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There are times, in the process of answering questions, that there is a mismatch between what I am hoping to convey, and how some site participants interpret what I am trying to convey. (I will give an example below). That leads me to search for better formulations. Sometimes I can find such formulations on my own, but sometimes I would like input from others. My question here is, are questions of the form "How might I best express concept X?" where X is related to electrical engineering, appropriate for EESE? EESE-Meta? Both? Neither?

Such questions are not directly about electronic design, but rather about pedagogy about electronic concepts. Does that make them inappropriate for the main EESE list? Would like to know other's opinions.


Now for the example. In answer to a question by a student who disagreed with his professor I wrote in part

Current always flows in a circuit. (Well almost always, "static" electricity being the exception.)

to which someone replied

"(Well almost always, "static" electricity being the exception.)". Wrong, static electricity is no exception.

That person is right, of course. But he is right because of displacement current. When current flows through a capacitor, in the conductor leading to one of the plates, there is conduction current (actual flow of charges), in the dielectric there is a displacement current, and in the conductor of the opposite plate there is another conduction current. All three of these currents are exactly equal, so we often, or even generally, ignore the transition of one type of current to another and then back again. We just speak about the current "through" the capacitor, lumping conduction current and displacement current together.

Now let us consider a capacitor which is charged. Through the weakness of the dielectric, electrons flow between the plates. This is a conduction current. As the capacitor discharges, there is a displacement current that is exactly equal in magnitude to the conduction current. The two algebraically sum to 0. So it is just as correct to say that the current in the capacitor is 0, as it was to say that the current in every part of a simple circuit is equal (if that circuit includes capacitance across a dielectric).

However, there is something intuitively funny about describing the internal discharge of a capacitor with 0 current. Here, I think we want "current" to mean "conductive current" and not "conductive current + displacement current", and we want to describe the current as non-zero. To a technical audience, we can say "conductive current". But to someone unfamiliar with the concept of displacement current, "current = the flow of charges".

What I was trying to express with my comment about static electricity being "the exception" is that in "static" electricity, conductive current does not flow in a circuit. (Current does, but not conductive current). Of course in a circuit with capacitors, the same is true. I am thus wondering what might have been a better way to express my intent.

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  • \$\begingroup\$ The chat would be best for this kind of question, meta is for questions about the site. So this question would fall under 'neither' \$\endgroup\$
    – Voltage Spike Mod
    Jan 29 at 4:55
  • \$\begingroup\$ If you want to know what is on topic for the regular site please visit here: electronics.stackexchange.com/help/on-topic \$\endgroup\$
    – Voltage Spike Mod
    Jan 29 at 4:56

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