AQA GCSE Physics

Revision Notes

7.2.2 The Motor Effect

Higher Tier Only

The Motor Effect

  • The motor effect occurs:

When a wire with current flowing through it is placed in a magnetic field and experiences a force

  • This effect is a result of two interacting magnetic fields
    • One is produced around the wire due to the current flowing through it
    • The second is the magnetic field into which the wire is placed, for example, between two magnets
  • As a result of the interactions of the two magnetic fields, the wire will experience a force

The Motor Effect Magnetic Field, downloadable IGCSE & GCSE Physics revision notes

The magnetic field between opposite poles of magnets interact with the magnetic field produced around a current-carrying wire

The-motor-effect, IGCSE & GCSE Physics revision notes

The motor effect is a result of two magnetic fields interacting to produce a force on the wire

Factors Affecting Force

  • The size of the force exerted by the magnetic fields can be increased by:
    • Increasing the amount of current flowing through the wire
      • This will increase the magnetic field around the wire
    • Using stronger magnets
      • This will increase the magnetic field between the poles of the magnet
    • Placing the wire at 90o to the direction of the magnetic field lines between the poles of the magnet
      • This will result in the maximum interaction between the two magnetic fields
  • Note: If the two magnetic fields are parallel there will be no interaction between the two magnetic fields and therefore no force produced
Higher Tier Only

Calculating Magnetic Force on a Current-Carrying Conductor

  • The size of the force acting on a current-carrying wire in a magnetic field can be calculated using the equation:


  • Where:
    • F = force acting on current-carrying wire in Newtons (N)
    • B = magnetic flux density (which is the strength of the magnetic field) in Tesla (T)
    • I = current flowing through the conductor in Amps (A)
    • L = length of the conductor that is in the magnetic field in metres (m)

Force on conductor (2), downloadable AS & A Level Physics revision notes

Force on a current carrying conductor (directed into the page)

Worked Example

A 5 cm length of wire is at 90o to the direction of an external magnetic field. When current of 1.5 A flows through the wire it experiences a force of 0.06 N from the motor effect.

Calculate the magnetic flux density of the magnet.


Author: Katie

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.

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