1. Not finding help here? Sign up for a free 30min 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!

Discharging Capacitor

  1. Feb 25, 2014 #1
    1. The problem statement, all variables and given/known data

    The voltage across the capacitor is 10V when t<0
    The switch is closed at t=0

    I have to find the diff eq. in terms of Vs, C, R by balancing the current at node Vc(t). But i am not sure whether the Vs is 0 or 10V at t<0

    3. The attempt at a solution

    I am assuming Vs = 0 at t<0 since the voltage across capacitor is 10V. Also, I am not sure what current balance at the node means...I mean when the switch is closed the current through the capacitor is the same as current through the resistor. So,

    iR = iC
    (Vc/R) = C (dVc/dt)

    How do i write this in term of Vs? Isn't the current through the resistor Vc/R since Vs is 0 at the beginning?
     

    Attached Files:

    • rc.jpg
      rc.jpg
      File size:
      5.3 KB
      Views:
      67
  2. jcsd
  3. Feb 25, 2014 #2
    I'm not sure what you mean when you say "I am assuming Vs=0...". I think you mean that you are assuming that the voltage at the negative end of Vs is zero - which would be a fine assumption. It doesn't really matter what point you take as your "ground" or zero voltage level.

    If you take Vs- as zero, then obviously Vs+ will be "Vs" and Vc(0) will be at (Vs-10v). Or, perhaps, (Vs+10V), since the polarity of the voltage across the capacitor is not explicitly specified.
     
  4. Feb 26, 2014 #3

    NascentOxygen

    User Avatar

    Staff: Mentor

    Are you sure this is what it says--"across the capacitor"? Can you quote precisely the wording of the question?
     
  5. Feb 27, 2014 #4

    rude man

    User Avatar
    Homework Helper
    Gold Member

    You have to specify the polarity of the 10V across C before proceeding with the problem. It could be +10V or -10V.

    Since the source is not mentioned in the problem statement you should asume it stays at +10V for all time -infinity to +infinity.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted