How an electric circuit works
Battery, wires, switch, bulb and resistor drawn as one unbroken loop — voltage, current and resistance explained, series against parallel, and why a short circuit is the one thing you never try.
Battery: the energy source
A battery is a tin of chemistry waiting for permission. Inside, one metal is desperate to give away electrons and another is happy to take them, but the only road between them is the wire you connect. A common torch cell pushes with one and a half volts.
Voltage: the push
Voltage is the push, and it is best pictured as the height of a waterfall rather than the amount of water falling. A torch cell gives one and a half volts, a car battery twelve, and a wall socket around two hundred and thirty, which is why one is harmless and one can kill you.
Wires: the path
Wires are the road, and they are nearly always copper, because copper's outer electrons are barely attached and wander freely. The electrons themselves shuffle along more slowly than a snail, yet the push travels the whole length of the wire at close to the speed of light.
Current: the flow
Current measures how much charge passes a point each second, and it is counted in amps. One amp is roughly six million million million electrons going by every second. A phone charger delivers about one amp, while an electric kettle hauls something like ten.
Switch: opens the loop
A switch is nothing more than a deliberate gap. Closed, metal touches metal and the loop is whole. Open, the current stops everywhere at once, not just at the switch, because electricity flows only when there is an unbroken path all the way round and back again.
Bulb: the load
The bulb is the load, the thing the circuit exists for. Current squeezes through a coiled tungsten thread thinner than a hair and heats it past two thousand degrees until it glows white. An old style bulb turns only about five parts in a hundred into light, and wastes the rest as heat.
Resistor: the brake
A resistor is a deliberate narrowing of the road. It converts some of the electrical energy into heat and holds the current down to a level the rest of the circuit can survive. Every small light emitting diode you own has one hiding beside it, or it would burn out in seconds.
Ohm's law
One small equation ties the whole thing together. Current equals voltage divided by resistance. Double the push and you double the flow. Double the resistance and you halve it. Georg Ohm published that in eighteen twenty seven and was mocked for years before it was accepted as a law.
Series: one single loop
A series circuit is one single loop, so exactly the same current passes through every part in turn. Old fashioned fairy lights were wired like this, which is why one dead bulb plunged the entire string into darkness and left somebody testing every bulb on the tree.
Parallel: branches
In parallel the current splits, runs down separate branches and joins up again at the end. Each branch gets the full voltage, so every bulb burns at full brightness and switching one off leaves the rest untouched. Every room in your house is wired exactly this way.
Short circuit: the danger
A short circuit is a shortcut with almost no resistance in it. Join the two ends of a battery with bare wire and the current becomes enormous, the wire gets hot enough to start a fire, and a lithium cell can burst or catch alight. This is one to read about, never to try.
Fuses and breakers
That is precisely what a fuse is for. It is a thin wire built to be the weakest link, melting and breaking the circuit in a fraction of a second when too much current flows. A circuit breaker does the same job with a magnet and a spring, and can simply be switched back on.
Conductors and insulators
Metals conduct because their electrons roam free. Rubber, glass, dry wood and plastic grip their electrons tightly, so nothing moves. Every part of an appliance you are meant to touch is chosen to be an insulator, and the plastic sleeve on a cable is all that stands between you and the copper.
From torch to power grid
The loop inside a torch is the same loop that runs a city. Power stations take the place of the battery, pylons take the place of the wires, and every switch you flick quietly asks the grid for a little more. Understand one circuit and you understand the shape of all of them.
Watch it explain itself
Every step above is narrated aloud. Play it, or open it in the editor and make it yours — no account needed.