Electricity

Where does the charge on a rubbed balloon come from? Every atom holds positive protons in its centre and light, negative electrons around the outside. Normally the two balance, so the atom as a whole is neutral. Rubbing presses two surfaces into close contact, and some loosely held outer electrons cross from one material to the other.

Rub a rubber balloon on wool and electrons pass from the wool to the balloon. The balloon gains them and turns negative; the wool has lost them and turns positive, by exactly the same amount. The protons stay where they are. In everyday static, only electrons move.

So rubbing creates no charge at all. It moves charge between materials, and the total stays the same. This rule, that charge can be moved around but its total never changes, is called conservation of charge.

Shuffle across a nylon carpet on a dry day and your body collects extra charge. Reach for a metal doorknob and, with a zap, the built-up charge has suddenly escaped, leaping the last gap of air as a tiny spark. That jolt is a discharge: stored charge rushing to where it can balance out.

A thunderstorm does the same thing on a vast scale. Updrafts carry small ice crystals up while heavier pellets of soft hail fall, and the two collide over and over. Each collision separates charge, so one region of the cloud turns positive and another negative. When the imbalance grows too great, the air gives way and lightning carries the charge across.

In 1752 people proved the two are the same. Sparks were drawn from storm clouds down a tall iron rod in France, then down the wet string of Benjamin Franklin's kite: lightning is electricity.

When charge flows steadily instead of leaping in a spark, it makes an electric current. In a metal wire the charges that move are electrons, drifting along the wire together. Current is measured in amps: one amp means about six billion billion electrons passing a point every second.

Here is a puzzle. Those electrons drift remarkably slowly, typically less than a millimetre per second, slower than a snail. So why does a lamp light the instant you flip the switch? Because the wire is already full of electrons from end to end. Close the switch and a push races along the wire at close to the speed of light, and every electron in the loop moves at once, the way a whole row of marbles shifts when you nudge one end. The electrons already inside the lamp start working straight away.

Why does copper carry current easily while rubber stops it dead? The difference lies in how tightly each material holds its electrons. In a metal, every atom lets go of one or more of its outermost electrons. These free electrons belong to no atom in particular and wander through the whole metal like a shared sea, so the slightest push sets them flowing. That makes metals good conductors.

In rubber, glass and most plastics, every electron stays bound to its own atom. With nothing free to move, current cannot flow, and these materials are called insulators. That is why the copper core of a cable wears a plastic coat, and why the handles of an electrician's tools are wrapped in rubber.

Electrons in a wire will not flow on their own; something has to push them. That push is voltage, measured in volts. A useful picture is water in a pipe: voltage is like the pressure a pump supplies, and current is how much water flows past each second.

Voltages in daily life differ a lot. An AA battery gives 1.5 volts, a car battery 12, and the mains supply at a wall socket in Europe about 230. The mains differs in another way too. A battery pushes current steadily in one direction, which is called direct current. The mains push swings back and forth 50 times every second, driving the electrons to and fro in what is called alternating current. Power-station generators make it naturally, and its voltage is easy to step up or down.

For most of the 1700s, electricity meant sparks: a charge built up, then gone in an instant. That changed in 1800, when the Italian physicist Alessandro Volta described a stack of zinc and copper discs, each pair separated by cloth soaked in salty water. Join the top of the stack to the bottom with a wire, and a current flowed and kept flowing. Volta's pile was the first battery, and the volt is named after him.

Inside, a chemical reaction between the metals and the salty water pushes electrons out of one end of the stack and draws them in at the other. A modern battery works the same way. It stores chemical energy and turns it into electrical energy as the current runs, until its chemicals are used up and it goes flat.

Put the same voltage across two different wires and different currents flow. A thick copper wire lets electrons through easily. A thin wire of a poorer conductor, such as the nichrome in a toaster, makes them struggle, because they keep bumping into its atoms. How strongly a wire or any other part opposes the flow of current is its resistance, and it is measured in ohms.

Resistance is not simply a nuisance; engineers choose it. A small part called a resistor is put in a circuit to limit how much current reaches a delicate component, and a heater's coil is made resistive on purpose. Household wiring, by contrast, is thick copper because its resistance is low, so little of the push is wasted along the way before it reaches the appliance at the end.

In 1827 the German physicist Georg Ohm published a simple rule that still runs nearly every circuit calculation. For a wire kept at a steady temperature, the current equals the voltage divided by the resistance. Double the push and the current doubles; double the resistance and the current halves.

The rule turns electricity into arithmetic. Put the 230 volts of the mains across a heater element with a resistance of 23 ohms, and 10 amps flow through it. Drawn as a graph with the resistance held fixed, current rises with voltage along a straight line that starts at zero: no push, no flow. That straight line is how an engineer checks that a part obeys Ohm's law. A filament lamp, whose resistance climbs as it heats up, bends the line into a curve.

Whenever current flows through resistance, the moving electrons jostle the atoms they pass, and electrical energy turns into heat. A kettle, a toaster and an electric heater do this on purpose. An old filament bulb did it by accident: its thin wire glowed white-hot, but about 90 per cent of the energy it drew became heat rather than light.

More current means more heat. Electrical power, the energy used each second, is voltage times current, measured in watts. A 3,000-watt kettle on 230-volt mains draws about 13 amps, which is why its lead is thick. If a fault lets far more current flow, wiring could overheat and start a fire. A fuse prevents that. It holds a short, thin wire made to melt first, breaking the circuit before anything else gets dangerously hot.

A battery and a bulb joined by a single wire do nothing. Current flows only around a circuit: an unbroken loop that leaves one end of the battery, passes through the wire and the bulb, and returns to the other end. Electrons are not used up on the way. The same electrons go round and round, and what the bulb takes from them is energy.

Break the loop anywhere and the whole current stops at once. That is all a light switch does. Flicking it off pulls two metal contacts apart, leaving a small gap of air between them. Air is an excellent insulator, so a few millimetres of it are enough to stop the flow. Flick the switch back on and the contacts touch, the gap closes, and the loop is whole again.

Most circuits hold more than one device, and there are two basic ways to connect them. In a series circuit the parts sit one after another on a single path, so the same current passes through each in turn. Old strings of Christmas-tree lights were wired this way, and they had a famous flaw: when one bulb burned out, its break opened the only path, and every light on the string went dark.

In a parallel circuit each device sits on its own branch between the same two supply wires. The current divides among the branches, and each branch gets the full voltage. Switch one off, or let one fail, and the others carry on. The sockets and lights in a house are wired in parallel, which is why unplugging the toaster does not switch off the fridge.

A bird can perch on a bare overhead power line at thousands of volts and come to no harm. The reason is that a high voltage alone drives nothing. Current flows only between two points at different voltages, and only if there is a path between them. What matters is the voltage difference.

Both of the bird's feet grip the same wire, so both are at the same voltage. With no difference across its body, no current passes through it. The danger comes when something bridges two different voltages. A large bird that touches the line with one wing and a grounded pole with the other completes a path to the ground, and so does a person who touches a live wire while standing on the earth. Then current flows through the body, and it can kill.

Your body runs on electrical signals too, though not by pushing electrons down wires. A nerve cell sends its message as a travelling pulse of voltage, made by charged atoms crossing its outer skin. That is far slower than a push along copper. The fastest nerve fibres, wrapped in an insulating layer of fat, carry signals at up to about 120 metres per second; thin unwrapped fibres manage about one.

The heart is electrical as well. Each beat is triggered electrically, starting in a small patch of pacemaker cells, which fire a wave of charge that sweeps across the heart muscle and makes it squeeze in order. If that wave breaks into chaos, the heart only quivers and pumps almost no blood. A defibrillator's single strong shock stops the chaos all at once, giving the pacemaker cells the chance to restart a normal beat.