Electricity and Magnetism

Rub a balloon on your hair and it sticks to the wall. Hold a compass and the needle swings north. For thousands of years these looked like two unrelated curiosities. In the nineteenth century a series of experiments revealed that they are two faces of a single force, that this force can be made to turn wheels and send messages, and that light itself is made of it. The modern world runs on that discovery.

Charge

Matter is made of atoms, and atoms contain charged particles: positive protons in the nucleus and negative electrons around it. Normally the two balance. Rubbing a balloon scrapes electrons from your hair onto the rubber, leaving the balloon negative and your hair positive, and because opposite charges attract and like charges repel, the balloon clings to a wall and your hair stands on end. That is static electricity. Lightning is the same thing on a vast scale: ice and water in a storm cloud rub together, charge builds up until the air can no longer hold it apart, and hundreds of millions of volts discharge in a channel of air heated to five times the temperature of the Sun's surface.

The force between charges is enormous. It is what holds atoms together and makes solids solid; the floor pushes back on your feet because its electrons repel yours. Gravity feels stronger only because the positive and negative charges in ordinary matter cancel so completely.

Current

In metals, some electrons are not tied to any one atom and can wander. Connect a wire between the two ends of a battery and the battery's chemical push, its voltage, sets those electrons drifting. That flow is an electric current. The electrons themselves move slowly, less than a millimetre a second in a typical wire, but the push travels along the wire at nearly the speed of light, so a lamp lights the instant the switch closes. Resistance, the difficulty electrons have getting through a material, turns some of their energy into heat, which is how a toaster, a kettle and the filament of an old-fashioned bulb work. Materials with no free electrons, such as rubber, glass and dry air, are insulators and carry no current.

Magnetism

Every magnet has a north and a south pole, and like poles repel while opposite poles attract; cut a magnet in half and each piece has both poles again. The Earth is itself a huge magnet, its field produced by the churning of liquid iron in its core, and a compass needle is a small magnet lining up with it.

In 1820 the Danish physicist Hans Christian Ørsted noticed that a compass needle twitched when he switched on an electric current nearby. Moving charges, it turned out, create magnetism. Wind a wire into a coil and pass a current through it and you have an electromagnet, as strong as you like and switchable at will. At the atomic level, permanent magnets work the same way: the electrons in iron behave as tiny circulating currents, and when enough of them line up, the whole piece is magnetised.

The discovery that lit the world

If electricity could make magnetism, could magnetism make electricity? Michael Faraday, a bookbinder's apprentice who became the greatest experimenter of his age, found the answer in 1831. A steady magnet does nothing. But move a magnet through a coil of wire, or move the coil past the magnet, and a current flows for as long as the motion lasts. He had discovered electromagnetic induction.

Nearly all the electricity in the world is made this way. In a power station, steam, water or wind spins a turbine, the turbine spins coils of wire inside powerful magnets, and current flows out along the grid. Run the process backwards, feeding current into coils inside a magnet, and the coils are pushed round: that is an electric motor, in every fan, drill, train and electric car. Transformers, which step voltages up for long-distance transmission and down for homes, are induction too, with one coil's changing field driving a current in another.

Light is electromagnetism

In the 1860s the Scottish physicist James Clerk Maxwell wrote down four equations summarising everything known about electricity and magnetism. Together they implied something new: a changing electric field creates a magnetic field, a changing magnetic field creates an electric field, and the two can chase each other through empty space as a wave. He calculated the wave's speed and found it was the speed of light. Light, he concluded, is an electromagnetic wave. Twenty years later Heinrich Hertz produced such waves with sparks and detected them across his laboratory, and radio, television, radar, mobile phones, wi-fi and X-rays all followed. Every one of them is light at a wavelength the eye cannot see.

Why it matters

Electromagnetism holds atoms together, drives every chemical reaction, carries every nerve signal in your body and makes up all the light in the universe. Harnessed, it powers civilisation: the grid, the motor, the computer, the radio and the lamp above your head trace back to a handful of nineteenth-century experiments with wires, magnets and compass needles.