Electric motors

A wire carrying a current in a magnetic field feels a force: the motor effect. In an electric motor, the forces on the two sides of a coil push in opposite directions and turn it, and a split-ring commutator keeps it turning the same way. Loudspeakers use the same effect to make a cone vibrate.

Key facts

  • The motor effect: a wire carrying a current across a magnetic field feels a force. Fleming’s left-hand rule gives its direction.
  • Motor: electrical → kinetic energy. Generator: kinetic → electrical energy.
  • A d.c. motor’s split-ring commutator reverses the coil current every half turn, keeping it spinning one way.
  • A motor spins faster with more current, more turns, or a stronger field; reverse the current to reverse it.

Worked examples

Example 1

What energy transfer happens in an electric motor?

  1. A motor is supplied with electrical energy.
  2. It turns, so it gives kinetic energy.
  3. Some energy is wasted as heat.

Answer: electrical → kinetic (plus some heat)

Example 2

How can you make a d.c. motor turn in the opposite direction?

  1. The direction of the force comes from Fleming’s left-hand rule.
  2. Reversing the current (or the field) reverses the force on each side of the coil.
  3. So the motor turns the other way.

Answer: reverse the current (or swap the magnet’s poles)

Example 3

How does a loudspeaker make sound?

  1. A loudspeaker has a coil in a magnetic field.
  2. An alternating current in the coil makes a force that keeps changing direction (motor effect).
  3. The coil and cone vibrate, making sound waves.

Answer: An alternating current in a coil in a magnetic field makes the coil and cone vibrate

Example 4

A loudspeaker is fed a 440 Hz alternating current. How often does its cone vibrate?

  1. The force on the coil reverses each time the current reverses.
  2. The current completes 440 cycles each second.
  3. So the cone vibrates 440 times a second.

Answer: 440 times a second

Example 5

A small electric motor lifts a 1 kg load through 4 m. How much useful energy does it transfer to the load? (g = 9.8 N/kg)

  1. The useful energy is the GPE gained by the load.
  2. GPE = mgh.
  3. E = 1 × 9.8 × 4 = 39.2 J

Answer: 39.2 J

Example 6

What is the job of the split-ring commutator in a d.c. motor?

  1. Without a commutator, the forces would push the coil back after half a turn.
  2. The split-ring commutator reverses the current in the coil every half turn.
  3. So the forces keep turning the coil the same way.

Answer: it reverses the current in the coil every half turn so it keeps spinning the same way

Example 7

Which change would make a d.c. motor spin faster?

  1. The force on each side of the coil is F = BIL.
  2. Increasing the current increases the force.
  3. Bigger forces make the motor spin faster.

Answer: increasing the current

Example 8

A 12 V motor draws 2.0 A for 5.0 s while lifting a 4.0 kg mass through 1.5 m. Find its efficiency. (g = 9.8 N/kg)

  1. Find the electrical energy in. E = VIt = 12 × 2.0 × 5.0 = 120 J
  2. Find the useful energy out (GPE gained). E = mgh = 4.0 × 9.8 × 1.5 = 58.8 J
  3. Efficiency = useful ÷ total × 100. 58.8 ÷ 120 × 100 = 49%

Answer: 49%

Example 9

A 12 V motor draws 3.7 A for 8 s while lifting a 2.5 kg load through 3 m. Find its efficiency, as a percentage to 1 decimal place. (g = 9.8 N/kg)

  1. In: 12 × 3.7 × 8 = 355.2 J
  2. Out: 2.5 × 9.8 × 3 = 73.5 J
  3. 73.5 ÷ 355.2 × 100 = 20.69%

Answer: 20.7%