Voltage: Energy per Charge
Separating charges stores energy, like lifting a weight. Voltage measures how much energy every coulomb gets.
Builds on: 0.1 Electric Charge & the Atom
Pushing charges apart stores energy
Coulomb’s law from the last lesson cuts both ways. Opposite charges pull toward each other — so if you drag them apart, you have to do work against that pull, exactly like lifting a rock against gravity. That work doesn’t vanish: it is stored as electric potential energy. Let go, and the charge “falls” back, converting that stored energy into motion.
Here’s the key move: instead of talking about the total energy (which depends on how much charge you have), we talk about the energy per unit of charge. That quantity is voltage, also called potential difference:
A 9 V battery gives every coulomb that passes through it 9 joules of energy. A 1.5 V AA cell gives 1.5 joules per coulomb. Notice what voltage is not: it is not a thing that flows, and it is not energy by itself. It is a measure of how hard charges are being pushed from one point to another.
Always between two points
“The voltage at this wire is 5 V” is secretly a comparison — 5 V relative to somewhere else. Voltage is always measured between two points, like height: the top of a ladder is 2 m above the floor, but 0 m above the top of the ladder. In circuits we pick one reference point, call it ground (0 V), and quote everything relative to it.
What a battery actually does
A battery is a chemical charge pump. Reactions inside push electrons toward one terminal (making it negative) and pull them from the other (making it positive), and keep pumping until the potential difference between the terminals reaches the battery’s rated voltage. Connect a wire path between the terminals and the pump drives charge around the loop, spending its chemical energy at a rate set by the circuit. The battery doesn’t store electrons — it stores energy and hands it to each coulomb passing through.
Voltage is like water pressure: a taller water tower pushes water harder through a pipe. It’s a genuinely useful mental picture — higher voltage, stronger push. Just remember the electrons don’t get “used up” any more than water disappears in a water wheel; it is the energy that gets delivered.
Fields: how the push travels
Between two oppositely charged plates there is an electric field — at every point in the gap, a positive charge feels a force from the + plate toward the − plate. The field is the invisible machinery behind voltage: a charge moving with the field gains energy, and moving it against the field costs energy. In the lab below you can feel this directly.
⚡ Lab — The Potential Playground
Two charged plates make a uniform field. Drag the test charge around, then let it fly.
- Drag the charge close to the + plate — watch its potential energy climb.
- Move it straight up and down along a dashed line. Does its energy change?
- Press Release at different voltages — higher voltage, harder launch.