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What is the Speed of Electricity?
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What is the Speed of Electricity?

by pdxNat on Tue Aug 31, 2010 5:13 pm

Howdy y'all,

Props to Adafruit + Ladyada for turning me into a soldering fool. :) Thanks!

I'm a non-engineer hacker with a question about the speed of electricity traveling through a copper wire. This is not critical to a project, just hypothetical, curious if anyone can help verify what I've googled. Is it true that electricity traveling through a copper wire is basically traveling at 60%-96% the speed of light?

For the sake of creating a general test scenario, imagine a copper wire 100 feet long, insulated, outdoors, on an overcast day, about 70 degrees F. At one end of the wire is a power-source and at the other end a multimeter (or buzzer, or light.) When I "flip the switch" how long will it take the juice to travel to the other end?

Totally appreciate any answers, comments, links, formula, book references, fan fiction, etc. Thank you for helping a brother out.
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Re: What is the Speed of Electricity?

by westfw on Tue Aug 31, 2010 5:39 pm

Yep; for purposes of your example, you're looking at substantial fractions of the speed of light. I don't know that 96% of c is possible; more typically you see about 70% c for signals in regular coax. (for example, there are ethernet chips that will tell you that there is a break in the coax cable (back when ethernet used coax!) so many clock ticks away, and you can go and actually calculate and find the bad spot with impressive accuracy.)

In one nanosecond, light travels about a foot in vacuum. Adm Grace Hopper used to give talks about computing where she would hand out "nanoseconds"; pieces of wire about a foot long.

The actual electrons that carry the signal apparently move much more slowly. It's like a hose full of marbles; you push in a new marble quite slowly, but one comes out the other end right away...

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Re: What is the Speed of Electricity?

by uoip on Tue Aug 31, 2010 5:44 pm

Here's a site that describes it with perhaps more detail than you wanted to hear.

http://www.allaboutcircuits.com/vol_2/chpt_14/3.html

Interestingly, it just depends on the type of insulation used. It's normally somewhere around 2/3 to 4/5 the speed of light.

Velocity factor is a fractional value relating a transmission line's propagation speed to the speed of light in a vacuum. Values range between 0.66 and 0.80 for typical two-wire lines and coaxial cables. For any cable type, it is equal to the reciprocal (1/x) of the square root of the relative permittivity of the cable's insulation.
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Re: What is the Speed of Electricity?

by zener on Tue Aug 31, 2010 6:34 pm

There are some semantics here. What speed are you measuring? The speed between when you apply electrical potential (voltage) on one end of the conductor, and when you receive electrical potential at the other end? Or the actual speed of the electron movement through the conductor? The latter is MUCH slower.

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Re: What is the Speed of Electricity?

by Entropy on Fri Sep 03, 2010 12:53 pm

pdxNat wrote:Howdy y'all,

Props to Adafruit + Ladyada for turning me into a soldering fool. :) Thanks!

I'm a non-engineer hacker with a question about the speed of electricity traveling through a copper wire. This is not critical to a project, just hypothetical, curious if anyone can help verify what I've googled. Is it true that electricity traveling through a copper wire is basically traveling at 60%-96% the speed of light?

For the sake of creating a general test scenario, imagine a copper wire 100 feet long, insulated, outdoors, on an overcast day, about 70 degrees F. At one end of the wire is a power-source and at the other end a multimeter (or buzzer, or light.) When I "flip the switch" how long will it take the juice to travel to the other end?

Totally appreciate any answers, comments, links, formula, book references, fan fiction, etc. Thank you for helping a brother out.

In a situation like that, the capacitance of the wire (and the time needed to charge it) may dominate over propagation delay.

Usually velocity factor of cabling does not matter unless you're sending RF signals over transmission lines. Velocity factor becomes critical when trying to create tuned circuits from transmission lines (such as half-wave or quarter-wave matching stubs.) Typical coax has a velocity factor of 66% to 80% depending on its construction and the dielectric in use.
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Re: What is the Speed of Electricity?

by pstemari on Sat Sep 04, 2010 12:57 pm

Yes, it's dependent on the distance between the conductors and the dielectric constant of the insulator between them. Anything like a ferrite bead or loops in the wire that affects inductance will also have an effect.
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