# The Naked Scientists Forum

### Author Topic: Can electricity travel faster than light?  (Read 2829 times)

#### jeffreyH

• Global Moderator
• Neilep Level Member
• Posts: 3913
• Thanked: 53 times
• The graviton sucks
##### Re: Can electricity travel faster than light?
« Reply #25 on: 20/07/2016 00:58:04 »
I thought it would be a good academic exercise for us to do the calculation:

Let's imaging a notional conductor, a cylindrical piece of wire.

Its cross-sectional area is A, the length is d and there are n charge carriers, e, moving at velocity v.

Therefore the total mobile charge, Q, in the cable is given by Adne

Time, t, for this charge to move along the conductor = d/v

Since current, I, = Q/t we can substitute for the above:

I = Adne / d/v = Avne

So v = I / Ane

Now i need some numbers for my notional copper wire including - number of charge carriers etc...

Who would like to take up the baton...

So if we take the cross sectional area A = 0.003m, velocity v = 1 m/s, number of charge carriers n (in copper) = 8.5 x 1028 per m3. Then we need d to derive the volume of the wire to proceed. If we just assume it is 1 cubic metre anyway this simplifies things. Electron charge is 1.6 x 10-19C. So now we have the figures to calculate current and subsequently drift velocity. Then all we need do is change the value for current to see how this affects drift velocity. We could even calculate a hypothetical current needed to bring drift velocity close to c, just for fun.

#### jeffreyH

• Global Moderator
• Neilep Level Member
• Posts: 3913
• Thanked: 53 times
• The graviton sucks
##### Re: Can electricity travel faster than light?
« Reply #26 on: 20/07/2016 01:11:58 »
For v = 1 m/s you appear to get 40.8 million amps. This may be wrong! Very likely so.

#### chris

• Neilep Level Member
• Posts: 5337
• Thanked: 65 times
• The Naked Scientist
##### Re: Can electricity travel faster than light?
« Reply #27 on: 20/07/2016 22:56:01 »
Whoops!

#### lightarrow

• Neilep Level Member
• Posts: 4586
• Thanked: 7 times
##### Re: Can electricity travel faster than light?
« Reply #28 on: 21/07/2016 18:27:34 »
I know that if we switch on a torch on a train, the speed of light cannot be added to that of the train. What about speed of electricity on a wire? Can it be added to the train's speed?
It depends on
1) what you intend with "speed" (speed of what?)
2) what you intend with "electricity"

Just to make an example, the phase velocity of electromagnetic waves in a wave-guide can be greater than c. Even the group velocity of an electromagnetic wave, in a suitable medium, can be faster than c.
What instead must always be less then or equal c is signal velocity
http://www.mathpages.com/home/kmath210/kmath210.htm

#### The Naked Scientists Forum

##### Re: Can electricity travel faster than light?
« Reply #28 on: 21/07/2016 18:27:34 »