Flip a switch, and a lamp lights up at once. Something rushes through the wire — but you can’t see it, hear it or smell it. What is it? Let’s find out, starting from the very beginning.
What is moving?
Everything around you is built from tiny pieces called atoms. Around the middle of every atom move even tinier particles: electrons.
In metals, like the copper inside a cable, some electrons are not tied to one atom. They wander freely between the atoms, like a crowd of people in a big hall.
Now give them all a push in the same direction. The whole crowd starts to drift along the wire, together. That drift is electric current: electrons flowing through a wire.
What pushes them?
A battery or a socket pushes the electrons. We call that push voltage.
Electrons carry a negative charge. A battery crowds electrons at its minus end and leaves too few at its plus end. The crowded electrons are pushed away from the minus end, through the wire, towards the plus end.
It only flows in a loop
Electrons can only flow if they have a complete path to travel around and back to the battery. This loop is called an electric circuit.
A switch is a small bridge in the loop. Close it, and the loop is complete, so current flows. Open it, and there is a gap. The electrons stop, and the lamp goes dark.
Why does the lamp light up at once?
Here is a surprise: each electron moves very slowly — often more slowly than a snail! So how can a lamp light up the moment you flip the switch?
A wire is like that tube: it is already full of electrons. When the push starts, electrons all along the loop start moving at almost the same moment. So the lamp lights up at once.
What can current do?
When current flows, it carries energy from the battery or the power station to whatever is in the loop. That energy can be turned into:
- light — in lamps and screens,
- heat — in kettles, toasters and irons,
- movement — in fans, drills and electric cars,
- sound — in speakers and headphones.
How fast a device uses that energy is its power. Some materials make it harder for current to flow; that is called resistance, and it turns some of the energy into heat.
How strong is a current?
We measure current in amperes, or amps for short, written A. The name comes from the French scientist André-Marie Ampère.
| What | About how much current |
|---|---|
| An LED lamp | 0.04 A |
| A phone charging | 1–2 A |
| A kettle | 9 A |
| A bolt of lightning | 30,000 A |
Want to know how these numbers are worked out? Read how we calculate electric current. And to see what current does in bigger circuits, read how electric current behaves.
Go deeper: Which way does current flow? for grown-ups and the extra curious
Electrons flow from the minus end of a battery to the plus end. But on circuit diagrams, current is drawn flowing the other way, from plus to minus. This “conventional current” was chosen in the 1700s, before anyone knew about electrons. Scientists guessed that something positive was moving. Both descriptions give the same answers in calculations, so the old habit stayed. Read more in the history of electricity.
Go deeper: How slow are the electrons, really? for grown-ups and the extra curious
In a household cable carrying a few amps, electrons drift forward at only about a tenth of a millimetre per second. They also jiggle randomly at enormous speeds, but those random movements cancel out. What travels fast is the push: an electric field that spreads along the wire at a large fraction of the speed of light. In a wall socket, the current also swings back and forth 50 times every second — that is alternating current — so the electrons mostly shuffle back and forth on the spot.
Check yourself
Why does a lamp light up the moment you flip the switch, even though electrons move slowly?
Show the answer
The wire is already full of electrons, and they all start moving at once — Like marbles in a full tube, the push travels through the electrons that are already in the wire, so the lamp reacts almost instantly.
Like marbles in a full tube, the push travels through the electrons that are already in the wire, so the lamp reacts almost instantly.