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Circuits (RC Circuits) Help!

  1. Dec 27, 2007 #1
    I get very confused when you have circuits involving Capacitors and Resistors

    Especially the concept of current travelling through the path of least resistance

    1) Let's say you have a circuit with a battery... with a resistor in parallel first, then a capacitor in parallel to the resistor.. does the current NOT pass thru the resistor at all.. since the capacitor route has no resistance?

    I've included an image called CIRCUIT 1

    2) Let's say you have the following scenario in CIRCUIT 2... diagram is provided... what happens when you close the switch?? does all of it pass into the capacitor ? or if it splits, how does it split :(

    Is there a simple conceptual way to understand these rules so that I don't have to keep memorizing what happens with diff. scenarios

    Attached Files:

  2. jcsd
  3. Dec 27, 2007 #2
    Well, the images are waiting for approval, yet, but I can say the current through the resistance is independent of the current through the capacitor, since it depends only on the resistance and battery values.
  4. Dec 27, 2007 #3


    Staff: Mentor

    This phrase has little value in circuit analysis. Current always travels through all paths, not just the path of least resistance. The goal of circuit analysis is to determine exactly how much current goes through each path.
  5. Dec 27, 2007 #4
    If you go ahead and actually hook up such a circuit and put ammeters in each branch, you'll see the resistor current go to a stable value almost immediately and then stay there. You'll see the capacitor current go to a peak value and then fall slowly toward zero (if you use a good digital meter, you'll be able to see how it approaches zero and that's interesting).
  6. Dec 27, 2007 #5
    Those circuits dont exist. You cant made such a circuit. The wires, the battery and the capacitor have resistance and inductance.
    You can analyze what happens if you connect real batterys to real capacitors but you must take into account the resistance, that is not 0.

    In fig.1 a capacitor that is connected to a battery ( you didnt draw the switch, but I suppose there is a switch ) draws infinite intensity.

    In fig.2 there is a short-circuit -> infinite intensity.
  7. Dec 27, 2007 #6
    I'm just confused as to what happens in a situation where a resistor is in parallel with a capacitor

    The only scenario I can understand so far is: what happens when a resistor is in series with a capacitor. That's it.
  8. Dec 27, 2007 #7
    The thing you need to always remember is that parallel circuits are essentially independent. (I am speaking here of voltage circuits). In a manner of speaking, the computer I am writing this on is possibly in parallel with the radio you might be listening to right now. So, the current through the parallel resistor is independent of the current into the parallel capacitor, etc. So, this allows you to look at parallel circuit branches as though they were little isolated circuits, provided you can figure out the voltage for that branch (usually the same in simple circuits). If you possibly can take a lab course in this, do so. It helps immensely to actually see this stuff.

    Anyway, the most important thing a resistor in parallel with a capacitor does is drain off the capacitor charge after the switch is opened. Keeps you from getting a shocking experience.