# Conductors and Coulombs law

• posted
Awl--
I seem to remember that current flow (or current density) on a solid
conductor resides *exclusively* on it's surface, ie, looking at the wire
cross-sectionally, you would see *only one* layer of electrons--not even
two, no matter how high you cranked up the voltage across the conductor.
This was explained on the basis of Coulombic repulsion, and something of
Gauss' law, which I can't quite remember.
This would mean that the current-carrying capacity of a wire is *strictly*
proportional to its circumference, not to its cross-sectional area.
But it seems to me that if the current density were high enough, and thus
the very outer layer of electron flow sufficiently crowded, indeed an "inner
layer" of electron flow should be forced, and that then the current carrying
capacity of a wire would be proportional to its cross-sectional area.
Or is it that before the electrons could be forced into an inner layer, the
repulsive forces of the electrons would be so strong as to force them to arc
into air, or across an insulating covering??
Any idears?
Thanks.
----------------------------
Mr. P.V.'d
formerly Droll Troll
• posted
Not exactly. The current in a solid conductor will tend to flow on the outside as the frequency of current increases. DC current is fairly evenly distributed through the entire cross section. The term you need to look for in order to learn more is "skin effect". Here is a good place to start:
Skin effect is very prominent at high frequencies. Microwave conductors are actually hollow (and called waveguides) because the signal only travels on (or very near) the surface.
Skin effect IS a problem at normal power frequencies (50 or 60 Hz). It is much less pronounced, but it still effects decisions on conductor choice and design.
Charles Perry P.E.
• posted
Thanks. This is not dissimilar to other recent opinions I've gotten.
Doesn't uniform charge distribution across the cross-sectional area violate Gauss' law (forgot what gauss' law actually sez, just remember that it dealt with these types of problems), and coulombic constraints? Wouldn't DC current *necessarily* start at the "skin", and 'build up" inward, if necessary, according to these laws? Thanks. ---------------------------- Mr. P.V.'d formerly Droll Troll
• posted
I have to warn you all before i respond to this: I was chastised and asked to leave the class and never come back after I privately showed one of my teachers my take on Electron Flow at an atomic level using a substrate like pictorial diagram I made employing the Electron Theory for Semiconductors. which is, As Is, an atomic level view of anything that conducts.
To start; I think you are thinking way too Conventional to visualize Electron Flow in a Conductor as over the surface tension only., that would be similar to the Magnetic Flux that would hold charge particles in an Orbit surrounding & through the wire surface.
I suggest we try visualizing how this works with the Molecular Structure of Solid 10 AWG for instance, the Magnetic portion of the applied EMF would travel through this outer portion of the wire, and the current flow ( hole transfer etc) would be totally through the thick matter of the inner wire., then, what about stranded AWG where the electron flow & magnetic flux is concentric to the conductor as a whole, but atomically each strand carries it's properties mho & rho alone simultaneous to a splice non the less ~>
• posted
I think the guy in the sci group where you posted also gave a very good & clear explanation., with the change in electron or current flow paths as you query occuring when higher frequencies are applied to the conductor. =AE
• posted
Thank god for consistency!!
But for the DC case:
Doesn't a *uniform distribution* of electrons in the cross section have them *unresponsive* to very powerful Coulomb forces, and seemingly violate Gauss' law(s)??
It seems to me, that on *very first principles*, the electrons should at least have a *variable distribution* in the cross section, heavily weighted toward the circumference. No?
I will post this on a.s.p. as well. Thanks. ---------------------------- Mr. P.V.'d formerly Droll Troll
I think the guy in the sci group where you posted also gave a very good & clear explanation., with the change in electron or current flow paths as you query occuring when higher frequencies are applied to the conductor.
®
• posted
The rule that there is no electric field in a conductor applies to static charges, in equilibrium. In a current carrying conductor, there is most definitely an electric field, parallel to the direction of current flow. In the case of steady DC, that electric field is uniform through the conductor's cross section, so that any cross section is an equipotential surface. Any deviation from this uniformity would be a distortion of the applied electric field, which would result in currents that would restore it to uniformity. This applies to a wire, or other cylinder carrying current along its axis. If it were a conductive sphere, it might be a little more complicated.
-- john

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.