AQA GCSE Physics

Revision Notes

7.2.4 Electric Motors

Higher Tier Only

Electric Motors

  • The motor effect can be used to create a simple d.c electric motor
  • The simple d.c. motor consists of a coil of wire (which is free to rotate) positioned in a uniform magnetic field:

Force acting on coil in DC motor, downloadable IGCSE & GCSE Physics revision notes

Forces acting on the coil in a d.c. motor

  • When the current is flowing in the coil at 90o to the direction of the magnetic field:
    • The current creates a magnetic field around the coil
    • The magnetic field produced around the coil interacts with the field produced by the magnets
    • This results in a force being exerted on the coil
    • The direction of the force can be determined using Fleming's left-hand rule
    • As current will flow in opposite directions on each side of the coil, the force produced from the magnetic field will push one side of the coil up and the other side of the coil down
  • This will cause the coil to rotate, and it will continue to rotate until it is in the vertical position
  • When the coil is in the vertical position there will be a force acting upwards and a force acting downwards

Forces Acting on Coil in the Vertical Position, downloadable IGCSE & GCSE Physics revision notes

Forces acting on the coil in the vertical position

  • The split ring commutator swaps the contacts of the coil
    • This reverses the direction in which the current is flowing
  • Reversing the direction of the current will also reverse the direction in which the forces are acting
  • As a result, the coil will continue to rotate

Forces on coil after commutator has reversed the direction of the current, downloadable IGCSE & GCSE Physics revision notes

Forces on coil after commutator has reversed the direction of the current

  • The commutator reverses the direction of the current in the coil every half turn
    • This will keep the coil rotating continuously as long as the current is flowing

Factors Affecting the D.C Motor

  • The speed at which the coil rotates can be increased by:
    • Increasing the current
    • Increasing the strength of the magnetic field
  • The direction of rotation of coil in the d.c motor can be changed by:
    • Reversing the direction of the current
    • Reversing the direction of the magnetic field by reversing the poles of the magnet
  • The force supplied by the motor can be increased by:
    • Increasing the current in the coil
    • Increasing the strength of the magnetic field
    • Adding more turns to the coil

Worked Example

A d.c motor is set up as shown below.

WE D.C Motor Question image, downloadable IGCSE & GCSE Physics revision notes

Determine whether the coil will be rotating clockwise or anticlockwise.

Step 1: Draw arrows to show the direction of the magnetic field lines

    • These will go from the north pole of the magnet to the south pole of the magnet

WE D.C Motor Step 1, downloadable IGCSE & GCSE Physics revision notes

Step 2: Draw arrows to show the direction the current is flowing in the coils

    • Current will flow from the positive terminal of the battery to the negative terminal

WE D.C Motor Step 2, downloadable IGCSE & GCSE Physics revision notes

Step 3: Use Fleming’s left hand rule to determine the direction of the force on each side of the coil

    • Start by pointing your First Finger in the direction of the (magnetic) Field
    • Now rotate your hand around the first finger so that the seCond finger points in the direction of the Current
    • The THumb will now be pointing in the direction of the THrust (the force)

WE D.C Motor Step 3, downloadable IGCSE & GCSE Physics revision notes

Step 4: Use the force arrows to determine the direction of rotation

    • The coil will be turning clockwise

WE D.C Motor Step 4, downloadable IGCSE & GCSE Physics revision notes

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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