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Lesson 1 of 5

Charge and current

Before voltage and resistance mean anything, you need to know what's actually moving in a wire, and how fast.

What is electric charge?

Everything is made of atoms, and atoms contain tiny charged particles called electrons. In a metal wire, some of these electrons are free to drift from atom to atom. That drifting is electricity.

Charge is measured in coulombs (C). You won't often deal with a single electron's charge — instead you deal with a "packet" of charge flowing past a point, the way you'd count litres of water flowing past a point in a pipe rather than counting molecules.

Current is the rate of flow of charge

Current isn't "how much charge" — it's how much charge flows past a point per second. That's the key idea people mix up: a current can be small even if a huge amount of charge eventually moves, if it just takes long enough.

I = Q / t

Where I is current in amperes (A), Q is charge in coulombs (C), and t is time in seconds (s). One ampere means one coulomb of charge is flowing past a point every second: 1 A = 1 C/s.

Worked example

A charge of 12 C flows past a point in a wire in 4 s. What is the current?

I = Q / t = 12 ÷ 4 = 3 A

Measuring current: the ammeter

An ammeter measures current, and it's always connected in series — directly in the path the charge travels, like a tollbooth on a single road. A good ammeter has almost zero resistance of its own, so it counts the charge going past without slowing it down or changing the circuit it's measuring.

Try it Drag the sliders below. Watch the ammeter reading and the speed of the dots — they move together. More current means more charge scoops passing the ammeter every second, so the dots visibly speed up.
6.0 V 3.0 Ω V 6.0 V A 2.0 A
6.0 V
3.0 Ω
Current I = 2.0 A p.d. across R = 6.0 V

Quick check

Q A current of 0.5 A flows through a torch bulb for 2 minutes. How much charge flows through it?

Careful — the formula uses seconds, and the question gives minutes. 2 minutes = 120 s.

Q = I × t = 0.5 × 120 = 60 C