# speed of electricity

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Hi..

if i have a power source, and a switch connected to a light bulb 900,000 KM away over a pure copper wire.

how long would it take since i switch on untill the light bulb actually lights?

Thanks.

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The speed depends a bit, wikipedia says, "Propagation speed is affected by insulation, so that in an unshielded copper conductor ranges 95 to 97% that of the speed of light." If the 900,000 km is not a straight line, the speed would probably be less.

http://en.wikipedia.org/wiki/Speed_of_electricity

Edited by EdEarl

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(1) You can't light a bulb at that distance, because of losses. The longest lines span very few 1000km with good efficiency but use 600kV DC or even 1MV DC (and cheaper aluminium, by the way). Not the slightest hope with 200V.

(2) This distance is the limit where RC gives about the same delay as LC, so we may still take LC, that is the speed of light since overhead lines are air-insulated, to estimate the propagation time. After 3s, you get your nothing at the end of the line.

(3) Propagation time does make a difference in electricity transport. The phase lags over the distance, but the whole network is synchronous over a continent. 1000km introduce 3.3ms lag and a quarter period of 50Hz is just 5ms. It has to be compensated.

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The OP did not specify wire size or AC.

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There is another effect in play at the distances considered.

Even 'across a continent' the distances are only of the same order as the wavelength (approx 5,000 km for 50/60 Hz) of powerline frequencies.

So the transmission of disturbances such as switching on or off is between a single pair of nodes and continuous and the velocity of transmission is close c, since the transmitting wave is largely surface.

If it were not for the skin effect and the electricity actually travelled through the copper wire it would slow dramatically. Powerline frequencies have a velocity of 300,000,000 m/s in air (skin effect velocity) and 3 m/s in pure copper.

However at 900,000 km there will be a significant number of complete standing waves.

Edited by studiot

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Standing waves would imply AC current. The OP did not specify either AC or DC.

Assuming DC, there would be transients initially, but no standing waves.

Edited by EdEarl

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Assuming DC, there would be transients initially, but no standing waves.

Unless the transient is 100% absorbed at the end of a DC line there will certainly be standing waves due to impedance mismatch induced reflections.

Depending whether we accept our copper wire to have zero or actual resistance such standing waves will not or will gradually die away.

Edited by studiot

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Copper is not a super conductor.

A light bulb has some resistance in addition to the wire.

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Copper is not a super conductor.

A light bulb has some resistance in addition to the wire.

So?

Edited by studiot

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So, resistance will dampen transients assuming DC.

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So, resistance will dampen transients assuming DC.

Which is what I said if you read my post properly. However that does not mean that reflections will not cause standing waves. Yes they will die away exponentially but you claimed an absolute - that they will not exist.

Edited by studiot

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Sorry, I am a bit dyslexic and sometimes misunderstand.

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No problem.

I don't think moses fully appreciated the mechanism of transmission of electric signals when he posed the question, so I hope we have all helped understanding.

Edited by studiot

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I hope we did not confuse him.

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The propagation speed of the electrical signal depends on the cable contruction and its dielectric.

That far away bulb takes two conductors to circulate a current trough it.

The speed can be somewhere between 60% to 99% of c.

What I cannot discern is if the total wiring lenght being 2 x 900,000 Km = 1,800,000 Km long; the light bulb will turn on when the wave reaches it or when the wave reaches the return starting point *

[All this assumming perfect ideal theoretical conduction]

If c is 300,000 Km/sec, and the propagation speed over the wiring is 66.666 % , the actual electrical signal propagates at 200,000 Km/sec

For a 900,000 Km trip, would take 4.5 seconds. * If the 1,800,000 Km round trip has to be considered, would be 9 seconds.

I think

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You are on the right track, the answer depends on whether your assumptions are accurate.

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Round trip distances are not required for this calculation.

It is not even necessary to have a return cable. Part of continental North America and Australia are still fed via the single wire system, using the earth itself as the 'return'.

But you are right and as several have already said, more specific calculations could be offered against the supply of more specific cable data, including configuration.

So if moses would like to provide more detail about what he had in mind?

Edited by studiot

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