What's the relative speed of electricity?

  • Thread starter Magma828
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  • #1
I've searched for this question on these forums and google and everyone seems to have the same question as me, but hasn't stated it clear enough for people to understand exactly what they want to know.

I'm wondering how long it takes for electricity to turn something on. I don't want to know how fast electrons flow because I know all about electron drift velocity, and this is clearly nowhere near the speed at which electricity takes effect.

Put it this way:
I have a massive extension cable which I connect from from my house in St Andrews to London. The dude in London has a light plugged into the end of the extension cable. When I switch the plug on, how long will it take for the guy to see the light turn on?

Also, please don't reply stating the flaws in my experiment - just imagine everything is perfect and we have super-accurate measuring equipment.

Answers and Replies

  • #2
Science Advisor
The simple answer to your question is that the extension cord will behave like a transmission line, and the signal will propagate through the line at somewhat less than the speed of light, with the ratio of the speed of the signal to the speed of light depending on the relative dielectric constant of the insulating material between the wires in the extension cord. Try this link for an explanation:


  • #3
Vanadium 50
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2019 Award
The signal speed in a high quality cable is about 2/3 the speed of light.