# EEs & Physicists: Why does current flow in a wire? **URL:** \ **Category:** Factual Questions\ **Created:** [March 31, 2003, 3:29pm UTC](https://boards.straightdope.com/t/ees-physicists-why-does-current-flow-in-a-wire/165453 "2003-03-31T15:29:03Z")\ **Posts on this page:** 12\ **Page:** 3
**Author:** ![David\_Simmons](https://avatars.discourse-cdn.com/v4/letter/d/9de053/32.png) [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)\ **Post date:** [April 1, 2003, 6:12am UTC](https://boards.straightdope.com/t/ees-physicists-why-does-current-flow-in-a-wire/165453/41 "2003-04-01T06:12:34Z")
> [@](#): > > \*Originally posted by Desmostylus \* > \*\*Just a minor nitpick, **David Simmons**. The EM field doesn’t just penetrate to the skin depth. The field strength decays exponentially. The skin depth is the depth at which the field strength has been reduced to 1/e, or about 37%, of the strength at the surface. \*\* Correct, and I should have said that. The field is down 95% by 3 and 98% at 4 skin depths. > [@](#): > > - > > Originally posted by Ring\* > **And another minor nitpick.The penetrating EM field is an evanescent wave and since in an evanescent wave the E and B fields are ninety degrees out of phase it cannot transport energy. The energy just sort of sloshes around.** I can’t argue with this because, as I said, I haven’t looked at this stuff for a long, long time. However, the text by Ott that I quoted says that for a conducting shield 1 skin depth thick, the _"absorption loss is 9 dB._ Doubling the thickness of the shield doubles the loss in dB." This indicates that the energy in the field that penetrates the conductor is being dissipated inside the shield somehow. > [@](#): > > _Originally posted by Desmostylus_ > > \*\* > > Since were going to get right into it, I’ll add another one. > > Conductors only act as conductors up to a certain frequency. Beyond that critical frequency, displacement current exceeds conduction current. > > For some common materials, the frequency is: > > Silver… 1.1x1018 Hz > Aluminium… 6.7x1017 Hz > Solder… 1.2x1017 Hz > Sea Water… 9x108 Hz \*\* True. And even before those frequencies, the circuit model gets dicey because the components can’t be considered as lumped elements having on the characteristic of resistance or capacitance or inductance. Components first begin to act as resonant circuits and then with increased frequency have to be regarded as distributed elements in which case the circuit approach is awfully cumbersome. Although with computer modeling it might not be that bad. Things are beginning to fall apart around 1 gigaHz for ordinary circuit elements as I remember it. Elements in integrated circuits can be made small enough that the frequency can be higher before you run into trouble but I have no idea what it is. ---
**Author:** ![Desmostylus](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@Desmostylus](https://boards.straightdope.com/u/Desmostylus)\ **Post date:** [April 1, 2003, 6:38am UTC](https://boards.straightdope.com/t/ees-physicists-why-does-current-flow-in-a-wire/165453/42 "2003-04-01T06:38:18Z")
There’s no real practical significance to my last post as far as circuits go. 10[sup]18[/sup] Hz means a wavelength of 3x10[sup]-10[/sup] m, i.e. x-ray type wavelengths. ---
**Author:** ![Desmostylus](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@Desmostylus](https://boards.straightdope.com/u/Desmostylus)\ **Post date:** [April 1, 2003, 12:01pm UTC](https://boards.straightdope.com/t/ees-physicists-why-does-current-flow-in-a-wire/165453/43 "2003-04-01T12:01:41Z")
> [@](#): > > \*Originally posted by Ring \* > \*\*And another minor nitpick.The penetrating EM field is an evanescent wave and since in an evanescent wave the E and B fields are ninety degrees out of phase it cannot transport energy. The energy just sort of sloshes around. \*\* After thinking about this one for a while, I still can’t work out what it means. If you come back to this thread, **Ring** : - In a perfect dielectric, the E and B fields are 90[sup]o[/sup] out of phase, and so no energy can be _dissipated_, but that doesn’t mean that energy cannot be _transported_. - “Evanescent waves” aren’t something I’ve come across regarding field penetration in _conductors_, but rather total internal reflection in _dielectrics_. One of us is misunderstanding something. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [April 1, 2003, 1:20pm UTC](https://boards.straightdope.com/t/ees-physicists-why-does-current-flow-in-a-wire/165453/44 "2003-04-01T13:20:48Z")
> [@](#): > > \*Originally posted by Desmostylus \* > \*\*There’s no real practical significance to my last post as far as circuits go. 10[sup]18[/sup] Hz means a wavelength of 3x10[sup]-10[/sup] m, i.e. x-ray type wavelengths. \*\* I make it 3 x 10[sup]-11[/sup] m, which is gamma rays (10[sup]-10[/sup] - \>10[sup]-15[/sup]). Mostly, however, I’m curious about what property of conductors this phenomenon is related to. I’ve not come across this in any of the literature, since, as you say it’s meaningless for practical circuit design. So, what _is_ it for? ---
**Author:** ![Desmostylus](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@Desmostylus](https://boards.straightdope.com/u/Desmostylus)\ **Post date:** [April 1, 2003, 2:13pm UTC](https://boards.straightdope.com/t/ees-physicists-why-does-current-flow-in-a-wire/165453/45 "2003-04-01T14:13:39Z")
> [@](#): > > \*Originally posted by Q.E.D. \* > **I make it 3 x 10[sup]-11[/sup] m** What do you want me to say? I know how to use a calculator, so do you. 3x10[sup]8[/sup] ms[sup]-1[/sup]/1x10[sup]18[/sup] s[sup]-1[/sup]=3x10[sup]-10[/sup] m. The distinction between x-rays and gamma rays is somewhat artificial. X-rays come from electron transition, gamma rays come from nuclear interactions. The frequencies overlap. What’s it for? It demonstrates the boundary between conductor and dielectric. It explains why gamma rays penetrate metals. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [April 1, 2003, 2:23pm UTC](https://boards.straightdope.com/t/ees-physicists-why-does-current-flow-in-a-wire/165453/46 "2003-04-01T14:23:30Z")
My mistake. I must have used 10[sup]18[/sup]instead of 1x10[sup]18[/sup]. ---
**Author:** ![Desmostylus](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@Desmostylus](https://boards.straightdope.com/u/Desmostylus)\ **Post date:** [April 1, 2003, 2:35pm UTC](https://boards.straightdope.com/t/ees-physicists-why-does-current-flow-in-a-wire/165453/47 "2003-04-01T14:35:51Z")
> [@](#): > > \*Originally posted by Q.E.D. \* > \*\*My mistake. I must have used 10[sup]18[/sup] instead of 1x10[sup]18[/sup]. \*\* What? What’s the difference between 10[sup]18[/sup] and 1x10[sup]18[/sup]? Seriously, please think before posting. Especially in threads like this. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [April 1, 2003, 2:42pm UTC](https://boards.straightdope.com/t/ees-physicists-why-does-current-flow-in-a-wire/165453/48 "2003-04-01T14:42:05Z")
That should have read “10x10[sup]18[/sup] instead of 1x10[sup]18[/sup]”. As you can plainly see, I’m horrible at math. :smack: ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [April 1, 2003, 4:02pm UTC](https://boards.straightdope.com/t/ees-physicists-why-does-current-flow-in-a-wire/165453/49 "2003-04-01T16:02:11Z")
> [@](#): > > \*Originally posted by Desmostylus \* > \*\* > > One of us is misunderstanding something. \*\* And it’s probably me. After checking textbooks and searching the Web I can’t seem to find any globally agreed upon definition of what an evanescent wave even is. Roy McCammon is a guru at [IEEE.org](http://IEEE.org) and here’s what he says: (Please notice he uses the same scientific term I did (slosh)) > [@](#): > > I must admit that I cannot cite an authoritative reference that says an evanescent field is such and such; all I can find is examples that have been labeled evanescent. There could be more than one correct definition in general use, however, every case that I have encountered has these features: The time phase of the E and H fields everywhere are at 90 degrees difference from each other, and the time phase the field is not a function of position (everywhere the same). The amplitude usually attenuates with distance. The fact that E and H are at 90 degrees tells you the field cannot transport net energy (averaged over one cycle) but energy can slosh back and forth. The field inside the copper does transport energy into the copper where it is absorbed as heat. E and H inside the copper are at 45 degrees (over a significant frequency range) so you might say that the wave is half evanescent. But the essence of an evanescent wave is that it only stores energy and does not transport it, hence my reluctance to characterize the wave inside the copper as evanescent ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [April 1, 2003, 5:14pm UTC](https://boards.straightdope.com/t/ees-physicists-why-does-current-flow-in-a-wire/165453/50 "2003-04-01T17:14:29Z")
I can back up **Roy McCammon**. I did a quick look through several of my EM books. They all use _evanescent_ only when the fields are non-lossy, with E and H 90 degrees out of phase. Attenuation is used when the fields are propagating and decaying. Using this definition, the fields in a good conductor are attenuating, but not evanescent. ---
**Author:** ![bbeaty](https://avatars.discourse-cdn.com/v4/letter/b/c0e974/32.png) [@bbeaty](https://boards.straightdope.com/u/bbeaty)\ **Post date:** [April 2, 2003, 6:57am UTC](https://boards.straightdope.com/t/ees-physicists-why-does-current-flow-in-a-wire/165453/51 "2003-04-02T06:57:21Z")
About surface currents… One source of misconception is the fact that the imbalanced charges on conductors can only exist on the conductor surface. E-fields and circuit voltages are created by this net-charge or “charge-imbalance,” and it drives electric currents within wires. Some people wrongly conclude that since all the charge is on the wire surface, all the current must also be on the surface, since current is just charge-flow. Wrong. In wires, electric current is a flow of cancelled-out charge. The population of electrons inside the wire is mixed in with a population of protons of the metal atoms, so the net charge within the metal is zero. Yet the negative charges can flow along through the positive ones. Even though the NET charge is zero, the wires still contain an enormous amount of movable charge. If the electrons flow through the protons (or vice versa) then we have a large current even though we still have zero net charge. A net charge does exist on the surface of wires. Yet this charge is negligable when talking about the flowing charges in a current. The surface charge is on the order of picoCoulombs, while the electron population with a wire is on the order of tens of Coulombs. The surface charge is only important in generating e-fields and causing voltage. Here’s a short paper on this stuff: Electrostatics and circuits, R. Chabey, B Sherwood (PDF) > **[Ruth Chabay](https://ruthchabay.net/)** > > Physics, models, computation These authors wrote a physics text which unlike other books explains circuits correctly and in great detail: [http://www.amazon.com/exec/obidos/tg/detail/-/0471078476/](http://www.amazon.com/exec/obidos/tg/detail/-/0471078476/) > [@](#): > > \*Originally posted by Crafter\_Man \* > **Well I think we have it covered, whuckfistle, but thanks for the offer. What’s bothersome is that I’ve taken “EM field” and “solid state devices” classes, yet I’ve never acquired a “feeling” for what’s going on with these damn electric and magnetic fields.** You’re not alone! I spent lots of hobby time re-teaching myself electronics from the ground up, rooting out errors and misconceptions, etc. Electricity is fouled up because it’s taught wrong in grade school, and even PhD people still have those wrong concepts stuck in their heads. We’d have to perform some intentional “un-learning” to root them out of there. Most people never do this. See: [Electricty Misconceptions Spread By K6 Textbooks](http://amasci.com/miscon/elect.html) Here’s a personal favorite: a great website at MIT that shows us the EM fields around a dipole antenna as the antenna emits radio waves. (Large MPEG download, but worth it!) [http://ocw.mit.edu/8/8.02/f02/visualizations/index.html](http://ocw.mit.edu/8/8.02/f02/visualizations/index.html) PS Current doesn’t flow in a wire. The name of the stuff that flows within wires is CHARGE. The word “current” means “charge flow.” What flows in rivers, water? Or “current?” What flows in wires? Charge? Or “current?” To help defeat this error, just call them “currents” instead of “current.” “Current” sounds like the name of a substance, while “Currents” are like motions. This stuff might seem like a minor quibble, but I’ve found that it has major consequences. It leads to sloppy thinking on the part of textbook authors, and they end up putting many other errors in texts on intro electrical physics. ---
**Author:** ![Desmostylus](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@Desmostylus](https://boards.straightdope.com/u/Desmostylus)\ **Post date:** [April 2, 2003, 8:32am UTC](https://boards.straightdope.com/t/ees-physicists-why-does-current-flow-in-a-wire/165453/52 "2003-04-02T08:32:08Z")
Dunno about you, **bbeatty** , but _I_ find it conceptually useful to talk about things like “a current of 100 A flowing from a to b”, even though it’s AC and the charge isn’t flowing _anywhere_. And apologies to the others for my earlier, sloppy use of the symbol “B” in place of “H”. [Previous page](https://boards.straightdope.com/t/ees-physicists-why-does-current-flow-in-a-wire/165453.md?page=2)