How does electricity flow?
The short answer
A wire is already full of free electrons. A battery pushes all of them round a closed loop at once, so the light comes on immediately, even though each electron only drifts about 0.04 mm a second in a 1 mm² copper wire carrying 0.5 A. The current is the voltage divided by the resistance: double the voltage and it doubles, add resistance and it falls. In a bulb's thin filament the electrons move faster and collide with its atoms, turning energy into heat and light.
Chapters and transcript
- 0:00
Flip the switch. The light comes on at once.
Close the switch and the bulb lights at once.
- 0:13
Inside, the wire is already full of electrons.
A copper wire is full of free electrons, jittering at random between its fixed atoms.
- 0:19
Each one drifts along at a crawl: 0.04 mm a second.
In a 1 mm² copper wire carrying 0.5 A, each electron drifts about 0.04 mm a second, shown here at real speed against the scale bar. The speed follows from copper's 8.5 × 10²⁸ free electrons per cubic metre (HyperPhysics).
- 0:27
Leave the light on for an hour: it moves 13 cm.
At that speed an electron moves about 13 cm in an hour.
- 0:34
So how does the light come on at once?
If the electrons crawl, how does the light come on at once?
- 0:41
The push runs round the loop at nearly light speed.
The battery's push, the change in the electric field, travels along the wire at nearly the speed of light. The overview speeds the drift up; the push is drawn slowed down.
- 0:47
Every electron in the loop starts moving at once.
Every electron in the loop starts drifting almost at the same moment, so the electrons already in the bulb start moving straight away.
- 0:55
Double the push, the voltage: twice the current.
Current is voltage divided by resistance. With 3 V instead of 1.5 V across the same 3 Ω bulb, the current doubles from 0.5 A to 1 A, and the bulb is brighter. These are the film's own example values.
- 1:07
Add resistance, and the current falls.
A second bulb doubles the resistance to 6 Ω, and the current falls back to 0.5 A.
- 1:18
In the thin filament, they squeeze through faster.
The filament is much thinner than the wire. The same current has to pass, so the electrons move faster there.
- 1:26
They hit its atoms. That becomes heat and light.
As they go, they keep hitting the filament's atoms. That energy comes out as heat and light.
- 1:38
Break the loop, and everything stops at once.
Open the switch, and with no closed loop the current stops everywhere at once.
- 1:46
Electricity is not electrons racing. It is a push round a loop.
The electrons barely move. What travels fast is the push, shared by every electron in the loop.
Sources
Every figure in the film comes from these. The animation is illustrative.