Voltage: the push that moves electricity
Voltage is the push a battery gives to electricity, like pressure in a water pipe. See what volts measure and why more push means more flow.
A battery says 1.5 V on its side. A phone charger says 5 V. The socket in the wall might say 230 V or 120 V, depending on your country. What do those numbers mean?
They measure a push. That push is called voltage, and it is the reason electricity moves at all.
The pipe picture, again
In the last lesson we pictured electricity as water in a loop of pipe. Voltage is the pressure from the pump.
A strong pump gives a big push, and the water rushes. A weak pump gives a small push, and the water trickles. In a circuit, a battery is the pump, and its voltage is how hard it pushes the charges round.
Volts, and what they measure
Voltage is measured in volts (V), named after the Italian scientist Alessandro Volta, who made one of the first batteries in 1800.
What does a volt actually count? It counts energy given to each bit of charge. One volt means each unit of charge (a coulomb) is handed one joule of energy. A 3 volt battery gives each bit of charge three times as much energy as a 1 volt battery.
| Thing | Typical voltage |
|---|---|
| A single AA battery | 1.5 V |
| A phone's USB charger | 5 V |
| A car battery | 12 V |
| Wall socket (varies by country) | 120 V or 230 V |
| A lightning bolt | millions of volts |
Voltage is always between two points
Pressure in a pipe only makes water move when it is higher at one end than at the other. It is the difference that matters.
Voltage works the same way. It is always measured between two places, such as the two ends of a battery. A bulb with the same voltage at both ends has no push across it, so nothing flows.
That is also why a battery has two ends, plus and minus. The plus end has the higher "pressure", and the minus end the lower. The charges are pushed round the loop from one to the other.
Try it
This is the pipe picture again, but now with real numbers. Change only the voltage and leave the resistance alone.
6 V ÷ 12 Ω = 0.50 AMore voltage means more current. More resistance means less.
- Set the voltage to 0. What happens to the flow?
- Double it from 3 V to 6 V. What happens to the current?
- Slide it all the way up. When does the lamp get dangerously bright?
More voltage means more current. With the pipe unchanged, twice the push gives twice the flow.
Batteries add up
Put two batteries end to end, plus to minus, and their pushes add up: 3 V + 3 V = 6 V. This is exactly what you did in the Power boost mission of Circuit Rescue. Put one the wrong way round and they push against each other.
People often mix these up. Voltage is the push. Current is the flow that the push causes. You can have a big voltage and no flow at all, like pressure in a pipe with a closed tap. Next lesson: the flow.
Words to know
- Voltage
- The push that moves charge round a circuit. Measured between two points.
- Volt (V)
- The unit of voltage. One volt gives each coulomb of charge one joule of energy.
- Battery
- A device that keeps a voltage between its two ends by turning chemical energy into electrical energy.
Check yourself
In the water-pipe picture, what does voltage match?
Show the answer
The pressure from the pump.
You join two 1.5 V batteries end to end (plus to minus). What is the total push?
Show the answer
3 V. Their pushes add up.
Can there be a voltage but no current?
Show the answer
Yes. If the loop is broken (for example, an open switch), the push is still there but nothing flows, like pressure behind a closed tap.
Why does a battery have a plus end and a minus end?
Show the answer
The voltage is a difference between two places. The plus end is at a higher "pressure" than the minus end.