Why the Lamp Lights at Once
Close the switch and two completely different speeds are at work. The electric field sweeps round the loop at close to the speed of light and has the lamp glowing within nanoseconds, while the electrons themselves barely crawl. Nothing had to travel from the battery to the lamp — the electrons were already there.
How to use this simulation
- Press Close the switch and watch the field front sweep round the loop
- Follow a marked electron inside the filament and see it start with the rest
- Compare the time the field takes with the distance any electron has covered
- Reopen and close the circuit to run the two timescales side by side again
What to look for
- The lamp is lit long before any electron from the battery has gone anywhere near it
- Electrons everywhere in the loop begin to drift at effectively the same instant
- The field front crosses the whole circuit in nanoseconds; the drift is a fraction of a millimetre per second
- There were already free electrons inside the filament waiting to be pushed
- The battery supplies energy and a field, not the particular electrons that do the lighting
The physics behind it
Closing the switch establishes an electric field along the whole conductor, and that field propagates at a speed close to c. For a loop about a metre round the field is in place in a few nanoseconds, and every electron in the circuit — including those already sitting in the lamp filament — starts drifting at essentially the same moment. Those electrons move at a drift velocity of order 10⁻⁴ m/s and would take hours to cover the distance from the battery. The lamp lights because of the field's arrival, not because of any electron's journey.