Turn on a tap and water rushes out. Nobody is pouring it. The water is pushed out by the water behind it. Electricity needs a push too, and that push has a name: voltage.
What is being pushed?
Wires are made of metal, and metal is full of tiny particles called electrons. They are far too small to see, even with a microscope.
When something pushes on these electrons, they start to drift along the wire, all together. This flow of electrons is called electric current.
Where does the push come from?
A battery has two ends. One is marked − (minus) and the other + (plus). Inside the battery, a chemical reaction crowds electrons together at the minus end and takes them away from the plus end.
Electrons carry a negative charge, and they push each other away. So the crowded electrons at the minus end want to escape to the plus end, where there are too few. Connect the two ends with a wire, and off they go around the loop.
How strong this push is — that is the voltage. We measure it in volts, written V. The name comes from Alessandro Volta, an Italian scientist who built the first battery in 1800. You can meet him in the history of electricity.
A push between two places
Voltage works the same way. It is always a difference between two points, like the two ends of a battery. No difference means no flow.
That is why birds can sit on a power line without getting hurt. Both of their feet touch the same wire, so there is no difference in voltage between them. Read more in why birds are safe on wires.
Small and big voltages
| Where | Voltage |
|---|---|
| One AA battery | 1.5 V |
| A USB phone charger | 5 V |
| A car battery | 12 V |
| A wall socket in Europe | 230 V |
| The biggest power lines in Slovakia | 400,000 V |
| A bolt of lightning | hundreds of millions of volts |
More push, more flow
Take the same wire and push twice as hard: double the voltage. Now twice as many electrons flow through it every second.
How much a wire or a lamp holds the electrons back is called resistance. The rule that connects voltage, current and resistance is Ohm’s law.
Go deeper: What a volt really measures for grown-ups and the extra curious
Voltage tells how much energy each bit of electric charge gains or loses between two points. One volt means one joule of energy for every coulomb of charge:
A coulomb (C) is the charge of about 6.2 billion billion electrons (). So a 1.5 V battery gives every coulomb that travels around the circuit 1.5 joules of energy. The bulb takes that energy and turns it into light and heat.
The exact name for voltage is potential difference. It compares the electric potential at two points, the same way the height of two places on a hill can be compared.
Go deeper: Where the water picture goes wrong for grown-ups and the extra curious
- A battery does not store electrons and then run out of them. The electrons are already in the wires; the battery keeps pushing them around the loop. What gets used up is the battery’s chemical energy.
- The push travels along a wire almost as fast as light, but each electron only creeps forward — often more slowly than a snail.
- Water pours out of a cut pipe, but electrons do not pour out of a cut wire. Without a closed loop, they simply stop moving.
Check yourself
Why doesn't a bird sitting on a power line get a shock?
Show the answer
Both feet touch the same wire, so there is no difference in voltage — Electricity only flows when there is a difference in push between two points. Both feet are on the same wire, so there is no difference.
Electricity only flows when there is a difference in push between two points. Both feet are on the same wire, so there is no difference.