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## What is the Speed of Electricity? Moderators: adafruit_support_bill, adafruit

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What is the Speed of Electricity?

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.
pdxNat

Posts: 16
Joined: Wed Apr 28, 2010 8:34 am

Re: What is the Speed of Electricity?

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...

westfw

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Location: SF Bay area

Re: What is the Speed of Electricity?

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

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.
uoip

Posts: 127
Joined: Wed Sep 23, 2009 5:48 pm

Re: What is the Speed of Electricity?

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.

zener

Posts: 4567
Joined: Sat Feb 21, 2009 2:38 am

Re: What is the Speed of Electricity?

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.
Entropy

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Location: Owego, NY USA

Re: What is the Speed of Electricity?

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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pstemari

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Please be positive and constructive with your questions and comments.