Electronics
What is voltage?
Voltage is the electrical push that makes charge flow round a circuit. Its proper name is potential difference, it's measured in volts (V), and one volt means one joule of energy for every coulomb of charge. More voltage across the same circuit drives more current.
A hill for charge
Picture a ball at the top of a hill: the higher the hill, the harder it pushes to roll down. A battery lifts charge up to its + end. From there the charge wants to get back down to the − end, through whatever you connect between them.
Voltage is always a difference between two points. That's why a meter has two leads, and why "the voltage at" a point really means its voltage measured from a reference, usually the − end.
Live experiment
Turn up the push
A battery, a 330 Ω resistor and Pip, an LED, in one loop. Step the battery from 1.5 V up to 24 V: below about 2 V Pip stays dark, 9 V gives a bright 21 mA, and too much push burns him out.
“Voltage: the push” is lesson 2 of 6 in Copper Flats, Electronics World 1. Free with an account. Lessons unlock in order, so you’ll do the one before it first.
What a volt really is
Every coulomb that passes through a 9 V battery picks up 9 joules of energy and gives it away on the trip round. Energy, charge and voltage are linked by W = Q × V, so moving 2 C through 9 V transfers 18 J.
Everyday voltages
- An AA or AAA cell: 1.5 V
- A phone's lithium-ion battery: about 3.7 V
- A USB port: 5 V
- A "9 V" battery: 9 V
- A car battery: about 12 V
A red LED needs about 2 V before it lights at all, which is why one 1.5 V cell can't light it and two can.
Cells in series add up
Connect cells end to end, + to −, and their voltages add: two AA cells make 3 V, which is why a TV remote holds two. Flip one cell round and it pushes the other way, subtracting its voltage instead.
Voltage alone doesn't decide the current. That depends on what the push is pushing through: its resistance. Ohm's law ties the three together.
Free lesson
Now build it yourself
Each lesson comes in three short parts: see the idea, try it on a live board, then build it yourself, with quick checks as you go.
“Voltage: the push” is lesson 2 of 6 in Copper Flats, Electronics World 1. Free with an account. Lessons unlock in order, so you’ll do the one before it first.
Or start from the very first lesson, no account neededWork it out
Practice questions
Three quick questions from the lessons. Pick an answer to check it.
Show the answer
Answer: A push between two points. Voltage is the push between two points, like the two ends of a battery. The bigger the push, the harder charge is driven round the loop.
- Why not “The colour of the wire”? Wire colours are only a labelling convention, like red for + and black for −. They don't change the voltage.
- Why not “How fast charge moves”? That describes the flow, which is current. Voltage is the push that drives it.
- Why not “How much charge a wire holds”? Voltage isn't an amount of charge. It's the difference in push between two points, like the two ends of a battery.
Show the answer
Answer: Twice as hard as one. Stacked + to −, each cell adds its push. Two cells push twice as hard as one, which is why a TV remote holds two.
- Why not “Just as hard as one”? That's what two cells side by side, in parallel, give: the same voltage, lasting longer. Stacked + to −, the pushes add.
- Why not “Not at all”? Two cells only cancel when one is flipped round. Facing the same way, + to −, they push together.
- Why not “Half as hard as one”? Cells in series add their pushes. Two 1.5 V cells + to − give 3 V, not 0.75 V.
Show the answer
Answer: 0 V. The flipped cell pushes the other way, so the two pushes cancel. 1.5 − 1.5 = 0 V, and nothing flows.
- Why not “1.5 V”? The flipped cell doesn't just sit idle. It pushes the other way with its own 1.5 V and cancels the first.
- Why not “3 V”? You'd get 3 V only with both cells facing the same way, + to −. Flipped, the second one pushes against the first.
“Voltage: the push” is lesson 2 of 6 in Copper Flats, Electronics World 1. Free with an account. Lessons unlock in order, so you’ll do the one before it first.
Questions people ask
What is voltage in simple terms?
Voltage is the electrical push that drives charge round a circuit. Its proper name is potential difference, and it's measured in volts (V). More voltage across the same circuit means more current flows.
What does 1 volt mean?
One volt is one joule of energy for every coulomb of charge. So each coulomb that passes through a 9 V battery picks up 9 joules, and gives that energy away to the parts on its trip round the circuit.
Is voltage the same as current?
No. Voltage is the push, in volts. Current is the flow it causes, in amps. They're linked by the resistance in between: Ohm's law, V = I × R.
Why do two AA batteries make 3 V?
Cells connected end to end, + to −, add their voltages. Two 1.5 V cells give 3 V, four give 6 V. Turn one cell round and it pushes the other way, subtracting its voltage instead.
Related topics
- What is electric current?How much charge flows past a point each second, in amps, and why it's the same all round a loop.
- Ohm's law explainedV = I × R: set any two of voltage, current and resistance and the third follows.
- Voltage divider explainedTwo resistors that split a voltage: the formula, potentiometers, and why a load drags the output down.