Current in a (series) circuit with a capacitor

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Discussion Overview

The discussion revolves around the behavior of current in a series circuit containing a capacitor, addressing questions about the nature of current flow, the application of Kirchhoff's laws, and the differences between alternating current (AC) and direct current (DC) in relation to capacitors.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions how current can exist in a series circuit with a capacitor, given the insulating material between the plates.
  • Another participant explains that alternating current can flow through a capacitor due to the electric field, while direct current cannot, as electrons only wiggle back and forth in AC.
  • A participant notes that their physics book discusses capacitors in the context of direct current circuits, raising a point of confusion.
  • There is a clarification that while capacitors do not conduct DC, they can still be used in series DC circuits for other purposes.
  • One participant asserts that there is current flow sufficient to charge and discharge the capacitor, suggesting a different perspective on current behavior in this context.
  • A mathematical expression for current flow into an uncharged capacitor is provided, indicating a time-dependent behavior.
  • A mechanical analogy involving a water pipe and a condom is presented to illustrate the behavior of a capacitor in DC and AC circuits.
  • Another participant discusses the implications of charge buildup on capacitor plates and how it affects the application of Kirchhoff's Current Law (KCL), suggesting a need for different analytical approaches when dealing with capacitors.

Areas of Agreement / Disagreement

Participants express differing views on the nature of current flow in circuits with capacitors, particularly regarding the applicability of Kirchhoff's laws and the behavior of capacitors in DC versus AC circuits. No consensus is reached on these points.

Contextual Notes

There are unresolved questions regarding the definitions of current flow in the context of capacitors, the assumptions underlying the use of Kirchhoff's laws, and the conditions under which capacitors operate in different types of circuits.

broegger
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I've just been reading about capacitors and I have some (stupid?) questions... First, how can there be a current in a (series) circuit with a capacitor when there is an insulating material or vacuum between the plates (or whatever makes up the capacitor)?? i don't get it.. Second (pretty much the same as the first), how can you apply Kirchoff's closed loop law to a circuit in which there is a capacitor - it isn't closed when there can be no current through the insulator or vacuum in the capacitor??
 
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Alternating current can flow through a capacitor (but direct current cannot). Consider that in an alternating current, the electrons don't actually flow all the way around the circuit -- they basically just wiggle back and forth in place. The capacitor couples its plates together through the electric field, which is exactly what causes electrons to move through wires.

- Warren
 
but in my physics book they discuss these things before even introducing alternating current.. in fact, i think it's in a chapter called Direct Current Circuits..
 
Well, capacitors do not conduct DC. So I don't know what to tell you.

- Warren
 
So, you're saying that capacitors can't be used in (seriel) dc circuit?
 
No, I'm not saying they can't be "used." There are many ways a capacitor could be used in a series DC circuit -- but conducting current is not one of them.

- Warren
 
I've posted this in the other thread - there is current flow sufficient to charge/discharge the capacitor.
 
The current flow into a uncharged cap is like

[tex]i(t)= i_0 e^{-kt}[/tex]

where the k is determined by the capacitor.
 
condom capacator analogy

The mechanical analogy of a capacator, is a water pipe with a condom traped in it. If water tries to flow in one direction (DC), the condom will streach until it stops the flow of water. If the water oscilates back and forth in the pipe (AC), the condom stretchs back and forth, representing little or no effect on the oscilating water.

All the best
john
 
  • #10
Ironically, a capacitor kind of violates KCL, at least significantly more so than the other two passive elements. Charge builds up on the plates. To apply KCL, you have to "draw a black box" around the capacitor and treat it as an isotropic 2-port device. The same can be done with the other two passives. You can still apply KCL and KVL outside the elements, but, instead of Ohm's law, you have to realize that capacitors are reactive and a differential equation or Laplace transform must be used.
 

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