Electric Motors (Cambridge (CIE) IGCSE Physics): Revision Note

The d.c. motor

• The motor effect can be used to create a simple d.c. electric motor

  • The force on a current-carrying coil causes it to rotate in a single direction

• A simple d.c. motor consists of:

  • a coil of wire (which is free to rotate) between the poles of a permanent magnet

  • a split-ring commutator and brushes connected to a source of d.c.

• The brushes and commutator are designed to reverse current every half turn:

  • the brushes ensure current is maintained without tangling wires

  • the coil rotates continuously in the same direction

• As current flows through the coil, it produces a magnetic field which interacts with the external magnetic field

• Forces act in opposite directions on each side of the coil, causing a turning effect

  • The greater the force on the coil, the greater the turning effect and the faster it will turn

• The turning effect is increased by increasing:

  • the number of turns on the coil

  • the current in the coil

  • the strength of the magnetic field

Operation of a d.c. motor

Extended Tier Only

• In a d.c. motor, when the coil of wire is horizontal, it forms a complete circuit with a cell

  • The coil is attached to a split-ring (a circular tube of metal split in two)

  • This split-ring is connected in a circuit with the cell via contact with conducting carbon brushes

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

• Current flowing through the coil produces a magnetic field

  • This magnetic field interacts with the uniform external field, so a force is exerted on the wire

• Forces act in opposite directions on each side of the coil, causing it to rotate:

  • On the blue side of the coil, current travels towards the cell so the force acts upwards (using Fleming’s left-hand rule)

  • On the black side, current flows away from the cell so the force acts downwards

• Once the coil has rotated 90°, the split-ring is no longer in contact with the brushes

  • No current flows through the coil so no forces act

Coil in the vertical position in a d.c. motor

• Even though no force acts, the momentum of the coil causes the coil to continue to rotate slightly

• The split-ring reconnects with the carbon brushes and current flows through the coil again

  • Now the blue side is on the right and the black side is on the left

• Current still flows toward the cell on the left and away from the cell on the right, even though the coil has flipped

  • The black side of the coil experiences an upward force on the left and the blue side experiences a downward force on the right

  • The coil continues to rotate in the same direction, forming a continuously spinning motor

Forces on the coil when rotated 180°

Factors affecting the d.c. motor

• The speed at which the coil rotates can be increased by:

  • increasing the current

  • using a stronger magnet

• The direction of rotation of the coil in the d.c. motor can be changed by:

  • reversing the direction of the current supply

  • 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