Electromagnetic induction and the generator
| English | Chinese | Pinyin |
|---|---|---|
| generator | 发电机 | fā diàn jī |
| electromagnetic induction | 电磁感应 | diàn cí gǎn yìng |
| induced | 感应的 | gǎn yìng de |
| a.c. generator | 交流发电机 | jiāo liú fā diàn jī |
| slip rings | 滑环 | huá huán |
| brushes | 电刷 | diàn shuā |
Making electricity from movement
- The motor effect turns electricity into movement. Run it backwards — movement into electricity — and you have a generator 发电机.
- This is electromagnetic induction 电磁感应, and it powers almost every power station on Earth.
- Let's see how movement makes a current.
Electromagnetic induction
- When a wire moves across a magnetic field — or the field through a coil changes — an e.m.f. is induced 感应的.
- If the wire is part of a complete circuit, that e.m.f. drives a current.
- Move a magnet in and out of a coil joined to a meter, and the needle flicks.
The generator
V = a sin(bt)
A spinning coil induces a sinusoidal voltage — the AC output.
How can you induce a current in a coil with a magnet?
A current is induced only when the field through the coil is changing — for example by moving the magnet.
A magnet held still inside a coil induces a steady current.
There must be movement or a changing field. A still magnet causes no change, so no e.m.f. is induced.
Making the induced e.m.f. bigger
- The induced e.m.f. is bigger when you use a stronger magnet, move faster, or use more turns on the coil.
- There must be movement or change — a magnet sitting still inside a coil induces nothing.
- The induced e.m.f. always acts to oppose the change that causes it.
An a.c. generator 交流发电机
Which change makes the induced e.m.f. bigger?
A stronger magnet, faster movement, or more turns all increase the induced e.m.f.
Match each idea to what it means.
Each item links the term to its correct meaning.
The a.c. generator
- An a.c. generator spins a coil in a magnetic field. As it turns, the field through the coil keeps changing, so an alternating e.m.f. is induced.
- Slip rings 滑环 and carbon brushes 电刷 carry the current out while the coil keeps turning.
- The e.m.f. is largest when the coil is flat (cutting field lines fastest) and zero when it is upright — giving a wave-shaped (sine) output.
The output of a simple a.c. generator is:
As the coil spins, the field through it changes back and forth, giving an alternating (sine-wave) e.m.f.
What do the slip rings and brushes do in an a.c. generator?
Slip rings and brushes keep a sliding contact, carrying the a.c. out without the wires twisting. (A commutator, by contrast, reverses the current — that is for a motor or d.c. generator.)
You've got it
- moving a wire across a field (or changing the field through a coil) induces an e.m.f.
- bigger e.m.f.: stronger magnet, faster movement, more turns — and there must be change
- an a.c. generator spins a coil → alternating e.m.f.; slip rings + brushes carry it out
- output is a sine wave: max when the coil is flat, zero when upright