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: RC circuits voltage

  1. Sep 12, 2012 #1
    1. The problem statement, all variables and given/known data


    Relevant infor
    V(out) stands for the voltage divider I believe

    3. The attempt at a solution

    I am just going to get the equation first.

    For the first part, applying the loop rule I got

    [tex]12 - 10^5q' - 10^7 q = 0[/tex] with q(0) = 0. Solving I get

    [tex]Q = \frac{3}{25000}(1 - e^{-100t})[/tex]

    Since V = IR = q'R = q'(105) I should get [tex]V = 12e^{-100t}[/tex]. So the plot would be a exp curve, curving down.

    For the question that follows after. The loop rule is

    [tex]100 - q'(10^6) - q/C = 0[/tex]. Solving, I get

    [tex]q(t) = 100C - 100Ce^{-10^{-6}t}[/tex]

    Since V = IR = q'R = q'(106) I should get [tex]V = 100e^{-10^{-6}t/C}[/tex]

    Now [tex]V_{out} = 70 = 100e^{-10^{-5}/C}[/tex], solving I get C = 2.8

    Here is the problem, the answer is supposed to be 8.3μF. All the ODEs were solved with Maple

    What did I do wrong?
  2. jcsd
  3. Sep 12, 2012 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Huh? :confused: What voltage divider? Vout is the output voltage, which is the voltage across the capacitor, in this circuit.

    You're solving for the WRONG V here. The voltage given by IR is the voltage across the resistor (let's call it VR). You're looking for the voltage across the capacitor. To get the capacitor voltage (VC), use the fact that q = CVC for a capacitor. OR, use the loop rule to solve for VC in terms of VR and V0. Either method should give you the same answer.

    Also, whenever you get an answer to a problem, always ask yourself, "does this make any sense?" A decaying exponential for the capacitor voltage makes no sense, because after the switch closes, the capacitor is charging, not discharging. :wink:

    EDIT: and you made the exact same mistake in part 2. Your equation for the capacitor voltage vs. time is just wrong.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook