A generator works by electromagnetic induction: when a coil of wire turns in a magnetic field (or a magnet turns inside a coil), a voltage is induced in the wire, and that voltage pushes a current round a complete circuit. A generator therefore transfers energy from movement to electricity. Knowing how generators work explains almost every light you switch on in the UK, because somewhere a magnet is spinning to power it.
How does a generator work? The short answer
Every generator, from a bicycle light to a huge power station machine, follows the same energy chain. Something makes a shaft turn: an engine, steam, falling water, the wind or a person pedalling. The turning shaft moves a coil relative to a magnetic field. The result is electrical work done on a circuit.
In the language of GCSE physics, a generator moves energy from a kinetic (mechanical) store to electrical work. If you want a refresher on those stores, our guide to the different kinds of energy and their uses covers them.
Notice what is missing from that description: nothing in a generator makes energy from nothing. The electrical energy that comes out has to be paid for by the mechanical energy that goes in.
You can see this with a hand-cranked torch. Crank it with the bulb disconnected and it spins easily. Connect a bright bulb and the handle suddenly feels stiffer, because the generator is now doing work on a circuit.
Electromagnetic induction: the generator effect
The key idea has a formal name in GCSE physics: the generator effect. The AQA specification describes it like this. If an electrical conductor moves relative to a magnetic field, or if the magnetic field around a conductor changes, a potential difference is induced across the ends of the conductor. If the conductor is part of a complete circuit, a current flows.
Two things can create that change. Either the wire moves through the field, cutting the field lines, or the field itself gets stronger or weaker around a wire that stays still. A generator uses the first method. A transformer, which has no moving parts, uses the second.
The classic classroom demonstration needs only a coil, a sensitive meter and a bar magnet:
- Connect the coil to the meter so that the circuit is complete.
- Push the magnet into the coil. The meter needle flicks briefly in one direction.
- Hold the magnet still inside the coil. The needle returns to zero, even though the magnet is right there.
- Pull the magnet out. The needle flicks the opposite way.
- Repeat more quickly. The flick is bigger.
This tells you that it is the change that matters, not the magnet's mere presence. If you would like to try it yourself, the electromagnetic induction practical on this site walks through the method.
The experiment also shows two rules that come back in every generator. Reversing the direction of movement reverses the induced current, and moving faster induces a larger potential difference. A stronger magnet helps too, which is one reason engineers look for powerful magnets. Our article on the strongest magnets explains what makes some magnets so much stronger than others.
Who invented the generator?
The story begins with the English scientist Michael Faraday in 1831. On 29 August 1831 he wound coils on an iron ring and found that a brief current appeared in one coil only while he switched the current in the other coil on or off. The effect lasted just for the change, which fits what you saw in the magnet-and-coil experiment.
The Royal Institution in London still holds Faraday's coil-and-bar-magnet apparatus, dated October 1831, which it describes as his "first-ever electric generator". Faraday also built the Faraday disc: a copper disc spun between the poles of a magnet, which gave a small, steady direct current. It is usually called the first electromagnetic generator.
The disc was never a practical power source. It was very inefficient, because currents swirled around inside the disc itself and wasted energy. But it proved the idea that movement and magnetism could make electricity continuously.
Hippolyte Pixii then built one of the first dynamos in 1832. Self-excited dynamos, which used some of their own output to power their electromagnets, followed in the 1860s: Werner Siemens and Charles Wheatstone both announced the idea in early 1867. Today generators produce nearly all of the electric power for the world's electricity grids. The main exceptions are solar cells, batteries and fuel cells.
How an AC generator works, step by step
The simplest useful generator is a single loop of wire turning between the north and south poles of a magnet. The loop is connected to an external circuit (here, a voltmeter) through slip rings, which you will meet properly in the next section. The axle the loop turns on is at right angles to the magnetic field.
Follow one complete turn and watch what the two long sides of the loop are doing:
- Start: the plane of the coil is at right angles to the field lines. The sides are moving along the field lines rather than across them, so they cut no lines. The potential difference is zero.
- Quarter turn: the plane of the coil is parallel to the field lines. The sides now slice across the field lines at the greatest rate. The potential difference is at its maximum.
- Half turn: back to right angles. The sides again move along the field, so the potential difference falls to zero.
- Three-quarter turn: parallel again, but upside down. The side that moved up now moves down, so the potential difference reaches a maximum in the opposite direction.
- Full turn: back to the start. The potential difference is zero again, and the pattern repeats.
Plotted against time, this gives a smooth wave: up to a peak, down through zero, down to a negative peak, then back to zero. One full turn of the coil makes exactly one complete cycle of alternating current, which is why this machine is called an AC generator.
The current changes direction twice per turn, because the potential difference reverses every half-turn. This is what alternating means.
Why is the induced potential difference zero when the coil is at right angles to the field? (2 marks)
At that instant the sides of the coil are moving parallel to the magnetic field lines, so they are not cutting through any field lines. With no cutting of field lines there is no induced potential difference.
The main parts of a generator
A simple AC generator is made from only a handful of parts. Each does a clear job, and exam questions often ask you to name them and say what they do.
| Part | What it does |
|---|---|
| Magnet (the field) | Provides the magnetic field. It can be a permanent magnet or an electromagnet. |
| Coil (armature) | Many turns of insulated copper wire. The potential difference is induced in this wire as it moves relative to the field. |
| Axle (shaft) | Carries the turning movement from the engine, turbine or hand crank. It is at right angles to the magnetic field. |
| Slip rings | Two rings fixed to the coil that turn with it. They keep a sliding contact with the brushes. |
| Carbon brushes | Pressed against the slip rings so the current can pass to the external circuit without the wires twisting. |
| External circuit (load) | The lamp, motor or grid that the current flows through. Without a complete circuit there is a potential difference but no current. |
The slip rings and brushes solve a practical problem. If the coil's wires were attached straight to a meter, they would twist up after a few turns. Rings that rotate with the coil, with fixed brushes sliding on them, let the coil spin freely while the current still reaches the circuit.
Spin the magnet instead: how big generators are built
Most large generators turn this arrangement inside out. Instead of spinning the coil, they spin a magnet (usually an electromagnet on the rotor) inside fixed coils called the stator. The magnetic field still changes relative to the coil, so a potential difference is still induced.
There is a big advantage. The output, which may be very large, is taken from stationary coils with solid connections. The rotor needs only two slip-ring contacts to carry the small direct current that powers its electromagnet.
The diagram shows the idea in miniature: a magnet spinning inside a fixed coil, with a voltmeter showing a wave-shaped output. The shape is a sine wave, the same as for the turning loop earlier.
Alternator vs dynamo: AC or DC?
Two words appear on every GCSE specification: alternator and dynamo. Both use the generator effect. The difference lies in how the current gets out of the coil.
An alternator uses slip rings, so its output alternates: the current reverses direction every half-turn. A dynamo uses a split-ring commutator. This is a single ring cut into two halves, and it swaps the connections to the external circuit every half-turn, just as the coil's own potential difference reverses. The current in the circuit therefore always flows the same way. It is direct current (DC), although it pulses, rising to a peak and falling to zero twice per turn, and never reverses.
| Feature | Alternator | Dynamo |
|---|---|---|
| Output | Alternating current (AC) | Direct current (DC), pulsing |
| Connection to circuit | Slip rings and brushes | Split-ring commutator and brushes |
| pd–time graph | A sine wave that crosses the axis | A series of humps that stay on one side of the axis |
| Examples | Power station generators, car alternators | Early cars and 19th-century machines |
Cars used to have dynamos. They switched to alternators once cheap silicon diodes made it easy to convert the alternator's AC into the DC a battery needs. You will meet that arrangement in the car section later in this article.
What affects the voltage a generator makes?
Because the induced potential difference depends on how quickly the coil cuts through field lines, four changes make it bigger.
Turn it faster
Faster rotation means the field lines are cut more quickly. In the ideal case, doubling the speed doubles the peak potential difference. Speed has a second effect: the AC also gets a higher frequency, so more cycles happen each second. On a graph, the peaks become both taller and closer together.
Use a stronger magnetic field
A stronger field means more field lines to cut. Replacing a weak magnet with a strong one, or increasing the current in an electromagnet, raises the output.
Add more turns to the coil
Each turn of wire has a potential difference induced in it, and the turns are connected in series, so the effects add up. A coil with more turns gives a larger total.
Make the coil bigger
A larger coil area cuts through more field at once. This again raises the potential difference.
Reversing the direction of movement, or swapping the magnet's poles, reverses the direction of the induced current.
Give three ways to increase the potential difference from a simple generator. (3 marks)
Any three of: turn the coil (or magnet) faster; use a stronger magnet; increase the number of turns on the coil; use a coil with a larger area.
If you need to predict the direction of the current, Fleming's right-hand rule does it. Hold your thumb, first finger and second finger at right angles. The first finger points along the Field, the thumb along the Motion, and the second finger shows the direction of the Current. Our page on Fleming's right-hand rule has worked examples.
Generator vs motor: the same machine backwards
A motor and a generator are built from the same parts: a coil, magnets, and brushes or rings. They do opposite jobs. A motor uses a current in a magnetic field to make movement, which is the motor effect. A generator uses movement in a magnetic field to make a current, which is the generator effect.
Many machines can work both ways. Electric cars use their motors as generators when they slow down: the wheels turn the motor, which feeds electricity back to the battery. This is regenerative braking, and you can read more about it in our guide to how electric cars work.
The same pairing appears in everyday audio equipment. A loudspeaker uses the motor effect to turn current into sound, while a microphone uses the generator effect to turn sound vibrations into a varying potential difference.
How power station generators work
Most of the electricity you use comes from a generator driven by a turbine, a wheel with blades that is turned by a moving fluid. Only the way of turning the turbine changes from one power station to another.
- Thermal power stations (gas, coal, oil, biomass and nuclear) use heat to boil water. The high-pressure steam spins a turbine, and the turbine turns the generator.
- Hydroelectric stations let falling or flowing water, from the gravitational potential energy store, turn a turbine.
- Wind turbines use the kinetic energy of moving air to turn the blades, which turn a generator in the nacelle (the housing at the top of the tower).
Our guide to energy sources and their uses compares these options in more detail.
Large generators are remarkably efficient. They convert about 98% of the mechanical energy they receive into electrical energy. In a fossil-fuel power station the big losses happen earlier, as heat in the steam cycle, not in the generator itself.
One technology does not follow this pattern. Solar panels make electricity with no moving generator at all: light knocks electrons loose inside a solar cell. If you are curious, read how solar panels work.
Inside a wind turbine generator
A wind turbine's blades turn slowly compared with the speed a normal generator likes. Engineers have found two ways round this.
The first is a gearbox. It speeds up the slow rotor shaft so a smaller, faster generator can be used. Many turbines with gearboxes use a doubly-fed induction generator, a type of AC machine designed to work well as the wind speed changes.
The second is direct drive. There is no gearbox: a large, slow generator turns at the same speed as the rotor. Many direct-drive designs use permanent magnets made with rare-earth metals such as neodymium. Electrically excited designs avoid neodymium, though they have their own trade-offs.
In 2025, according to the Department for Energy Security and Net Zero's Digest of UK Energy Statistics, renewables supplied 52.1% of UK electricity. Wind, at 29.5% (86.4 TWh), was the largest renewable source, ahead of solar at 6.9% (20.1 TWh). For the pros and cons of wind power, see advantages and disadvantages of wind energy.
Why is UK mains electricity 50 Hz?
Frequency is the number of complete cycles per second, measured in hertz (Hz). UK mains electricity is 230 V at 50 Hz, so the current changes direction 100 times a second. The frequency is linked directly to how fast the generator spins and how many magnetic poles it has:
f = (number of poles × rpm) ÷ 120
Here rpm means revolutions per minute. Each pair of poles that sweeps past a coil makes one full AC cycle. Putting numbers in shows the pattern:
- A 2-pole generator must turn at 3,000 rpm to give 50 Hz.
- A 4-pole generator needs only 1,500 rpm.
- A 40-pole generator needs just 150 rpm, which suits slow hydroelectric turbines.
Countries with 60 Hz mains need a 2-pole generator to turn at 3,600 rpm. In short, more poles means the shaft can turn more slowly for the same frequency. Try some combinations yourself. Start with 4 poles at 1,500 rpm, then 40 poles at 150 rpm.
Keeping the grid in step
The big generators on the national grid all turn in step with each other, which is called being synchronised. Together they behave like one enormous machine. If demand for electricity rises faster than supply, the generators slow down very slightly and the frequency dips. Grid operators then add more power to bring it back.
Britain's grid operator, NESO, has to keep the frequency within a legal range. The Electricity Safety, Quality and Continuity Regulations 2002 allow it to vary by no more than 1% either side of the declared value. NESO also works to a narrower routine band.
| Quantity | UK value | What it means |
|---|---|---|
| Mains voltage | 230 V | Legal supply range is up to 10% above or 6% below, about 216–253 V |
| Mains frequency | 50 Hz | The statutory range is 49.5–50.5 Hz |
| NESO routine band | 49.8–50.2 Hz | The narrower band the operator normally works within |
| Transmission lines | 400 kV or 275 kV | Large power lines that carry electricity long distances |
| Local lines | 132, 66, 33 and 11 kV | Voltage is stepped down before it reaches homes |
The generators in a power station do not produce 400 kV directly. Transformers step the voltage up for transmission, and step it down again near your home. Very high voltages bring their own problems, such as corona discharges around high-voltage lines.
The result is a system where a spinning magnet in a hydro station in Wales, a gas turbine in the Midlands and a wind farm in the North Sea all run at the same 50 Hz, and a plug socket anywhere in the country delivers the same alternating supply.
Generators you can hold: portable, car and bike
The same principle works at every size. Some generators are small enough to carry, and you will probably meet at least one of these in daily life.
Portable and standby generators
A portable or standby generator has a petrol or diesel engine that turns an alternator. The engine supplies the mechanical work, and the alternator turns it into electricity. People use them on building sites, at events and during power cuts.
Inverter generators
An inverter generator adds some electronics. The alternator's AC is first rectified to DC and smoothed. An electronic circuit then inverts it back into clean AC at the correct frequency. Because the output frequency no longer depends on the engine speed, the engine can slow down when the load is light, which makes it quieter and uses less fuel.
Car alternators
In a car, a belt from the engine spins the rotor, an electromagnet, inside stator coils. The coils produce AC. Six diodes, which act as one-way valves for current, convert it to DC for the battery. A voltage regulator adjusts the current in the rotor's electromagnet to hold the output steady. A 12 V battery needs about 13.8–14.4 V to recharge properly.
Bike dynamos
Despite the name, a bike "dynamo" (a hub type or a bottle type pressed on the tyre) is really a small permanent-magnet alternator. A typical one gives 6 V and 3 W, which is plenty for lights. USB chargers convert the roughly 6 V output down to 5 V.
Using a portable generator safely
A portable generator burns fuel, and that creates a hazard that has nothing to do with electricity. The engine produces carbon monoxide gas. According to HSENI, Northern Ireland's health and safety body, it is completely colourless and odourless, so you cannot detect it with your own senses.
This checklist follows the HSENI guidance. Your ticks are remembered in your browser.
- Run the generator outdoors only. Never use it in a home, garage or partially enclosed space.
- Keep it away from open doors and windows. Fans and open windows do not stop carbon monoxide building up.
- Fit battery-powered carbon monoxide alarms in the home.
- Turn the generator off and let it cool before refuelling.
- Never plug it into your home wiring. It can push current back onto the public cable and endanger network engineers. A safe set-up uses a changeover switch fitted by a qualified electrician.
- If anyone feels dizzy, sick or short of breath, get into fresh air and call 999 if it is urgent.
As the UK adds more wind and solar, the mix of machines supplying the grid is changing, a story told in renewable energy: expectations versus reality. The generators themselves keep working on the same principle Faraday found in 1831.
Generator words: flip the cards
Frequently asked questions about how generators work
How does a generator work in simple terms?
A generator turns a coil of wire in a magnetic field, or turns a magnet inside a coil. The movement induces a potential difference in the wire, and if the wire is part of a complete circuit, a current flows. The energy comes from whatever turns the shaft, such as an engine, steam or wind.
Does a generator produce AC or DC?
It depends on how the current leaves the coil. A generator with slip rings, called an alternator, produces alternating current. A generator with a split-ring commutator, called a dynamo, produces direct current that pulses but never reverses. Power stations and car alternators produce AC first.
What are the main parts of a generator?
A simple AC generator has a magnet, a coil of insulated wire, an axle, two slip rings, carbon brushes and an external circuit. The magnet provides the field, the coil is where the potential difference is induced, and the slip rings and brushes carry the current out while the coil turns.
What is the difference between an alternator and a dynamo?
Both use the generator effect. An alternator uses slip rings and gives alternating current, with a sine-wave graph. A dynamo uses a split-ring commutator that swaps the connections every half-turn, so its output is direct current that pulses but never reverses direction.
What is the difference between a generator and a motor?
They are built from the same parts but work in opposite directions. A motor uses an electric current in a magnetic field to produce movement, which is the motor effect. A generator uses movement in a magnetic field to produce a current, which is the generator effect. Some machines do both.
Who invented the electric generator?
Michael Faraday made the first electric generators in 1831, including coils and a bar magnet, and his copper Faraday disc. Hippolyte Pixii built one of the first dynamos in 1832. Later engineers, including Werner Siemens and Charles Wheatstone, developed self-excited dynamos in the 1860s.
How can you increase the voltage from a generator?
You can increase the induced potential difference in four ways: turn the coil or magnet faster, use a stronger magnetic field, add more turns of wire to the coil, or use a coil with a larger area. Rotating faster also raises the frequency of the AC.
Why is UK mains electricity 50 Hz?
UK mains is 50 Hz because the grid's synchronised generators are run so that the current completes 50 cycles every second. A 2-pole generator needs to turn at 3,000 rpm for this. The frequency must stay between 49.5 and 50.5 Hz, which is within 1% of 50 Hz.
Do generators create energy?
No. A generator transfers energy from a kinetic (mechanical) store to electrical work, and energy is conserved. The electricity out is paid for by the engine, turbine, wind or person turning the shaft. The harder you draw current, the harder the shaft is to turn.
Is it safe to run a portable generator indoors?
No. Portable generators produce carbon monoxide, which is colourless and odourless, so you cannot detect it. Never run one in a home, garage or partially enclosed space, or near open doors and windows. Use it outdoors only, and fit battery-powered carbon monoxide alarms in the home.
