A generator transfers kinetic energy to electrical energy, and a motor transfers electrical energy to kinetic energy. Both use a coil and a magnetic field, but one uses induction while the other uses the force on a current-carrying wire.
This lesson follows explaining the change needed for induced voltage and is part of induction and transformers.
How does a generator work?
A coil is turned in a magnetic field by an outside source of energy, for example steam, flowing water or wind. As it turns, the coil cuts field lines and a voltage is induced. Slip rings and brushes connect the spinning coil to the outside circuit.
Because the coil cuts the lines in alternating directions as it rotates, the output is alternating. Turning the coil faster, using a stronger magnet or adding more turns all increase the voltage. A faster turn also raises the frequency.
How does a motor work?
A current in the coil inside a magnetic field gives a force on each side of the coil, in opposite directions, so the coil turns. A split-ring commutator reverses the current every half turn, so the force keeps turning the coil the same way.
A larger current, a stronger magnet or more turns each give a bigger turning effect. Reversing either the current or the field reverses the direction of rotation.
Side by side
| Feature | Generator | Motor |
|---|---|---|
| Energy in | Kinetic (turning the coil) | Electrical |
| Energy out | Electrical | Kinetic |
| Effect used | Induction (a voltage is induced) | Force on a current in a field |
| Connection to circuit | Slip rings and brushes (a.c.) | Split-ring commutator (d.c.) |
| Always wasted | Some thermal energy | Some thermal energy |
Worked example
Two fictional machines are tested. (Invented example data.)
Motor: a small motor runs on 12 V and draws 2.0 A. The mechanical (kinetic) power it delivers is 18 W. Find its efficiency and the power wasted.
Step 1, input power: P = I × V = 2.0 × 12 = 24 W.
Step 2, efficiency: useful output ÷ input = 18 ÷ 24 = 0.75, so 75%.
Step 3, wasted power: 24 − 18 = 6 W, mostly as thermal energy in the coil and from friction.
Generator: a small wind turbine turns a generator with 400 W of mechanical power. The electrical output is 340 W.
Efficiency = 340 ÷ 400 = 0.85, so 85%. Wasted power = 400 − 340 = 60 W.
In both cases the energy in equals the energy out plus the wasted energy. Nothing is created or destroyed.
The mistake to watch for
Mistaken answer: “A generator creates electricity, so the output is bigger than the input.”
The student forgot energy conservation. The output cannot exceed the input, and some always leaves as thermal energy.
The correction: a generator transfers energy from one store to another. Write “kinetic to electrical, with some wasted as thermal” and the marks follow. The same applies to a motor, in the other direction.
Check yourself
Try these, then open each answer.
1. State the main energy transfer in (a) a hand-turned dynamo, (b) an electric fan.
Show answer
(a) Kinetic to electrical. (b) Electrical to kinetic (with some thermal energy wasted).
2. A motor takes 50 W of electrical power and gives 40 W of useful kinetic power. Find the efficiency and the wasted power.
Show answer
Efficiency = 40 ÷ 50 = 0.80, so 80%. Wasted power = 50 − 40 = 10 W.
3. A generator coil is turned twice as fast. State two effects on the output.
Show answer
The induced voltage is larger because field lines are cut faster, and the frequency is higher, so the output alternates more quickly.
Where this leads next
Generators are the source of the alternating supply that transformers use to change voltage. You can test the energy ideas again in the induction practice set.
Being able to explain a diagram in words, not just label it, is a skill teachers can build with you live. That is part of online one-to-one Physics tuition.