Physics Narrative for 14-16
What generators do
Transformers use the principle that a changing magnetic effect linked to a coil induces a potential difference. This potential difference can then be used to drive a current. In transformers a changing current produces the changing magnetic effect.
Another possibility is to use the power in another pathway to change this magnetic effect and link these changes in effect to an output coil. This new device switches from the other pathway to the electrical pathway. Such devices are called electrical generators. Electrical charge is set in motion everywhere in an electrical loop connected to such a generator. It's just as if you'd inserted a battery to complete a circuit.
Here we'll focus on generators that switch from the mechanical pathway because these are quite common.
Large-scale power stations, whether nuclear, geothermal, fossil fuel or wind powered, all make a turbine spin, and this turbine is used to rotate an arrangement that varies the effect in the magnetic loop.
More details about which energy stores are depleted in achieving the spinning are given in the SPT: Energy topic.
Modelling a generator from what we know about a transformer
To explore an arrangement that can vary the magnetic effect, start with the transformer. The input coil has a constant current driven through the coil in order to make an electromagnet. The constant magnetic effect is converted to a changing magnetic effect by spinning the coil or magnet.
Careful design of the coils and the iron can make the whole process much more efficient by optimising the magnetic linkage between the two electrical loops (input and output). This is the essential design of the generators in many power stations.
Spinning a coil changes the magnetic field clinked to that coil
You don't have to spin the magnet – you could have a stationary permanent magnet and spin the coils. What you do need is a changing magnetic effect through the output coil.
The combination of a spinning coil and stationary permanent magnets is the way in which a generator is most likely to be introduced in the laboratory. First make an electric motor. Then disconnect the power supply and connect an ammeter. Spin the coils and you have a direct current generator.
Why direct current? That's because of the split-ring commutator – the position-reversing switch introduced in explaining the motor. Here it functions exactly as before, serving to connect the side of the coil moving upwards to one terminal of the motor, and the side of the coil moving downwards to the other.
A less useful, but perhaps simpler, generator
You could have an oscillating permanent magnet moving in and out of the output coil. This has so far proved impractical as a large-scale design but is often used to introduce
electromagnetic induction in the laboratory. Again, it's the changing magnetic effect that's important. You could simply rotate the permanent magnet – or the coil. Either will produce a change in the linkage between the magnetic link and the electrical loop, and so a change in magnetic effect, and this will in turn induce a potential difference.
Not all generators use magnetic fields …
As Earth has a magnetic field, you might think that rotating a coil would change the magnetic effect through the coil by changing the linkage. On this basis you might expect an induced potential difference. You'd be right! But you need the most sensitive meter in the school laboratory to detect the very small potential difference and a coil with a very large number of turns to convert the small rate of change of magnetic effect into even this tiny potential difference. (You could also try spinning the coil faster, but then you'd need a sensitive AC voltmeter: not so common.)
The principles you learnt about the magnetic loop linking input and output electrical loops when studying the transformer also apply to generators.
The more the current in the input loop, the more the magnetic effect. The greater the rate of change of magnetic effect, the larger the induced potential difference.
The more turns on the output coil, the larger the induced potential difference.
Not all generators switch from mechanical working to electrical working.
Solar photovoltaic cells are generators that switch from the heating by radiation pathway to the electrical working pathway as they absorb the incident photons of light and provide a potential difference. Usually this involves carefully layered semiconductors. Nothing has to get hot and make steam, unlike the solar energy stations that concentrate sunlight to boil water, and then drive a conventional turbine.
Thermoelectric generators are still largely in the development laboratories, except for specialised applications, but they mostly switch from the heating by particles pathway to the electrical working pathway. Again, semiconductors are involved and development continues, but so far the power made available to the electrical pathway is only small.