1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Kirchoff's Loop Rule as applied to Capacitors?

  1. Jan 19, 2008 #1
    1. The problem statement, all variables and given/known data

    How does the loop rule apply to capacitors? I can't find any examples of circuits containing capacitors and resistors where the loop rule is used. I know the loop rule measures potential differences, but I'm not quite sure if that has anything to do with capacitors? All the examples are 0 = V - IR - IR, etc.

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Jan 19, 2008 #2

    Tom Mattson

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Yes, the loop rule is used with capacitors all the time. The element law for a capacitor is [itex]v=q/C[/itex]. In more advanced (calculus-based) courses this is written [itex]i=C\frac{dv}{dt}[/itex]. Solving this for the voltage, one obtains:

  4. Jan 19, 2008 #3
    all the basics of RC circuits (RL and RLC circuits too) come from a basic application of Kirchoff's Loop principle.
  5. Jan 20, 2008 #4
    So when finding currents, the branch that a capacitor is on (in terms of current) is 0, right? since when a capacitor is full, no current flows.
  6. Jan 20, 2008 #5
    not necessarily. it depends on the situation. Since charging rate = current, current=0 if and only if the charge of the capacitor is constant. This happens when the capacitor has been (dis)charging for a long time, or when the circuit reaches steady state.
    Last edited: Jan 20, 2008
  7. Jan 20, 2008 #6
    the problem says that the currents reach equilibrium. isn't that steady state?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook