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
    • The magnetic field which is produced around the wire due to the current flowing through it will interact with the magnetic field into which it is placed
    • As a result of the interactions of the two magnetic fields the wire will experience a force

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)

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.

Step 1: List the known quantities

    • Length, L = 5 cm
    • Current, I = 1.5 A
    • Force, F = 0.06 N

Step 2: Convert the length to metres

L = 5 cm = 0.05 m

Step 3: Write out the equation linking the magnetic flux density (B) to the known quantities


Step 4: Rearrange the equation to make B the subject of the formula

Step 5: Substitute values into the equation

Step 6: Calculate a value for B and include the appropriate unit

B = 0.8 T

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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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