0

Distribution of surface charges with a constant gradient is present along the length of the wire which produces a constant electric field along the entire length of the wire. This results in constant current in all the sections of the wire.

I am aware of the constant distribution of the surface charges at equilibrium but when the circuit is complete, the distribution is not constant but the change in density of the surface charges is constant.

Can some one please be able to explain me, how exactly is this constant gradient of surface charges formed? I would like to know the entire process of going from equilibrium (open circuit) to the steady state (closed circuit) in terms of surface charges redistribution.

winny
  • 13,064
  • 6
  • 46
  • 63
  • It's a great question. I have written here a few times on the topic, but only in hand-waving ways. Most don't really care much about the physics going on (or, those who do are not raising their hands in the air here.) However, the topic is hard if you want *"the entire process"* explained. That's not going to happen here. It's a specialty. An introductory segue to the topic is covered in Matter & Interactions by Chabay and Sherwood. It's enough to get started on some modeling equations. The period of time for this transition is on the order of femptoseconds. So it's mostly ignored in EE. – jonk Jun 06 '21 at 03:50
  • I tried explaining it in this answer ..... See if this can help you (you can refer scenario 1 and scenario 2 part of the answer) https://electronics.stackexchange.com/a/567583/284706 –  Jun 06 '21 at 04:54
  • By the way, this is an area of interest for me. So if you are willing to work hard on this one, I will also work. I would enjoy it. ;) – jonk Jun 06 '21 at 05:00

1 Answers1

0

The question really could be written so: when we instantly apply a potential-difference to the ends of a resistor, how does the resistor respond over time, in order to eventually exhibit a constant current-density throughout its interior?

A "quick" intro is found in the 1999 paper by Chabay and Sherwood, author of undergrad physics text MATTER AND INTERACTIONS: A unified treatment..." The main conclusion is that, for the final DC case, the surface-charge is distributed in circles of constant surface-charge density, i.e. a resistor is like a stack of charged rings. (For the DC case, all charge-imbalance must exist only on the conductor surface.)

But for the full description, we really should use FEM finite-element simulators with animated field-diagrams, since each charged volume in the wire will experience a capacitance to distant Earth ...and also exhibit a capacitance to every other charged volume in the wire!

One method is to initially assume that the wire has zero resistance, and can be modeled as a thin hollow tube (pure skin-effect, all transient and no long-term DC.)

When the voltage-drop is first applied to a long thin conductor, EM waves dance back and forth along this hollow tube, creating the rings of charge and shoving them around, until they settle down and produce a uniform current density on the surface. This occurs at the speed of light, over fractions of nanoseconds.

Hmmm, rather than a wire in space, maybe it would simplify things to first examine the center-conductor of a piece of coax cable. That way the resistor becomes a simple transmission line, with all the capacitance only existing between the wire surface and the nearby inner surface of the shield-braid. In that case the wire becomes a looooong inductor, like a long coil, but with numerous capacitors connected between the wire surface and the nearby Earth-potential.

Then, once we know the nanosecond behavior of voltage-waves on this conductor, then for real conductors having finite resistance, next the current "oozes inside" over long microseconds. All metals are EM-shields of course, so the initial e-fields and currents are blocked from affecting the interior of the metal wire. The elements of the wire exhibit L/R time constants, with slower response for lower resistance. Or in other words, charge "has inertia," and the applied e-field can only, after a significant delay, create a current within the interior of the wire.

Or said differently, the speed of light inside copper is quite slow, on the order of tens of M/S, and "skin effect" is really about the electrical energy being able to "leapfrog" quickly across the space outside the metal surface, and only slowly to propagate inwards ...as if the wire was really an "onion layer" of concentric pipes. At first, the entire current only arises on the outside pipe. For a thick copper wire, an entire millisecond may pass by before the current becomes uniform throughout the metal. Yet the first initial current-pattern became established millions of times faster, in roughly a nanosecond.

See, we really really need animated diagrams!

But so few people are interested in this niche-topic, such things have never been done before. And with good reason. At MIT, a Dr. Belcher tried to introduce it to thir large undergrad EM course, with field-simulations giving a view of the internal workings of all components. Rumor is, the undergrad students rose up en masse and forced the administration to go back to the old way: obscure walls of equations, with none of these easily-understood animated pictures! To me the situation appears like that of medical doctors in the year 1300, if their med school suddenly started teaching courses in plain English (or French!) Nooooo! Then just anyone could understand the material! It turns the complex and obscure into "Physics for Poets!" BRING BACK THE LATIN RIGHT DAMN QUICK, OR THE STUDENT BODY WILL MARCH IN AND HANG THE LOT OF YOU!

Heh.

Here's the residue of the late-1990s MIT "T.E.A.L" project by JW Belcher and crew...

ALso: https://web.mit.edu/fnl/vol/162/belcher.htm

wbeaty
  • 10,761
  • 23
  • 39