Electromagnetic Induction

Turning magnetism into electricity

By 1820 scientists knew that an electric current produces magnetism.^[5] In 1831 Michael Faraday showed that the reverse also happens: magnetism can produce electricity.^{[1, 7]} The effect he found, electromagnetic induction, is how the generators in power stations make electricity.^[2]

Faraday’s experiments

Michael Faraday was a self-educated scientist from London. At 13 he took a job in a bookbinding shop, reading the books he worked on. Humphry Davy later hired him as a laboratory assistant, and although he never had a formal education or a degree, he became professor of chemistry at the Royal Institution.^[6]

Faraday pushed a bar magnet into a coil of wire. A voltage (an emf, short for electromotive force) appeared in the coil while the magnet moved. Pulling the magnet out produced a voltage of the opposite sign. The faster the motion, the greater the voltage, and there was none at all when the magnet was stationary.^[1]

He also showed that a changing current in one circuit induces a current in a second, nearby circuit, even though the two are not connected.^[1] What matters in every case is change: a changing magnetic field through a coil.

Faraday’s law and Lenz’s law

Physicists describe how much magnetic field passes through a loop of wire with a quantity called magnetic flux. Faraday’s law says that the induced emf equals the negative of the rate at which that flux changes.^[1]

The math (optional)

ε = −N × ΔΦ / Δt

Here ε is the induced emf in volts, N is the number of turns in the coil, and ΔΦ/Δt is how fast the magnetic flux changes.^[1] More turns, a stronger magnet, or faster motion all give a bigger voltage.^[2]

The minus sign carries Lenz’s law, which tells you the direction in which the induced emf drives current around the circuit.^[1]

Generators: from motion to electricity

An electric generator rotates a coil inside a magnetic field. As the coil turns, the magnetic flux through it keeps changing, so an emf is induced. The output rises and falls smoothly, like a sine wave, which is why generators naturally produce alternating current.^[2] The peak voltage depends on the number of turns, the area of the coil, the strength of the magnetic field and the rotation speed.^[2]

Faraday built the first generator himself, by spinning a copper disc between the poles of a horseshoe magnet.^[6]

Something has to turn the coil. In power plants, a turbine is spun, for example, by falling water, by steam produced by burning fossil fuels, or by the wind.^[2] Modern generators have a stationary cylinder of insulated wire coils, the stator, around a spinning electromagnet, the rotor.^[7] According to the US Energy Information Administration, turbine-driven generators like these produce nearly all of the electricity in the United States.^[7]

Motors and “back emf”

An electric motor also has a coil turning inside a magnetic field. So while it turns, an emf is induced in its coil too, and by Lenz’s law this back emf opposes the voltage driving the motor.^[2] That is why a motor draws the most current when it first starts and less once it is up to speed.^[2]

Transformers and the power grid

A transformer is two separate coils of wire wrapped around a soft iron core.^[4] It relies on Faraday’s second discovery: a changing current in one circuit induces a current in another nearby.^[1] Depending on the ratio of turns in the two coils, a transformer can step up the voltage or step down it.^[4]

Power companies use this to send electricity efficiently. Some energy is always lost as heat in long cables, and that loss depends on the square of the current. Sending the same power at a high voltage and a low current keeps these losses down, which matters over lines many kilometres long.^[4] Transformers then step the voltage back down in stages before it reaches homes.^[4]

Together, the power lines, substations and transformers that connect power plants to consumers make up the grid. In the United States it includes thousands of miles of high-voltage lines and millions of miles of low-voltage lines. The final, lower-voltage distribution lines are the ones that are safer to use in homes and businesses.^[8]

Induction all around you

Electromagnetic induction shows up in many everyday devices:^[3]

• Graphics tablets: the pen’s tip has a tiny magnetic field, and as it moves, the changing field induces an emf in wires under the screen.
• Card readers: the magnetic stripe on a card is read by a playback head, much like audio or video tape.
• Regenerative braking: electric and hybrid cars use the motor as a generator when braking, and store the energy in the battery.
• Medicine: transcranial magnetic stimulation places a rapidly changing, very localized magnetic field close to specific areas of the brain. It is used to treat conditions such as depression.

Going further

In the free simulation below, you can push a magnet through a coil and light a bulb, and see how speed and the number of turns change the result.