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!

Capacitor Being Discharged

  1. Apr 9, 2009 #1
    1. The problem statement, all variables and given/known data
    A capacitor is discharged through a 100[tex]\Omega[/tex] resistor. The discharge current decreases to 27.0% of its initial value in 2.80ms.

    What is the value of the capacitor?

    2. Relevant equations

    Qf=Qi*e([tex]\frac{-t}{CR}[/tex])

    3. The attempt at a solution

    ln(.27)=[tex]\frac{.0028s}{C100}[/tex]

    [tex]\frac{ln(.27)}{.0028}[/tex]=4.68×104

    Answer is wrong, which means I need help. :D Thanks!
     
  2. jcsd
  3. Apr 9, 2009 #2
    So you need to express C.

    Your equation is ok, just when you put in the numbers... try that part again ;)
     
  4. Apr 9, 2009 #3
    Firstly, be careful of the equation you use. The equation you state is for charge - but you're interested in current. In this case you're safe since the equations are exactly similar, but you might faulter in the future if you're not careful..

    It may be useful to rearrange the equation before you insert the values, then you will see where you've made the mistake. In your answer the value you've calculated is the recipricol of capacitance rather than capacitance itself.
     
  5. Apr 9, 2009 #4

    Redbelly98

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    You have

    4.68x104 = 1/C

    So what is C ?

    EDIT:
    Ah, I really should refresh my screen more often. Two people got their responses in before I got around to it :blushing:
     
    Last edited: Apr 9, 2009
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Capacitor Being Discharged
  1. Capacitor discharge (Replies: 2)

  2. Capacitor discharge (Replies: 3)

Loading...