The Motor Effect (AQA GCSE Physics)

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The Motor Effect (HT only)

  • 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-and-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

Examiner Tip

It is key to remember that the magnetic force on the magnetic and conductor is maximum when their field lines are perpendicular to each other.  The force is zero when they are parallel.

Calculating Magnetic Force on a Current-Carrying Conductor (HT only)

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

F = BIL

  • 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. 

Examiner Tip

For the maximum force, F, then B and I must be perpendicular to each other. If the question states that the wire and field are parallel then F = 0!

Make sure that the units for F is Newtons, B is Tesla, I is Amps and L is metres. Make sure to convert any units if you're given otherwise (e.g. cm to m in the worked example.)

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Katie M

Author: Katie M

Expertise: Physics

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.