You can slow the group velocithy of light. Can you slow electricity?

[cyber one]

There are various methods to slow and even stop visible light. Most of
these methods appear to be slowing the group velocity of light, not the
phase velocity.

Can you slow the velocity of propagation of electricity? I have seen
estimates that electricity, in a wire, propagates at 2/3 the speed of
light. T he drift velocity of electrons in the wire may be one meter
an hour, for DC current, but I am interested in the slowing the
propagation speed of the electricity instead.

[Tom Roberts]

On 4/1/12 4/1/12 5:51 AM, cyber one wrote:
> There are various methods to slow and even stop visible light. Most of
> these methods appear to be slowing the group velocity of light, not the
> phase velocity.

Yes. Anomalous dispersion can reduce the phase velocity of light.

> Can you slow the velocity of propagation of electricity?

Certainly. Well, in the sense that a signal propagating in a transmission line
of length L is delayed by more than L/c.

When timing in high-speed electronics, we routinely prepare a
set of coax cables labeled by their delay (1 ns, 2 ns, 5 ns, ...)
so we can easily select the required delay. The speed of light
in vacuum is about 1 foot per nanosecond, but a 1 ns cable is
about 8 inches long (2/3 of a foot).

> I have seen
> estimates that electricity, in a wire, propagates at 2/3 the speed of
> light.

This depends on the geometry and physical configuration, but that is a
reasonable rule of thumb. Remember, this is for a signal propagating in the
wire, NOT the speed of electron drift.


> The drift velocity of electrons in the wire may be one meter

> an hour, for DC current, but I am interested in the slowing the
> propagation speed of the electricity instead.

We don't call this the "speed of electricity", instead we call this the speed of
electrical signals, or the drift speed of the electrons. That first phrase is
too ambiguous. Note that current starts flowing with the speed of the signals.


Tom Roberts