A battery is a tiny chemical pump. Inside, a reaction at the two ends shoves electrons out of the minus end, through the wire, through whatever you've plugged in, and back into the plus end. The bulb glows because all those electrons crowding through it have to work hard to get past. The reaction is the push, that's what voltage means, and a bigger push moves the electrons faster and lights the bulb brighter. Inside the battery, charged bits called ions shuffle the opposite way to keep the loop complete. The reaction has only so much fuel, so as it's used up the push fades and the bulb dims. Slide the voltage in the simulator and watch the battery slowly drain.
Most people think a flat battery has run out of electrons, like an empty tank. In fact it is still packed with electrons; what has run out is the reactant chemistry doing the pushing, so the voltage collapses.
What's actually happening
The everyday picture of a battery is a little tank of electricity that slowly empties, like a bucket with a hole. That picture is wrong in a useful way. A battery doesn't store electricity at all — it stores chemicals that are eager to react, and it only lets them react if you give the electrons a path to travel. Connect a wire and the reaction starts; disconnect it and the reaction politely waits, sometimes for years.
The trick is that the battery keeps the two halves of the reaction physically apart. At one terminal, a material gives up electrons (it oxidises); at the other, a material is hungry for them (it reduces). The only way for the electrons to get from the generous side to the hungry side is the long way round — out through your wire and your bulb. That detour is the whole point. The chemical drive to react is the voltage, the pump pressure, and it pushes electrons out of the minus terminal and pulls them into the plus terminal. Meanwhile, inside the battery, the circuit is completed not by electrons but by ions drifting through the electrolyte, carrying charge the opposite way. Electrons through the wire, ions through the guts: both have to flow or nothing moves.
This is why a dead battery is such a strange object. It is still full of electrons, matter is made of them, but the specific reactants that were doing the pushing have been converted into products, and the reaction has nowhere left to go. The pump has lost its pressure. In a rechargeable battery you can shove current backwards and force the reaction to run in reverse, rebuilding the reactants, which is why a phone battery survives hundreds of cycles before the chemistry stops cooperating cleanly. A single AA cell pushes about 1.5 volts; stack them and the pushes add up, which is the only reason your TV remote and a car both run on the same idea at different scales.
A battery stores eager chemicals, not electricity, and voltage is the chemical pump pressure shoving electrons round your circuit.
- 1Push a copper coin or strip and a galvanised (zinc-coated) nail into a lemon, a few centimetres apart, without letting them touch.
- 2Clip a wire from each metal to the legs of a small LED. The acid pulls electrons off the zinc and feeds them to the copper through your wire — a complete circuit.
- 3String two or three lemons in a line, copper of one to zinc of the next, to add their pushes together until the LED glows. You've just stacked cells like Volta did.
Common questions
A flat battery is still full of electrons, because matter is made of them. What has run out is the reactant chemistry doing the pushing; the reactants have turned into products, so the pump loses its pressure and the voltage collapses.
Inside the battery, charged particles called ions drift through the electrolyte, carrying charge the opposite way to the electrons. Electrons through the wire and ions through the guts must both flow or nothing moves.
You shove current backwards through it, forcing the reaction to run in reverse and rebuilding the original reactants. That is why a phone battery survives hundreds of cycles before the chemistry stops cooperating cleanly.