What really happens when you flip a switch?

1Charge

What is everything made of?

Everything around you is made of atoms, far too small to see. A single grain of salt holds about a billion billion of them.

Each atom has a small, heavy middle, called the nucleus, with much lighter electrons around it. What keeps the electrons there is a property called electric charge. It comes in two kinds, which we call plus and minus. The nucleus is plus. Electrons are minus.

Charges follow one rule. Opposites pull toward each other. Matching charges push each other away. Drag these two around, and tap one to flip its charge:

Flip the right one Start again

A plus and a minus. They pull toward each other.

Watch the arrows as you bring them together. Halve the distance, and the pull gets four times stronger. Bring them ten times closer, and it’s a hundred times stronger.

That pull is what holds every atom together. The pushes and pulls between the atoms hold your chair together too, and stop you from sinking through it.

I’ve drawn the charges as little balls. Real electrons are more like a blur around the nucleus, but balls will do for everything in this guide.

Opposites pull. Matching charges push apart. Closer means much stronger.

2Static

Why does a balloon stick to your hair?

If charges push and pull this hard, why don’t we feel them all the time?

Because they cancel out. An atom normally has just enough electrons to match the plus charge of its nucleus. Up close the pushes and pulls are strong, but from a little way off, the plus and the minus balance. Nearly everything around you is balanced like this, so things ignore each other.

Unless you move some electrons. Rub a balloon on your hair, and some electrons rub off the hair and onto the balloon. Drag the balloon back and forth over the hair, then lift it away:

Start again

Rub the balloon on the hair.

Now the balloon has more electrons than plus charge, so it’s minus overall. The hair has lost some, so it’s plus. The two pull on each other. And every hair is plus, so the hairs push each other apart and stand on end.

Each little sign in the picture stands for a huge number of electrons. Even so, a rubbed balloon moves only a tiny fraction of the hair’s electrons. A tiny fraction is plenty.

A static shock is the same thing. Walk across a carpet on a dry day and your body picks up extra electrons. As your finger nears a metal door handle, they jump across in a tiny spark. Lightning is a static spark on the scale of a storm cloud.

Electricity starts when charges are pulled out of balance.

3Metals

Why are wires made of metal?

The balloon’s extra electrons stay where they were rubbed on. To carry electricity anywhere, electrons have to be able to move.

In most materials they can’t. Each electron belongs to an atom and stays close to it. Metals are different. In copper, every atom lets go of one electron. Those loose electrons wander through the whole piece of metal, in the gaps between the atoms. So a lump of copper is a fixed frame of atoms, filled with loose electrons.

Bring a charged balloon near a copper bar, then switch to plastic and try again:

Copper Plastic

Drag the balloon toward the bar.

The balloon’s extra electrons push the copper’s loose electrons away. They crowd into the far end, and the near end, now short of electrons, is left plus. In plastic, nothing goes anywhere. Each electron leans away a little and stays with its atom.

Materials full of loose electrons are called conductors: copper, aluminum, silver, gold. Materials without them are insulators: plastic, rubber, glass, dry wood. A cable is both. Copper inside to carry the electrons, plastic outside to keep them in, and to keep your fingers off the copper.

A metal is full of loose electrons. That’s what makes a wire a wire.

4Current

What is an electric current?

Let’s look inside a copper wire. The orange circles are copper atoms, fixed in place. The blue dots are the loose electrons.

Even with nothing connected, the loose electrons are never still. They race around at about 1,600 kilometers every second, which would take you across India in two seconds. They crash into atoms and bounce off in new directions. But they go every which way, so on average they get nowhere. (Why they move this fast is a story from quantum physics, and we’ll skip it.)

Now give them a push. Drag the slider, and count the electrons crossing the dashed line:

Push

No push. As many electrons cross to the left as to the right.

With a push, each electron still jiggles as fast as before. But between crashes, it gets nudged a little to the right. The nudges add up to a slow drift. Follow the electron with the yellow ring: it still zig-zags, but it creeps to the right.

That drift is an electric current. We measure it by counting how many electrons cross a line each second. The unit is the amp, and one amp means about 6 billion billion electrons crossing every second. A phone charger cable carries 2 or 3 amps.

In the picture, I’ve slowed the jiggle way down and sped the drift up, or you’d never see the drift at all. In a real wire, the jiggle is about ten billion times faster than the drift.

A current is a slow drift, riding on a fast jiggle.

5Speed

If electrons are so slow, why is the light instant?

How slow is the drift? Take a copper wire a little over a millimeter thick, carrying one amp. Its electrons drift about 0.07 millimeters per second. That’s slower than the tip of a clock’s minute hand. In an hour, they cover about a quarter of a meter.

So when you flip a switch, the electrons that light the bulb can’t be ones that started at the switch. They’d take hours to get there.

The answer is that the wire is already full. It’s packed with loose electrons from the switch all the way to the bulb. Push the electrons at one end, and they push their neighbors, which push theirs, all along the wire. The push travels, even though each electron barely moves.

Here’s the same idea with marbles in a tube. Push one more in at the left, then try it with the tube half empty:

Push one in

Packed full Half empty

The tube is packed full.

When the tube is packed, a marble drops out of the far end the moment you push. The yellow marble you pushed in didn’t travel there. The push did. With gaps in the tube, each marble has to roll before it bumps the next, and the far end waits.

A wire is always packed full, and its push travels at more than half the speed of light. It crosses a two-meter lamp cord in about a hundredth of a millionth of a second.

Marbles push by touching. Electrons push each other with their charge, from a distance. That’s why the push in a wire is so much faster than in a tube of marbles, and it’s the one way this picture is wrong.

The electrons are slow. The push is fast.

6Voltage

What pushes the electrons?

So far, the push has come from a slider. In real life, it often comes from a battery.

Inside a battery, chemicals move electrons from one end to the other. Extra electrons crowd into the flat end, marked minus. The end with the bump, marked plus, is left short of them. Join the two ends with a wire, and the crowd at the minus end pushes the wire’s loose electrons along, toward the plus end.

As electrons arrive at the plus end, the chemicals carry them back through the battery to the minus end. So a battery doesn’t make electrons. It keeps them going round, until its chemicals are used up.

How hard a battery pushes is called its voltage, measured in volts. An AA battery gives 1.5 volts. Put two in a row, and you get 3.

A good way to picture voltage is as height. The battery is a lift that carries each electron up a hill. The electron then rolls back down, through the wires and the bulb, giving up the energy the lift gave it. Add batteries to make the hill taller:

1 battery 2 3 4

1.5 volts. The bulb glows dimly.

A taller hill does two things. Each electron brings more energy down through the bulb, and a harder push means more electrons arrive every second. So the bulb gets brighter on both counts. The hill is only a picture: the electrons don’t really climb anything, but each one carries more energy.

Some voltages you meet

An AA battery	1.5 volts
A USB phone charger	5 to 20 volts
A car battery	12 volts
The socket in your wall	230 volts in India, Europe and most of the world; 120 in North America
A static spark you can feel	about 3,000 volts or more
Lightning	about 300 million volts

One odd thing. On circuit diagrams, the arrows for current point from plus to minus, the opposite way to the electrons. Benjamin Franklin chose which kind of charge to call plus in 1747, a hundred and fifty years before anyone knew electrons existed. He had a fifty-fifty chance, and the moving charges turned out to be the minus ones. Nobody wanted to redraw every diagram, so the old arrows stayed.

Voltage is the push. Current is the flow.

7Difference

Why can a bird sit on a power line?

Height has a catch. It’s always measured from somewhere. A ball on a high table won’t roll anywhere unless there’s a slope, a lower place right next to it.

Voltage works the same way. It’s a difference between two places, and electrons only flow when there’s a difference to roll down. The wires on a pole can be 11,000 volts above the ground, yet birds sit on them all day. Drag the bird:

Both feet on the same wire. No difference between them, so no current.

With both feet on one wire, each foot is at the same voltage, so nothing pushes electrons through the bird. Spread across two wires, the bird has a difference of 11,000 volts across its body, and current rushes through it. Big birds with wide wings really do get hurt this way, which is why many power companies space their wires apart or cover them.

It’s also why you must never touch a fallen power line, or anything touching one. Your feet are on the ground, and the ground is at zero volts. The whole difference would be across you.

Voltage is a difference. No difference, no flow.

8Resistance

What slows the current down?

Hook the same battery up to different wires, and some let through far more current than others. Why?

Remember the crashes. The push nudges each loose electron along, then it crashes into an atom and loses its progress. The more crashes, the slower the drift. This holding back is called resistance.

Change the wire, and watch the meter:

Length

Thickness

Copper Iron Nichrome

A longer wire gives a weaker push to each bit of it, so every electron drifts more slowly. A thicker wire has more room, so more electrons can drift at once. And some metals crash more than others. Iron resists about six times as much as copper. Nichrome, the metal in toasters and heaters, resists about 65 times as much.

Resistance is measured in ohms, and it gives us a simple rule. The current, in amps, is the voltage divided by the resistance. A 3-volt battery on a 10-ohm bulb pushes 0.3 amps through it. Double the volts, double the amps. Double the ohms, half the amps.

More push, more flow. More resistance, less flow.

9Power

Why does a bulb get hot, but not its wire?

Every crash gives an atom a little kick. The atoms shake harder, and shaking atoms are hot atoms. So resistance turns the electrons’ energy into heat.

An old-style bulb uses this on purpose. Inside the glass is a filament, a coil of tungsten wire thinner than a hair. The same current flows through every part of the loop, because it has nowhere else to go. But the thick copper wires barely resist it, while the thin filament resists a lot. So nearly all the crashes, and nearly all the heat, happen in the filament.

Turn the dimmer:

Dimmer

At full power, the filament reaches about 2,500 °C and glows white. Tungsten melts at 3,422 °C, so it runs close to the edge. The copper wires carry the same current and stay cool.

Volts times amps is called power, measured in watts. It tells you how fast the energy arrives. A phone charger gives 5 volts at 2 amps, which is 10 watts. A filament bulb uses 60. A kettle uses about 2,000.

LED bulbs skip the heat. In an LED, each electron drops down a small energy step and gives off its energy as light, directly. That’s why a 9-watt LED bulb is about as bright as a 60-watt filament bulb, and stays cool enough to touch.

Your electricity bill counts energy: watts multiplied by time. A thousand watts for one hour is a kilowatt-hour, which an Indian electricity bill calls a unit. Boil a 2,000-watt kettle for three minutes, and you’ve used a tenth of a unit.

Volts times amps is watts: how fast the energy arrives.

10Circuits

Why does it need a loop?

Every electron that leaves the battery’s minus end has to come back to its plus end. If there’s a gap anywhere, electrons pile up at the gap and push back. Within a blink, the flow stops everywhere, not only at the gap. That’s all a switch is: a gap you can open and close.

A loop like this is called a circuit. Build some. Pick a layout, tap the switch, and tap a bulb to unscrew it:

One bulb Two in a row Side by side

Add a shortcut wire

Two bulbs in a row share the push. Each gets half the voltage and half the current, so each gives a quarter of the power, and glows dimly. Unscrew either one and the gap stops the flow through both. Old strings of fairy lights were wired like this, which is why one dead bulb killed the whole string.

Side by side, each bulb gets the battery’s full push, so both shine as brightly as a bulb on its own. Unscrew one and the other doesn’t notice. The battery has to work twice as hard, though, so it runs down twice as fast. Every light and socket in your home is wired side by side, which is why switching off the TV doesn’t switch off the fridge.

Now add the shortcut. A plain wire has almost no resistance, so nearly all the current takes the shortcut and the bulb goes dark. The current shoots up, and the wires and the battery get hot. In a home this is called a short circuit, and it can start fires. That’s what fuses and circuit breakers are for. When the current gets too big, they open the loop.

In this lab, each bulb has a fixed resistance. Real filament bulbs resist less when they’re cold, but the pattern is the same.

No loop, no flow.

11Generators

Where does the push come from?

Batteries are handy, but small. A city runs on a different trick, found by Michael Faraday in 1831.

Move a magnet near a coil of wire, and the loose electrons in the wire get pushed. Drag the magnet into the coil and out again, at different speeds:

Spin it

Drag the magnet into the coil.

The push only lasts while the magnet moves. Move it faster, and the push is bigger. Hold it still, even deep inside the coil, and the needle drops back to zero. Pull it out, and the push goes the other way.

Now press Spin. A spinning magnet swings past the coil again and again, so the push swings one way, then the other, then back. That’s called alternating current.

Almost every power station works like this. Coal, gas and nuclear plants boil water, and the steam spins a turbine, which spins a magnet inside huge coils of copper. Dams and wind turbines spin the magnet directly. Solar panels are the odd one out. In a solar panel, light knocks electrons loose directly, with no magnet at all.

Move a magnet past a wire, and its electrons get pushed.

12The grid

What comes out of the wall?

Because the generators spin, the push from your wall socket swings back and forth. In India and most of the world, it makes 50 full swings every second. In North America, 60.

So the electrons in your lamp cord never travel anywhere. They shuffle back and forth, each less than a thousandth of a millimeter, hundreds of times thinner than a hair. The bulb doesn’t mind. A filament heats up whichever way the electrons crash through it.

The swinging also makes the push easy to change. A transformer, two coils of wire wrapped around an iron ring, can turn a low voltage into a high one and back again. It only works on a push that keeps changing.

Why would you want a higher voltage? Because of the trip. Power stations are often hundreds of kilometers from the cities they serve. The long wires in between have resistance, so they heat up, and every bit of heat is energy lost on the way.

The heat depends on the current, and steeply: twice the current makes four times the heat. So the trick is to send the same power with less current. Watts are volts times amps, so raise the volts and the amps go down. Send power to this town at different voltages:

230 V 11,000 V 66,000 V 400,000 V

At 400,000 volts, the town’s power needs only 25 amps, and almost nothing is lost. That’s why the tall steel towers carry hundreds of thousands of volts. Near your street, another transformer brings it back down to 230.

The town and the line here are made up, but the pattern is real.

High voltage for the trip, low voltage for the home.