termselectric motor

Must Know

  • The motor effect: a current-carrying conductor in a magnetic field experiences a force; no force if parallel to the field
  • Force increases with stronger magnetic field, larger current, and longer conductor in the field
  • Reversing current direction or field direction reverses the direction of the force
  • Fleming's left-hand rule gives the direction: First finger = Field, seCond finger = Current, thuMb = Motion (force)
  • A current-carrying coil in a magnetic field experiences opposite forces on each side, causing it to rotate — the basis of an electric motor

That's the notes covered.

Carry on to the next subtopic.