# Does electricity travel at the speed of light? **URL:** \ **Category:** Factual Questions\ **Created:** [December 4, 2002, 11:15am UTC](https://boards.straightdope.com/t/does-electricity-travel-at-the-speed-of-light/140407 "2002-12-04T11:15:57Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![KenP](https://avatars.discourse-cdn.com/v4/letter/k/c0e974/32.png) [@KenP](https://boards.straightdope.com/u/KenP)\ **Post date:** [December 4, 2002, 11:15am UTC](https://boards.straightdope.com/t/does-electricity-travel-at-the-speed-of-light/140407/1 "2002-12-04T11:15:57Z")
If not, what’s the speed of electricity? ---
**Author:** ![Cisco](https://avatars.discourse-cdn.com/v4/letter/c/ccd318/32.png) [@Cisco](https://boards.straightdope.com/u/Cisco)\ **Post date:** [December 4, 2002, 11:22am UTC](https://boards.straightdope.com/t/does-electricity-travel-at-the-speed-of-light/140407/2 "2002-12-04T11:22:11Z")
Electricity travels the speed of light in a vacuum but in everyday situations it is typically slowed down by what is carrying it (ie copper.) ---
**Author:** ![Nishroch\_Order](https://avatars.discourse-cdn.com/v4/letter/n/b4bc9f/32.png) [@Nishroch\_Order](https://boards.straightdope.com/u/Nishroch_Order)\ **Post date:** [December 4, 2002, 11:22am UTC](https://boards.straightdope.com/t/does-electricity-travel-at-the-speed-of-light/140407/3 "2002-12-04T11:22:14Z")
Erm…erm…_quick quick…get someone here to define electricity!!_ ---
**Author:** ![Cisco](https://avatars.discourse-cdn.com/v4/letter/c/ccd318/32.png) [@Cisco](https://boards.straightdope.com/u/Cisco)\ **Post date:** [December 4, 2002, 11:32am UTC](https://boards.straightdope.com/t/does-electricity-travel-at-the-speed-of-light/140407/4 "2002-12-04T11:32:22Z")
Upon further inspection I’d like to retract my statement. Search google for “speed of electricity” and you’ll see why. This is a rather more complicated matter than I had thought, one of the reasons being, as Nirhroch Order mentioned, the word ‘electricity’ has no single definition. ---
**Author:** ![Desmostylus](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@Desmostylus](https://boards.straightdope.com/u/Desmostylus)\ **Post date:** [December 4, 2002, 12:02pm UTC](https://boards.straightdope.com/t/does-electricity-travel-at-the-speed-of-light/140407/5 "2002-12-04T12:02:06Z")
The actual _electrons_ moving in the conductor move at the “drift velocity”, which depends on current and conductor cross-sectional area, but would typically be of the order of a few meters/sec. In an alternating current, of course, all they do is shuffle back and forth and never actually go anywhere. The speed of the _information_ carried by the electricity, which may be as simple as “switch is now on, make light work”, travels at the speed of light in the dielectric that the conductors are operating in. For air, that speed is just under the speed of light in a vacuum ©. For a polymer insulator, that speed would be something like 2/3 c. So the answer, is “a large fraction of c”. ---
**Author:** ![Mort\_Furd](https://avatars.discourse-cdn.com/v4/letter/m/8e8cbc/32.png) [@Mort\_Furd](https://boards.straightdope.com/u/Mort_Furd)\ **Post date:** [December 4, 2002, 12:03pm UTC](https://boards.straightdope.com/t/does-electricity-travel-at-the-speed-of-light/140407/6 "2002-12-04T12:03:03Z")
If you are talking about the actual motion of electrons in the conductor, then no electricity does not move at the speed of light. If it di, all you would need to make an electron gun would be to make a sharp bend in a piece of wire with current flowing. The electrons wouldn’t be able to make the turn and, presto, instant electron beam. If you mean the propagation rate of the force that causes the elextrons to move, then the answer is “almost.” The actual speed depends on the conductor and its density. RF cables actually have a factor that you can look up and use to calculate wavelengths for different frequencies in the cable. It is basically a percentage of c. A rough value for most 50 ohm cable is 0.9 - but that won’t get you very far if the cable is connected to the wrong impedance. You’ll also have to do a lot of fiddling to get the length just right if you make an antenna or balun using this value. To do it right, you need the correct value for the cable you are using. ---
**Author:** ![Mort\_Furd](https://avatars.discourse-cdn.com/v4/letter/m/8e8cbc/32.png) [@Mort\_Furd](https://boards.straightdope.com/u/Mort_Furd)\ **Post date:** [December 4, 2002, 12:06pm UTC](https://boards.straightdope.com/t/does-electricity-travel-at-the-speed-of-light/140407/7 "2002-12-04T12:06:13Z")
Simulpost. Desmostylus explains it better. ---
**Author:** ![Desmostylus](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@Desmostylus](https://boards.straightdope.com/u/Desmostylus)\ **Post date:** [December 4, 2002, 12:08pm UTC](https://boards.straightdope.com/t/does-electricity-travel-at-the-speed-of-light/140407/8 "2002-12-04T12:08:31Z")
**Mort Furd:** Almost right. It doesn’t depend on the conductor but on the insulator. The length stuff and impedance matching has nothing to do with speed. ---
**Author:** ![Desmostylus](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@Desmostylus](https://boards.straightdope.com/u/Desmostylus)\ **Post date:** [December 4, 2002, 12:11pm UTC](https://boards.straightdope.com/t/does-electricity-travel-at-the-speed-of-light/140407/9 "2002-12-04T12:11:45Z")
Sorry, **Mort\_Furd** my last post didn’t come across right. Those other factors you mention have to do with maximising power transfer rather than speed. ---
**Author:** ![Mort\_Furd](https://avatars.discourse-cdn.com/v4/letter/m/8e8cbc/32.png) [@Mort\_Furd](https://boards.straightdope.com/u/Mort_Furd)\ **Post date:** [December 4, 2002, 12:37pm UTC](https://boards.straightdope.com/t/does-electricity-travel-at-the-speed-of-light/140407/10 "2002-12-04T12:37:17Z")
No, the length doesn’t have anything to do with it. I was just trying to provide an example of why the speed of electricity in a conductor is significant. The balun is also just an example. Impedance matching has an effect on the length of the antenna in that is changes the length at which the impedance of the antenna matches that of the cable - which is a must for a transmitter antenna if you want to avoid getting power reflected back at the transmitter. It is also important for a receiver, though not as critical. A receiver won’t burn itself up because of a poor match the way a transmitter can. Sorry. I tend to look at these things from a radio technician’s point of view. Physics class is 17 years gone.🙂 ---
**Author:** ![Mort\_Furd](https://avatars.discourse-cdn.com/v4/letter/m/8e8cbc/32.png) [@Mort\_Furd](https://boards.straightdope.com/u/Mort_Furd)\ **Post date:** [December 4, 2002, 12:39pm UTC](https://boards.straightdope.com/t/does-electricity-travel-at-the-speed-of-light/140407/11 "2002-12-04T12:39:11Z")
> [@](#): > > \*Originally posted by Desmostylus \* > \*\*Sorry, **Mort\_Furd** my last post didn’t come across right. Those other factors you mention have to do with maximising power transfer rather than speed. \*\* Your last post was quite correct. We were discussing speed of electricity, not power transfer. I got rather far afield.🙂 ---
**Author:** ![Desmostylus](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@Desmostylus](https://boards.straightdope.com/u/Desmostylus)\ **Post date:** [December 4, 2002, 12:42pm UTC](https://boards.straightdope.com/t/does-electricity-travel-at-the-speed-of-light/140407/12 "2002-12-04T12:42:04Z")
**Mort Furd:** Your first posting does make perfect sense. I just didn’t read it carefully enough.:smack: ---
**Author:** ![zut](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/zut/32/2875_2.png) [@zut](https://boards.straightdope.com/u/zut)\ **Post date:** [December 4, 2002, 12:58pm UTC](https://boards.straightdope.com/t/does-electricity-travel-at-the-speed-of-light/140407/13 "2002-12-04T12:58:48Z")
> [@](#): > > \*Originally posted by Desmostylus \* > **Mort Furd:** Almost right. It doesn’t depend on the conductor but on the insulator. Can you 'splain this further? Why does speed depend on the _insulator_? As a practical matter, what would happen af a conducter were half-imbedded in polymer and half in air? Or, better yet, what happens in a stranded wire where the insulator doesn’t touch the entire perimeter of each strand? Or did I misinterpret your statement? ---
**Author:** ![Desmostylus](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@Desmostylus](https://boards.straightdope.com/u/Desmostylus)\ **Post date:** [December 4, 2002, 1:11pm UTC](https://boards.straightdope.com/t/does-electricity-travel-at-the-speed-of-light/140407/14 "2002-12-04T13:11:48Z")
The best example is a high frequency signal in a coaxial cable. The energy transmitted is actually in the form of an electromagnetic field which _exists in the insulator between the inner and outer conductor_. If the insulator is all air, you’ll get one speed. If it’s all, say, polyethylene, you get a lower speed. If it’s a combination of the two, and lots of cables are deliberately made with air spaces in polyethylene, you’ll get a speed in between that of air and polyethylene. Ask again if you want more. ---
**Author:** ![Algernon](https://avatars.discourse-cdn.com/v4/letter/a/a183cd/32.png) [@Algernon](https://boards.straightdope.com/u/Algernon)\ **Post date:** [December 4, 2002, 1:40pm UTC](https://boards.straightdope.com/t/does-electricity-travel-at-the-speed-of-light/140407/15 "2002-12-04T13:40:14Z")
I’ll ask for more. “The energy transmitted is actually in the form of an electromagnetic field which exists in the insulator between the inner and outer conductor.”??? Could you please explain this again, in a different way? Inner and outer conductors? Doesn’t the energy “flow” in the conductor itself? ---
**Author:** ![Desmostylus](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@Desmostylus](https://boards.straightdope.com/u/Desmostylus)\ **Post date:** [December 4, 2002, 2:04pm UTC](https://boards.straightdope.com/t/does-electricity-travel-at-the-speed-of-light/140407/16 "2002-12-04T14:04:28Z")
A coaxial cable has two concentric conductors. Like the cables from you TV antenna to your VCR, from the VCR to the TV. Like the stuff “cable TV” comes in. Electric current has to “flow” in the conductors, but that’s really just electrons shuffling back and forth a short distance. At low frequencies, like the 50 or 60 Hz used for electric power transmission, it’s usual for people to talk about power flowing through the conductors, etc., because that’s a useful and logical sounding way of describing it. But at higher frequencies, and this is where the “speed of electricity” question becomes important as **Mort Furd** noted, those useful and logical sounding descriptions aren’t so useful anymore. The real situation, at any frequency, is as I’ve described it, with energy being transmitted through the dielectric. Sounds silly, but that’s how it is. I’m struggling to keep the discussion simple. ---
**Author:** ![Algernon](https://avatars.discourse-cdn.com/v4/letter/a/a183cd/32.png) [@Algernon](https://boards.straightdope.com/u/Algernon)\ **Post date:** [December 4, 2002, 2:12pm UTC](https://boards.straightdope.com/t/does-electricity-travel-at-the-speed-of-light/140407/17 "2002-12-04T14:12:53Z")
Thanks for attempting to dumb it down for me **Desmostylus.** In a simple insulated copper wire, what exactly is the dielectric? ---
**Author:** ![Mort\_Furd](https://avatars.discourse-cdn.com/v4/letter/m/8e8cbc/32.png) [@Mort\_Furd](https://boards.straightdope.com/u/Mort_Furd)\ **Post date:** [December 4, 2002, 2:18pm UTC](https://boards.straightdope.com/t/does-electricity-travel-at-the-speed-of-light/140407/18 "2002-12-04T14:18:50Z")
The insulation. ---
**Author:** ![Desmostylus](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@Desmostylus](https://boards.straightdope.com/u/Desmostylus)\ **Post date:** [December 4, 2002, 2:24pm UTC](https://boards.straightdope.com/t/does-electricity-travel-at-the-speed-of-light/140407/19 "2002-12-04T14:24:25Z")
Dielectric basically equals insulator, but it’s slightly complicated. You can’t just have one insulated copper wire and transmit energy through it. You need a return path. The dielectric is that combination of insulators that exist between the two paths. So, if you have two insulated copper wires running side by side, the dielectric is the combination of the insulation surrounding the wires, any air between the copper and the insulation, and the air surrounding the two insulated copper wires. ---
**Author:** ![Algernon](https://avatars.discourse-cdn.com/v4/letter/a/a183cd/32.png) [@Algernon](https://boards.straightdope.com/u/Algernon)\ **Post date:** [December 4, 2002, 2:27pm UTC](https://boards.straightdope.com/t/does-electricity-travel-at-the-speed-of-light/140407/20 "2002-12-04T14:27:37Z")
Wow. This is fascinating. It’s like when I discovered that Santa really didn’t exist. The energy is transmitted through the insulation rather than the conductor. Could they have been more misleading with the noun selections? (OK. I know this is sounding quite sarcastic, when that’s not really my intent.) I’m quite confident they didn’t cover this in my college physics courses. So if the energy transmission is through the dielectric, and insulated copper wire is slower than air, then what difference does it make what material the conductor is made out of? [Next page](https://boards.straightdope.com/t/does-electricity-travel-at-the-speed-of-light/140407.md?page=2)