4 ms·
> People often cite the speed of light when talking about electronics when actually electrons don't travel at the speed of light (they have mass) Actual electr
by SEMW 14y ago
> People often cite the speed of light when talking about electronics when actually electrons don't travel at the speed of light (they have mass)
Actual electrons move pretty slowly - of the order of millimetres per hour. That speed has nothing to do with the speed the signal propagates down the wire.
The speed the signal is propagated is the speed the electromagnetic wavefront moves along it. Which is mostly limited by the dielectric constant of the wire's insulator. (That actually is related to the speed of light in the insulator - they both depend on its permittivity). It's not related to the speed electrons move in the wire, which depends on its cross-sectional area and the current. In particular, the fact that electrons have mass isn't relevant to the wavefront propagation speed.
(C.f. fibre optic cables, which will have a wavefront propagation speed not dissimilar to copper wire (i.e. both will be a substantial fraction of c), even though, unlike copper wire, their carriers are massless and actually do move at the wavefront speed).
Analogy: imagine pushing the end of a very long, rigid broomstick. The actual wood in the broomstick moves pretty slowly (maybe you move it a cm in a second). But the person at the other end feels their end of the broomstick move almost immediately, limited only by a speed of light delay (or a little more if the broomstick isn't as rigid as possible).
- laumars 14y agoInteresting stuff. Thanks for the correction :)
- yebyen 14y agoDamn, you beat me to it. I think you did a better job explaining than I would have, too. An upvote for you, sir! For any skeptics or laymen who might have been lost at 'electromagnetic wavefront', or who are wondering why electrons would seem to move so slowly when we think of electricity (and even free electrons) as moving so fast, the analogy of the broomstick really does not do it justice, since you can't see the broomstick oscillating very quickly as you might have guessed the electrons must really do to satisfy Heisenberg: http://en.wikipedia.org/wiki/Particle_in_a_box http://en.wikipedia.org/wiki/Particle_in_a_box http://www.physicsforums.com/showthread.php?t=121456 http://www.physicsforums.com/showthread.php?t=121456 The keyword is "electron drift velocity" The important concept to understand is that information does not move trapped inside of individual electrons, and thus is not at all limited by electron drift velocity.
- SummonJetTruck 14y agoWith a broomstick, wouldn't it be the speed of sound, since it's a pressure wave?
- SEMW 14y agoFair point. For the analogy I was going for a hypothetical rigid broomstick, made of stuff that's as incompressible as physically possible. The speed of sound in that would be c. Yeah, that isn't very realistic, but I don't think that hurts the point it was illustrating.