How Electromagnetic Induction Generates Current

Imagine a coil of wire connected to a sensitive meter. If a bar magnet sits still beside the coil, the meter shows no continuous induced current. Move the magnet towards the coil, however, and the reading changes. Pull it away and the reading changes in the opposite direction.

The effect occurs because the magnetic flux through the coil is changing. Flux is a way of describing how much magnetic field passes through an area. A changing flux induces a potential difference across the conductor.

A current flows if the conductor forms a complete circuit. If the circuit is open, a potential difference can still be induced even though charge cannot travel around a closed path.

The induced potential difference becomes larger when magnetic flux changes more quickly. Moving the magnet faster, using a stronger magnetic field or increasing the number of turns in the coil can all increase the effect.

You can also create induction by moving the coil instead of the magnet. What matters is the changing relationship between the conductor and magnetic field. Rotating a coil inside a magnetic field is especially useful because the change can continue repeatedly.

The direction of the induced current depends on the direction of the change. Lenz's law states that the induced effect acts in a direction that opposes the change that produced it. This is connected with conservation of energy, because the system cannot create useful electrical energy without an input of work.

In a generator, mechanical work rotates a coil in a magnetic field or rotates magnets around conductors. The changing flux induces a potential difference, allowing electrical energy to be transferred through a circuit.

Power stations use different methods to provide the rotation. Steam turbines, flowing water and wind turbines can all provide mechanical motion. The generator stage then uses the same induction principle to produce electrical output. The same principle can work on a smaller scale. Bicycle dynamos, hand-crank generators and some microphones all rely on motion and changing magnetic conditions to produce electrical signals or energy. The designs differ, but the underlying relationship between changing flux and induced potential difference remains the same.

Remember the core ideas:

• •A changing magnetic flux is required.
• •A coil with more turns can produce a larger induced potential difference.
• •Faster change usually produces a larger effect.
• •Reversing the change reverses the induced direction.
• •Mechanical work supplies the energy in a generator.

Induction does not always need motion. A transformer has two coils of wire wound around a shared iron core. An alternating current in the first coil produces a magnetic field that keeps growing, shrinking and reversing. The iron core guides this changing flux through the second coil, where it induces an alternating potential difference. Nothing spins, yet the flux is always changing. If the second coil has more turns than the first, the output voltage is higher. With fewer turns, it is lower.

This explains why transformers need alternating current. A steady direct current produces a steady magnetic field, so after it is switched on there is no continuing change and no continuing induced voltage. Transformers matter in electricity networks because power stations step voltages up for long-distance transmission. At a higher voltage, the same power can be carried with a smaller current, and a smaller current wastes less energy heating the cables. Transformers near homes then step the voltage down to a safer level for everyday use.

Scientists knew by 1820 that an electric current produces a magnetic field, and many wondered whether a magnet could produce a current. Michael Faraday found the answer in 1831. He wound two separate coils of wire around opposite sides of an iron ring. One coil connected to a battery, the other to a galvanometer, a sensitive current meter. A steady current in the first coil produced no steady reading in the second. But the needle flicked briefly when the battery was connected and flicked the other way when it was disconnected.

That brief flick was the crucial clue. The current appeared only while the magnetic field was changing, which is why the idea of change sits at the heart of induction. Faraday soon built a copper disc that turned between the poles of a magnet, with contacts at its edge and centre. Turning the disc produced a continuous current, making it one of the first electric generators. Joseph Henry in the United States discovered induction independently around the same time. Later, James Clerk Maxwell expressed Faraday's ideas in the mathematical equations still used today.

Drop a strong magnet down a vertical copper pipe and something odd happens. Copper is not magnetic, yet the magnet drifts down slowly, taking far longer to emerge than a non-magnetic object of the same size. As the magnet falls, the flux through each ring of the pipe changes, inducing swirling loops of current in the copper called eddy currents. Following Lenz's law, those currents produce magnetic fields that oppose the motion, pushing up on the magnet from below and pulling on it from above. The energy the magnet would have gained ends up slightly warming the pipe.

Engineers use the same braking effect without any contact. Many rollercoasters and drop rides slow their cars with strong magnets and metal fins that pass close to each other, so there are no brake pads to wear out, and some high-speed trains use eddy current brakes too. An induction hob turns the idea into heat. A coil beneath the glass carries rapidly alternating current, and the changing field induces eddy currents in the base of the pan, heating it directly while the glass is warmed mainly by the hot pan. That is why induction hobs need pans made of magnetic materials such as cast iron or suitable stainless steel.

The frequency of mains electricity comes straight from the speed of the spinning generators. In the simplest design, each full turn of the rotor makes the magnetic flux through a coil rise, fall, reverse and return, giving one complete cycle of alternating current. A large steam turbine generator with a single pair of magnetic poles therefore spins at 3,000 revolutions per minute to produce the 50 cycles per second, or 50 hertz, used in Britain and most of Europe. In North America, where the grid runs at 60 hertz, the same type of machine turns at 3,600 revolutions per minute.

Water turbines turn far more slowly, so hydroelectric generators carry many pairs of poles around the rotor. The rule is that frequency equals revolutions per second multiplied by the number of pole pairs, so a machine with 20 pole pairs needs only 150 revolutions per minute to make 50 hertz. Every generator on a grid must spin in step. When demand suddenly exceeds supply, the extra energy is drawn from the rotating machines themselves, they slow slightly and the frequency dips. Grid operators watch frequency closely for this reason, adding or removing generation to hold it within a narrow band.

Electromagnetic induction happens when magnetic flux through a conductor changes, creating an induced potential difference. Faster changes, stronger fields and more coil turns can increase the effect. Follow what is changing rather than looking only for a magnet and wire, and the working principle of generators becomes much clearer.