# Assistance with visualization of alternating current

Sorry, I'm already ashamed to ask for your help. But I'm feel that I needed in some hints about how to imagine alternating current. I really realize I have trouble with it during reading this document (full but old version is here). There is image on p.29:

At first, this simple circuit consisting of a battery and a resistor was in some steady state with constant voltage and current all around. Then source voltage is doubled. And there is this transient phase on the image during which a new voltage is forming. Pluses/minuses represent surface charges on the wires. The change in charge distribution travels like a wave-front through the circuit with speed of light.

I explicitly divide all these states apart. Like, 1st steady state with some current in accordance with Ohm's law > transient phase > new steady state after doubled voltage reached resistor with increased current in wires and the resistor.

My problem is that when I try to image AC voltage source instead of DC the mess begins. Since, roughly speaking, there is no steady states now, voltage and charge distribution on the wires is constantly changing. Because of this its hard for me to imagine alternating current in the circuit.

Maybe it would be a little easier for me if I knew the following. On the image above, there are 2 regions, behind "wave front", where a new doubled voltage has already been formed and ahead of it, where voltage is still old.

So my question is about currents in these regions. Is there in the region behind "wave front" already increased current and its value satisfying I = V/R and ahead the front, correspondingly, old value of current ?

If so, I can imagine AC more or less as follows: as voltage propagates down the wires it cause appropriate currents (according to Ohm's law), lets say, in vicinity of wave front. And since length of wires is small in this case, one can think as in some instant of time wave front "ran" through all wire and reach it end. And so we have some concrete equal value of potential for entire length of the wire and appropriate current's value in it.

If this is a wrong idea then I'm lost and would like to hear some tips about how to imagine it. Thanks for the help.

Updated: Now I think this is most likely incorrect because in that case even in an open AC circuit there would be a "normal" current. Well, after a couple of days of reflection and re-reading I realize I'am confuse charge redistribution processes with "main" current so nevermind. I'm going to rephrase the question.

• What are the pluses/minuses here supposed to represent? In a typical conducting circuit, there is only one type of charge carrier, with only one charge (usually negatively-charged electrons in a metal). In your example above, the electric potential of a DC circuit will be decreasing from a maximum at the positive terminal of the battery, to a minimum at the negative terminal. The charge carriers will all be flowing with a constant bulk speed around the circuit. Jul 29, 2018 at 0:23
• Pluses/minuses represent surface charges on the wires. Jul 29, 2018 at 2:45
• Is there any caption under the image in the book, or other text in the book describing what's going on in the image? From the image alone I can't make heads or tails of it. Jul 29, 2018 at 17:33
• @SteveT. I think those diagrams in your reference are confusing and overly-simplistic. In a conducting circuit, there is only one type of charge carrier (negative electrons), which have a bulk flow velocity around the circuit, in response to an imposed electric field. Jul 30, 2018 at 17:12
• @ Time4Tea Thanks for attempt to help, I confuse myself and going to read more for now. Jul 30, 2018 at 17:58