Inside a Conductor — Random Motion to Drift
A closed copper loop opened up so you can see the free electrons inside. With the field off they are already travelling at around 10⁶ m/s, colliding with the lattice and going nowhere on average. Switch the field on and a slow, steady lean appears on top of that racing — and the lean is the entire current.
How to use this simulation
- Press Field off to watch pure thermal motion with no current flowing
- Press Field on to superimpose the drift and start charge crossing the section
- Follow a single highlighted electron through several collisions to see its zig-zag path
- Look along the loop to confirm the drift is the same everywhere in the circuit
What to look for
- With the field off electrons move fast but the cloud as a whole stays put
- With the field on the racing is unchanged — a slow one-way lean is simply added to it
- The drift is so slow beside the thermal speed that you must watch the cloud, not one electron, to see it
- Collisions with the lattice keep resetting the acceleration, which is why the drift stays constant instead of growing
- Every cross-section of the loop carries the same drift, so the current is the same all the way round
The physics behind it
Thermal speed in copper at room temperature is about 10⁶ m/s, but the directions are random, so the mean displacement is zero and no charge is transported. An applied field E adds an acceleration a = eE/m between collisions, which every collision resets. The result is a small constant average called the drift velocity, typically of order 10⁻⁴ m/s — ten orders of magnitude below the thermal speed. The current is I = nAv_d e, and only that tiny drift term appears in it.