| From: | Starjammer <starjammer@**********.COM> |
|---|---|
| Subject: | [OT] GT PHYS 3211 Electronics for Physicists |
| Date: | Tue, 20 Oct 1998 06:04:22 -0400 |
>
>The speed of light is not the same as the speed of electricity. Here's why:
I was going to give it a bye, but since someone else brought it up I might
as well pass on what the physics professors at Georgia Tech taught me...
>1) Light moves slower in different mediums. The speed of light in, say,
>copper is significantly slower than in vacuum.
No and yes. All EM waves move at the same speed: c. However, an EM wave
moving through a physical medium gets deflected by a certain amount
depending on the wavelength of the wave. Long-wavelength waves are
deflected the least. This is important, because...
>2) Electrons don't move at lightspeed anyway. They move near to that speed,
>but not at it.
Electrons actually only move a few centimeters per second. That's okay,
the speed at which electrons propagate really has very little to do with
how electricity works. That's because...
>3) Electrons don't move in a straight line. Even with a current, they'll
>bounce around, which slows down the speed of the electronic signal.
Electrons are the medium, not the message. It's the electric field that
propagates at light-speed and transmits energy through the system. So, for
example, when you flip on a light switch you're not sending the electrons
zipping up the wire. Rather, you're creating a closed circuit with a
relatively large degree of difference in electric potentials. This creates
an electro-motive force (EMF) which pushes energy through the system.
Where the electrons come in to it, quantum mechanically speaking, is that
they absorb the photons of EM energy, get excited and jump to a higher
energy-level, then radiate a photon and drop to a lower energy-level.
Chemically speaking, metals make better conductors because they have a
outer energy-shells containing only a few loose electrons; makes it easier
for them to get excited and jump energy-levels. This is also why all
plasmas are superconductors; a plasma by definition is the state of matter
where all electrons are stripped away from their nuclei, so all electrons
in a plasma are "free" electrons.
>IIRC, the speed used for how fast electronic signals propagate is about
>1/2c, but it's a statistical average that varies between different
>conductors.
Yes, but that's due to the dispersion factor of an EM wave moving through a
physical medium, not how fast an electron moves through a particular
material. Also, keep in mind that "signal" and "wave" are not the
same
animal. A wave's speed is defined as "phase speed." A signal moves at
what's called the "group speed" which is defined as the speed which a wave
pulse carrying a piece of information can travel, based on how much the the
medium causes the wave to disperse. In systems with no dispersion (such as
light moving through a vacuum) phase speed and group speed are the same.
>Robert Watkins -- robert.watkins@******.com
Starjammer | Una salus victus nullam sperare salutem.
starjammer@**********.com | "The one hope of the doomed is not to hope
Marietta, GA | for safety." --Virgil, The Aeneid
