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: Voltage Across Capacitor

  1. Oct 29, 2011 #1
    1. The problem statement, all variables and given/known data

    [PLAIN]http://img189.imageshack.us/img189/4555/capacitorh.jpg [Broken]


    2. Relevant equations

    [itex]I(t)=I_0 e^{-t/\tau}[/itex] (current as a function of time for a capacitor being charged)


    3. The attempt at a solution

    The time constant of the circuit is τ=RC=7s. And the initial current in the circuit is

    v/Req=300/1489=0.2A (I know this is correct)

    Now pluging into the equation:

    [itex]I(5)=0.2 e^{-5/7}=0.0979 \ A[/itex]

    [itex]V=IR= 0.0979 \ A \times 1489 \Omega = 145.73 \ V[/itex]

    But the correct solution must be 153V. I'm very confused, what's wrong with my working? :confused:
     
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Oct 29, 2011 #2

    ehild

    User Avatar
    Homework Helper

    Is V=IR the voltage across a capacitor???


    ehild
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook