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For beginning, sorry for my english and for long post. But maybe it would be useful for others newbies. I'm trying to understand basic things about electric circuits on my own, and at the moment I become confused, so have to ask for help.

First I would like to describe how I understand processes in DC circuit, and then ask question about AC which cause confusion.

So, we have simple circuit consisting of battery, switch, couple of wires and resistor.

When switch is closed, transient phase begins, current start to flow in both wires simultaneously:

enter image description here

At first I thought the current is "usual" current that we are measure with amperemeter, but then realize this is "charging" current instead. Because to "create" potential at some place we need to move charge at the place (i.e. we need a current). And amount of charge needed to create certain voltage depends on capacitance. So this small current is not "main" current, and required only to create surface charges on wires i.e. voltage.

Eventually, in a few nanoseconds, maybe after some reflections, steady state is established. No more charging current. Now, these surface charges create electric field not only between wires but also inside the wires and the resistor. And this inner electric field cause electrons to move, thus we got "main" current, which obeys the Ohm law:

enter image description here

The image from Matter and Interactions by Ruth W. Chabay, Bruce A. Sherwood.

Or you can read about surface charges in article by J. D. Jackson "Surface charges on circuit wires and resistors play three roles"

Now if battery is replaced with AC voltage source I become confused. Hence there are, roughly speaking, 2 kinds of currents - charging / discharging and "main" current.

Unfortunately, I can't find anywhere such detailed description for AC, so I have a few questions.

Question 1:

First, is my description of DC is correct ? Because if my understanding is wrong, there is no point to move further.

Question 2:

Now, if I imagine AC as follows:

In open circuit there is only constant charging / discharging currents and "main" is absent.

If some load is connected then there would be constant charging / discharging currents AND, along with it, "main" current. I.e. constant surface charges redistribution due charging currents would create constantly increasing, decreasing and flipping electric field inside wires and resistor. Again, as result, we got "main" current due this inner electric field. But since the field is constantly changing now, electrons just slosh back and forth.

I'm assume this is oversimplified view since in this case changes in "main" current will always happen after charge redistribution in the circuit.

Anyway, is this wrong or acceptable description ?

Or it would be appropriate only for low frequencies (because if frequency really high and wire potential is differs along it, I cant even imagine the picture, total mess).

Question 3:

In general, are these constant charging / discharging currents ignored for simplicity in circuit theory, despite the fact, the are always actually present ?

I mean if I measure current in AC circuit or look at graph for example of AC RC circuit where current curve lead voltage by 90 degrees, it always related to "main" current, and not these charging / discharging currents ?

Question 4:

Are these these charging / discharging currents flow on surface of conductors (since they create surface charge) ?

Thanks for any help.

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  • \$\begingroup\$ No, we don't need any current to flow to have a potential difference. The electrons actually flow very slowly through a circuit; you would be better to think about energy flowing through a circuit at almost the speed of light. And there aren't two kinds of current. Sorry. \$\endgroup\$ – Elliot Alderson Aug 6 '18 at 21:32
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    \$\begingroup\$ @ElliotAlderson not correct: insulators connected to power supply won't display potential difference like conductors do (zero current, zero surface charge.) Brief 'capacitive' currents are certainly necessary before potentials become established. (brief: nanosecond scale.) All circuits are composed of small parasitic capacitors ~pF and below, which are charged when the supply is connected. \$\endgroup\$ – wbeaty Aug 7 '18 at 1:03
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    \$\begingroup\$ @wbeaty No, you misread my statement. I said we don't need current to flow to have a voltage difference. Take any battery out of your parts box...there is a potential difference between the terminals whether current flows or not, whether you measure the potential or not. I'm talking about the definition of electric potential, not your common observations of it. \$\endgroup\$ – Elliot Alderson Aug 7 '18 at 10:56
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    \$\begingroup\$ @wbeaty The water analogy is also misleading because a conductor with a higher electric potential doesn't have more charge in it like the rising level of water in a tub. Analogies like this tend to confuse students because they lose sight of the important bit, which is the transfer of energy rather than matter. \$\endgroup\$ – Elliot Alderson Aug 7 '18 at 11:02
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    \$\begingroup\$ @Barleyman I didn't say anything about the "speed of electricity"...that phrase doesn't have much meaning in engineering. I said that the electrons move slowly and energy moves quickly. Yes, the propagation of the wave depends on the dielectric constant but this is still many orders of magnitude faster than electron motion. \$\endgroup\$ – Elliot Alderson Aug 7 '18 at 15:51
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I can see that this is going to be like the three blind men describing an elephant — each response is going to emphasize a particular detail, and it's going to be difficult to get a good understanding of the underlying principles. But here goes...

Yes, your understanding of the low-level physical details of what happens in a DC circuit is correct, but that's way too much detail for everyday circuit analysis.

Instead, we generally do a simplified analysis using lumped circuit elements, and ignore the effects associated with the charging and discharging of circuit nodes. This is the basis of Kirchoff's voltage and current laws.

AC analysis is simply a matter of allowing the value of the voltage (or current) source(s) to vary with time. This still ignores the wiring effects, but now you need to account for the reactive properties of actual capacitors and inductors in the circuit.

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  • \$\begingroup\$ Sorry, I did not quite understand. Does your answer mean that when I read about behavior of some current in circuit or measure it, I can be sure that this current always mean "main" current in wires / load, and not charging / disharging currents ? \$\endgroup\$ – Steve T. Aug 7 '18 at 14:23
  • \$\begingroup\$ Well, no. Any actual measurements will include both, but the "charging/discharging" current (we call those "transmission line effects") will be insignificant relative to the "main" current except in certain special cases. \$\endgroup\$ – Dave Tweed Aug 7 '18 at 15:40
  • \$\begingroup\$ Then should I think of current at some place as of total flow of charge in the place, with some portion of it "spent" on charging /discharging ? In circuits this portion is insignificant, but for transmission line, I assume it can be pretty large. \$\endgroup\$ – Steve T. Aug 7 '18 at 16:04
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First, is my description of DC is correct ? Because if my understanding is wrong, there is no point to move further.

Technically they are both AC circuits, there is no steady state current, which would make them AC. DC is 0Hz, AC is everything else (or more associated with sine waves, but doesn't have to be).

If some load is connected then there would be constant charging / discharging currents AND, along with it, "main" current. I.e. constant surface charges redistribution due charging currents would create constantly increasing, decreasing and flipping electric field inside wires and resistor. Again, as result, we got "main" current due this inner electric field. But since the field is constantly changing now, electrons just slosh back and forth.

Anyway, is this wrong or acceptable description ?

This is where your getting confused: The carrier in an AC circuit is not the electron it is the electric field on the inside (surface currents) and outside of the wire. This is where the bulk of the energy is being transferred, the wire acts like a wave guide. If you integrate all the energy being carried by electrons vs the electric field on the outside of the cable, very little of that is transferred by the electrons themselves. Electrons are not needed to carry electromagnetic energy, or radios would not work, the carrier is the photon.

The reason for this is the inductance of the wire itself, with higher frequencies they prefer lower impedance of the outside of the wire. This is called skin effect

enter image description here
Source: https://www.allaboutcircuits.com/textbook/alternating-current/chpt-3/more-on-the-skin-effect/

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