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# The speed of electricity

## The speed of electricity - Physics Forum

### The speed of electricity - Physics Forum. Discuss and ask physics questions, kinematics and other physics problems.

#1
01-09-2004, 08:25 PM
 Danny Guest Posts: n/a
The speed of electricity

Hi

What would be an appropriate figure for the speed of an electrical
signal? If there is no constant one, what factors does it depend on (how
long would it take a signal to go through a 1 km cable).

Danny

#2
01-09-2004, 09:22 PM
 Prai Jei Guest Posts: n/a
The speed of electricity

"Danny" <[Only registered users see links. ]> wrote in message
news:BZELb.493\$[Only registered users see links. ].net...

Through a continuous cable with no junctions you can normally expect
something of the order of 0.5c. For 1km of cable this gives a transit time
of 6.7 microseconds or so.

If you have a continuous spectrum of frequencies there will be some
dispersion where the speed is frequency-dependent, the main effect of which
will be to smooth out sharp edges in the signal. The math is quite complex,
depending on cable geometry as well as proximity to other cables and
conducting surfaces.
--
Paul V. S. Townsend
Interchange the alphabetic elements to reply

#3
01-09-2004, 11:49 PM
The speed of electricity

"Danny" <[Only registered users see links. ]> wrote in message
news:BZELb.493\$[Only registered users see links. ].net...

There are *two* different answers to that question, and which one is correct
depends on what you are *thinking of* when you use the word "electricity."

There is an analogy here that may help - air. The speed of sound
(mechanical energy) in air is pretty fast - about 300 m/second, and it
depends on the temperature of the air, the humidity, and several other
things. The speed of the individual molecules in air depends on the weight
of the molecule and the air pressure and temperature, varies over a broad
range, and the average is 1400 m/sec for oxygen (molecular weight 32) at 25°
C and 1 atm. The speed of the bulk movement of air ("wind") is variable and
typically runs less than 10% of the speed of sound.

If you think of "electricity" as the movement of *electrical energy* through
the wire, then it is pretty fast - equal to the speed of light c divided by
the index of refraction n(f) of the metal for waves *of that particular
frequency f* - the index of refraction is frequency-dependent. For example,
the value of n(0) is 3.16 for lithium, which gives us a speed of about
c/n(0) = 95,000 km/sec for electricity in lithium - a significant fraction
of the speed of light.

Theoretical arguments set the velocity of *the bulk motion of electrons*
(electron wind) at much lower speeds (meters per second), but measuring
those speeds is very difficult:
[Only registered users see links. ]

OTOH, the motion of "individual electrons" (treated as particles like the
molecules in air, which itself is problematical) is typically much higher,
but not as fast as the motion of electrical energy:
[Only registered users see links. ]

[Only registered users see links. ]

Tom Davidson
Richmond, VA

#4
01-10-2004, 09:56 AM
 Danny Guest Posts: n/a
The speed of electricity

Hi

Another thing i was thinking, what if i remove the signal source before
the signal has reached the end of the line, will the signal carry on to
the end, or will it just stop dead and never reach the end?

Danny

#5
01-10-2004, 10:57 AM
 Prai Jei Guest Posts: n/a
The speed of electricity

"Danny" <[Only registered users see links. ]> wrote in message
news:2RQLb.74\$[Only registered users see links. ].net...

That will depend on what's at the end of the line.

Suppose the line simply ends in mid-air with an open circuit. Now there can
be no current through an open circuit, and what will happen is that the
signal will be "reflected" by the open-circuit, with reversed current (so
that the currents of the original signal and its reflection cancel). This
reflection will travel back up the line, possibly interfering with following
signals or, if the other end of the line is also an open-circuit, bouncing
back and being seen as another signal.

The other extreme type of termination (equally bad) is a short-circuit. Here
it's the voltage that has to be zero, so again there will be a reflection,
this time with reversed voltage.

The ideal termination is somewhere in between. The ideal cable is one that
is infinitely long, so it never reaches any termination and the question
does not arise. In practice, the cable characteristics can be analysed
(again, the math is quite complicated) and the final result is that an
infinite length of cable would appear as a pure electrical resistance, the
precise value depending on cable geometry and (to a lesser extent) signal
frequency. The resistance value is usually referred to as the
"characteristic impedance" of the cable.

The important point is that the math works both ways. An infinite length of
cable looks like a particular electrical resistance. A particular electrical
resistance looks like an infinite length of cable. If the cable is
terminated with its own characteristic impedance, there will be no signal
bounce - the termination absorbs both the voltage and the current perfectly
as though the cable were infinitely long.

--
Paul V. S. Townsend
Interchange the alphabetic elements to reply

#6
01-10-2004, 04:50 PM
 N:dlzc D:aol T:com \(dlzc\) Guest Posts: n/a
The speed of electricity

news:[Only registered users see links. ]...
to
has

If you're really fast (or Barney Fife), it might come out the bottom of the
holster! ;>}

David A. Smith

#7
01-10-2004, 06:50 PM
The speed of electricity

"Danny" <[Only registered users see links. ]> wrote in message
news:2RQLb.74\$[Only registered users see links. ].net...

If you fire a gun and put it back into the holster before the bullet gets to
the target, will the bullet *continue* towards the target after the gun has
been holstered?

Tom Davidson
Richmond, VA

#8
01-10-2004, 06:53 PM
The speed of electricity

"Prai Jei" <[Only registered users see links. ].uk> wrote in message
news:btop8e\$ih8\$[Only registered users see links. ].pol.co.uk...

<snip>

can
following

is an open circuit. It carries AC and transients quite well, but it has a
high impedance to direct currents (DC).

Danny's signal is a transient. It will continue quite well.

Tom Davidson
Richmond, VA

 Tags electricity , speed

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