Electric Motors (CIE IGCSE Physics: Co-ordinated Sciences (Double Award))

Extended tier only

• 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.

Structure of a simple d.c. motor

In a simple d.c. motor, a coil placed in a magnetic field may experience a turning effect

• 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

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

Forces acting in opposite directions on each side of the coil, causing it to rotate. The split ring connects the coil to the flow of current

• 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

No force acts on the coil when vertical, as the split ring is not in contact with the brushes

• 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°

Even though the coil has flipped, the current still flows anticlockwise and the forces still cause rotation in the same direction

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