The motor effect

• Motor effect


  The force experienced by a wire carrying an electric current when placed in a magnetic field
• Calculating the size of the force generated by the motor effect

  Force (N) = Magnetic flux density (T) x Current (A) x Length of wire (m)
• Equation for calculating motor effect force

  • Applies to a wire at right angles to the magnetic field
  • Magnetic flux density is a measure of the strength of the magnetic field
• Sample question


  • Wire length = 2.5 m, Current = 1.5 A, Magnetic flux density = 0.05 T
  • Force = 0.19 N (to 2 d.p.)
• Factors affecting the size of the motor effect force

  • Magnetic flux density
  • Current
  • Length of conductor
• Using Fleming's left hand rule to determine the direction of the motor effect force

  1. Point first finger in direction of magnetic field (N to S)

  2. Point second finger in direction of conventional current (+ to -)

  3. Thumb points in direction of force/motion
• Conductor parallel to magnetic field

  No force experienced
• Conductor at right angles to magnetic field


  Force experienced