# How can electricity travel the speed of light?

**URL:** <https://boards.straightdope.com/t/how-can-electricity-travel-the-speed-of-light/197285>\
**Category:** Factual Questions\
**Created:** [August 24, 2003, 5:54am UTC](https://boards.straightdope.com/t/how-can-electricity-travel-the-speed-of-light/197285 "2003-08-24T05:54:45Z")\
**Posts on this page:** 8\
**Page:** 2

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**Author:** ![vasyachkin](https://avatars.discourse-cdn.com/v4/letter/v/c5a1d2/32.png) [@vasyachkin](https://boards.straightdope.com/u/vasyachkin)\
**Post date:** [August 27, 2003, 6:43am UTC](https://boards.straightdope.com/t/how-can-electricity-travel-the-speed-of-light/197285/21 "2003-08-27T06:43:28Z")

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> [@](#):
>
> \*Originally posted by billy \*  
> \*\*This is a nitpick but I’m gonna call you on this one.
> 
> If there are free electrons, and a potential difference(voltage), the electrons are moving in the wire…if the electrons have arranged themselves so that there is no longer a voltage difference in the wire, then they will not move at all. V=IR and all that…if I=0, then so does V.
> 
> I’m ignoring superconductors BTW…assume copper. \*\*

you’re right. as we flip the switch the electons will move instantaneously, but they there will not be a continuous flow or drift of them.

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**Author:** ![Wolfrick](https://avatars.discourse-cdn.com/v4/letter/w/bb73d2/32.png) [@Wolfrick](https://boards.straightdope.com/u/Wolfrick)\
**Post date:** [August 27, 2003, 6:44am UTC](https://boards.straightdope.com/t/how-can-electricity-travel-the-speed-of-light/197285/22 "2003-08-27T06:44:28Z")

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Yes, they’re moving, but not necessarily very fast, or in any particular direction.

Electrical current is a wave effect, not a particle effect.

Think of an ocean wave moving towards the beach. Each bit of water moves towards then away from the beach was each wave crest passes, but the motion of the water is much much less than the motion of the wave.

Electricity moves at the speed of light in whatever medium it travels in (because electricity IS light).  
The speed of light is not a constant. This is what makes eyeglasses and telescopes possible.

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**Author:** ![vasyachkin](https://avatars.discourse-cdn.com/v4/letter/v/c5a1d2/32.png) [@vasyachkin](https://boards.straightdope.com/u/vasyachkin)\
**Post date:** [August 27, 2003, 6:45am UTC](https://boards.straightdope.com/t/how-can-electricity-travel-the-speed-of-light/197285/23 "2003-08-27T06:45:15Z")

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> [@](#):
>
> \*Originally posted by bbeaty \*  
> \*\*The population of electrons act as electric shields which keep the EM fields inside the cable. \*\*

i like this explanation 🙂

i prefer consistent models to simple ones 🙂

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**Author:** ![billy](https://avatars.discourse-cdn.com/v4/letter/b/ecd19e/32.png) [@billy](https://boards.straightdope.com/u/billy)\
**Post date:** [August 27, 2003, 7:28am UTC](https://boards.straightdope.com/t/how-can-electricity-travel-the-speed-of-light/197285/24 "2003-08-27T07:28:09Z")

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> [@](#):
>
> \*Originally posted by Wolfrick \*  
> \*\*because electricity IS light \*\*

UH, you ever try to start your car using light? Can you power your computer with light?

from [Dictionary.com](http://Dictionary.com)  
e·lec·tric·i·ty n.  
The physical phenomena arising from the behavior of electrons and protons that is caused by the attraction of particles with opposite charges and the repulsion of particles with the same charge.

light n.  
Physics.

- Electromagnetic radiation that has a wavelength in the range from about 4,000 (violet) to about 7,700 (red) angstroms and may be perceived by the normal unaided human eye.
- Electromagnetic radiation of any wavelength.  
You’re over simplifying to the point of being wrong.

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**Author:** ![Desmostylus](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@Desmostylus](https://boards.straightdope.com/u/Desmostylus)\
**Post date:** [August 27, 2003, 1:23pm UTC](https://boards.straightdope.com/t/how-can-electricity-travel-the-speed-of-light/197285/25 "2003-08-27T13:23:43Z")

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> [@](#):
>
> \*Originally posted by Chronos \*  
> \*\*An interesting tidbit, by the way: Even though the electrons move so slowly, it’s still possible (and in fact quite easy) to measure relativistic effects from their motion. The magnetic field around a wire is a purely relativistic effect. All moving objects obey relativity; it’s just that in slow-moving things, the effects are miniscule. But there are a lot of electrons in a wire, and it adds up. \*\*

> [@](#):
>
> \*Originally posted by Chronos \*  
> \*\*I did, in fact, mean “relativistic” as in “approaching the speed of light”, or “close enough to the speed of light that you can notice non-Galilean effects”. \*\*

I have no idea how you reconcile these two statements. The first one appears in many textbooks, and can only be true if the term “relativistic” is interpreted as meaning "pertaining to _Special_ Relativity. To make the second statement simultaneously true with the first, you’d have to show that magnetism _only_ arises due to a _v[sup]2[/sup]/c[sup]2[/sup]_ term. That of course isn’t correct. Pre-Maxwell, and pre-Special Relativity, Ampere’s Law dealt with the subject adequately.

> [@](#):
>
> \*Originally posted by Chronos \*  
> **If you put Coulomb’s Law and the rules for relativity in a blender and mix for a few minutes, you’ll discover that two parallel wires with current going the same way will be attracted to each other, and by the experimentally-observed result, without ever explicitly putting in anything about magnetism. Magnetism follows necessarily from electrostatics, as a direct result of special relativity.**

This is a bit dodgy. The actual derivation is to take Coulomb’s law, and then _assume_ that the Lorentz transformation is the correct one to use (most correct? most useful?). You end up with Maxwell’s equations. You _don’t_ get the Lorentz force equation out of that. That equation comes from somewhere else.

But Coulomb’s Law, Ampere’s Law and Gauss’s Laws of Electricity and Magnetism were all known prior to Maxwell. Maxwell and Einstein “unified” them, and provided some useful tools along the way. The “put Coulomb’s Law and the rules for relativity in a blender” stuff, due to Freeman Dyson and Richard Feynman, is tautological rubbish.

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**Author:** ![Desmostylus](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@Desmostylus](https://boards.straightdope.com/u/Desmostylus)\
**Post date:** [August 27, 2003, 1:42pm UTC](https://boards.straightdope.com/t/how-can-electricity-travel-the-speed-of-light/197285/26 "2003-08-27T13:42:45Z")

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> [@](#):
>
> \*Originally posted by Desmostylus \*  
> \*\*tautological rubbish. \*\*

I should clarify that a bit. It’s possible to derive Maxwell’s equations that way, but that derivation is not an independent proof, it’s simply a proof of consistency with the underlying empirically derived equations.

P.S. I left Faraday’s law out of the list in the previous post.

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**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\
**Post date:** [August 27, 2003, 2:05pm UTC](https://boards.straightdope.com/t/how-can-electricity-travel-the-speed-of-light/197285/27 "2003-08-27T14:05:58Z")

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**Desmostylus** if you’re standing next to a stationary conductor you _won’t_ detect a magnetic field. Now start moving with respect to the conductor and all of a sudden, you _will_ detect a magnetic field. How do you explain this without special relativistic length/volume contraction?

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**Author:** ![Desmostylus](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@Desmostylus](https://boards.straightdope.com/u/Desmostylus)\
**Post date:** [August 28, 2003, 11:22am UTC](https://boards.straightdope.com/t/how-can-electricity-travel-the-speed-of-light/197285/28 "2003-08-28T11:22:00Z")

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I appreciate your patience, **Ring** (and yours too, **Chronos**.) 🙂

To show the relationship between Coulombs law (F=k[sub]e[/sub]q[sub]1[/sub]q[sub]2[/sub]/r[sup]2[/sup]) and the Lorentz force (F=qvB=k[sub]m[/sub]q[sub]1[/sub]q[sub]2[/sub]v[sub]1[/sub]v[sub]2[/sub]/r[sup]2[/sup]), you need to get a v[sub]1[/sub]v[sub]2[/sub] term from somewhere.

The simple straightforward way to obtain this term is to note that I.dl = qv.

The rather tortured alternatives I’d seen so far involved a length contraction term 1/√(1-v[sup]2[/sup]) - √(1-v[sup]2[/sup]) = v[sup]2[/sup]/(1-v[sup]2[/sup]) which is approximately v[sup]2[/sup] for v \<\< 1.

I’ve tended to regard this as bullshit. I mean, I could probably come up with an interpretation of the v[sup]2[/sup] term in E=1/2mv[sup]2[/sup] that involved length contraction. Wouldn’t necessarily be correct, though.

However, today, I read a few papers by an Italian dude, Cavalleri, and then looked up Purcell’s derivation using length contraction. It’s different to what I’d seen before, and seems to be kosher. I’ll shut up now.

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