Generators · Year 12
In Year 12 you get fluent with: alternators and dynamos; a.c. generator output: peak and frequency.
Moving a wire across a magnetic field, or changing the field through a coil, induces a voltage: the generator effect. A generator spins a coil in a magnetic field, so kinetic energy becomes electrical energy. An alternator gives alternating current through slip rings; a dynamo gives direct current through a commutator. Spinning faster gives a bigger, more frequent output.
Key facts
- The generator effect: moving a wire or coil through a magnetic field, or moving a magnet in a coil, induces a voltage.
- Motor: electrical → kinetic energy. Generator: kinetic → electrical energy.
- An alternator uses slip rings and gives a.c.; a dynamo uses a commutator and gives d.c.
- Spinning a generator faster raises both the peak e.m.f. and the frequency.
Worked examples
Example 1
What energy transfer happens in a generator?
Answer: kinetic → electrical
Example 2
A magnet is pushed into a coil joined to a sensitive ammeter, and the needle flicks to the right. What happens when the magnet is pulled out again?
Answer: The needle flicks to the left: the induced current reverses.
Example 3
How can you make a generator give a bigger voltage?
Answer: Spin it faster, use more turns on the coil, or use a stronger magnet.
Example 4
In a moving-coil microphone, what happens when sound hits the diaphragm?
Answer: The coil moves in a magnetic field and a voltage is induced
Example 5
What is the difference between an alternator and a dynamo?
Answer: An alternator uses slip rings and gives a.c.; a dynamo uses a split-ring commutator and gives d.c.
Example 6
The trace shows two cycles of a generator’s output taking 40 ms, with a peak of 12 V. Find the period and frequency.
Answer: T = 20 ms, f = 50 Hz
Example 7
A generator gives a peak of 8 V at 25 Hz. What happens if it spins three times as fast?
Answer: peak 24 V, frequency 75 Hz
Example 8
A generator coil turns at 1,200 revolutions per minute. What is the frequency of its a.c. output, and hence its period in ms? Give the period.
Answer: 50 ms
Example 9
In a generator, when is the induced e.m.f. largest?
Answer: when the coil’s plane is parallel to the field (cutting field lines fastest)