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Homework Help: Discharging DC Circuit Modeling Question

  1. Jul 30, 2010 #1
    1. The problem statement, all variables and given/known data
    Hello all, I just have a quick question regarding the modeling of the following circuit (see attached).
    I am trying to derive the equation for the voltage across the capacitor as a function of time. I come up with the correct result when the element polarities are the same (i.e. figure on the left) but do not when the polarities are opposite (right figure). I have attached a pdf file showing my work if you care to look it over. My resulting equations are:

    Left figure:
    v_c(t)=v_0*exp(-t/RC); CORRECT

    Right figure:
    v_c(t)=v_0*exp(t/RC); SHOULD BE NEG. IN EXP

    I am a mech. eng. and thought that the polarity setup was irrelevant for modeling. Am I missing something? Thanks for the help!



    2. Relevant equations



    3. The attempt at a solution
     

    Attached Files:

  2. jcsd
  3. Jul 30, 2010 #2
    Hey Guys,

    I found my error. I had inconsistent current directions--silly mistake. My apologies.
     
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