Current: Charge in Motion
Give charges a push and a path, and they flow. Current measures how much charge streams past per second.
Builds on: 0.1 Electric Charge & the Atom0.2 Voltage: Energy per Charge
Defining the flow
Put a voltage across a conductor and its free electrons drift. The amount of charge passing a point per second is the electric current:
One ampere (amp, A) sounds modest until you remember what a coulomb is: at 1 A, about 6.24 × 10¹⁸ electrons cross any cross-section of the wire every second. Typical currents you’ll meet: an LED sips about 0.02 A (20 mA), a phone charger supplies a couple of amps, a kettle draws around 10 A.
Slow drift, instant signal
Here is the misconception-buster of the whole unit. The electrons themselves crawl — in a typical copper wire their average drift speed is well under a millimetre per second. Yet the light turns on the instant you flip the switch. How?
Because the wire is already packed full of free electrons, end to end. Flipping the switch applies a field that starts all of them moving at once — like a tube completely full of marbles: push one in at this end and one pops out the far end immediately, even though each marble barely moved. The push travels near the speed of light; the electrons themselves amble.
Which way does it flow?
Benjamin Franklin guessed the direction of flow before anyone knew about electrons — and guessed wrong. By convention, current flows from + to − (“conventional current”), while the electrons actually drift from − to +. Engineers everywhere use conventional current; every formula, every datasheet, every arrow on every circuit symbol assumes it. It works perfectly, because negative charge moving left is mathematically identical to positive charge moving right. Follow the convention and forget the embarrassment.
Charge doesn’t pile up in a wire or vanish at the end — it circulates. Current only flows when there is an unbroken conducting path from one battery terminal, through the circuit, back to the other terminal. Break the path anywhere and the current stops everywhere. This is why the next unit is about circuits: complete loops are where electricity does its work.
⚡ Lab — The Charge Counter
A magnified view inside a copper wire, with a counter across one cross-section.
- Set 1 A and watch the electron counter — that’s 10¹⁸ electrons per second.
- Turn the current to zero. The electrons stop; the counter holds its total.
- Note the two arrows: conventional current one way, electron drift the other.