The motor effect

  • When a current carrying conductor lies in a magnetic field there is a force on the conductor. This is called the motor effect
  • When the coil is placed in another magnetic field that is created by two magnets. The interaction of the two fields creates a force that causes movement and runs the motor.

How movement is produced in a current carrying wire in a magnetic field

  • A conductor carrying current has a magnetic field around it.
  • When the wire is placed in another magnetic field, the interaction of the two fields will produce movement on the wire
  • Observation would confirm that the wire moved up when the current was in one direction and moved in the other direction when the current is reversed
  • Direction of induced force can be determined by using Fleming’s left hand rule. Stretch the thumb, index and middle finger of the left hand so that they are perpendicular to each other
    • Index or first finger shows direction of field i.e. north to south
    • Middle or second finger shows the direction of current
    • Thumb shows the direction of force or movement


The operation of a direct current (d.c.) motor

  • Consist of a coil placed in a magnetic field.
  • The magnetic field due to the current in the coil interacts with that of permanent magnet.
  • Using Fleming’s left hand rule, the forces are in the clockwise direction.
  • The coil overshoots vertical position because of the momentum it has gained.
  • The commutator reverse its contact with the ends of the coil thereby ensuring that current is always in one direction.
  • The motor converts electrical energy to mechanical energy

Factors that affect the rotation of the coil

  • Strength of magnetic field
  • Number of turns in the coil
  • Amount of current passed
  • Cross sectional area of the coil

The speed of a motor is increased by

  • Increasing current
  • Increasing number of turns in the coil to increase surface area
  • Using stronger magnets to increase density of magnetic field
  • Place a soft-iron core within the magnetic field lines

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