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!

Capacitor energy

  1. Nov 6, 2017 #1
    1. The problem statement, all variables and given/known data

    The initial voltage on the 0.5 microF capacitor shown in the figure is -20 V. the capacitor current has the waveform shown in the figure. https://i.imgur.com/P8jwI7s.jpg?1

    How much energy is stored in the capacitor at t = 870 micro seconds.

    2. Relevant equations

    v(t) = 1/c * integral from t to t0 i dtao + v(t0) EDIT: "integral from t to t0" is a definite integral with t on the top and t0 on the bottom
    w(t) = 1/2cv^2

    3. The attempt at a solution

    got v(t) = 50*e^-2000t + 30

    after plugging into w(t) tested the following answers:

    https://imgur.com/3P8ipf4
     
    Last edited: Nov 6, 2017
  2. jcsd
  3. Nov 6, 2017 #2

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Your integral calculated the change in voltage (except, you have a sign error) How does that relate to the change in energy?
     
  4. Nov 6, 2017 #3
    "from t to t0" is admittedly ambiguous but i mean that t is on the top of the definite integral and t0 is on the bottom. voltage relates to energy by the equation w(t) = 1/2cv^2. i plugged in my equation for v(t) and the time as 870 microseconds. please let me know what else is unclear/wrong. thank you for your reply.
     
  5. Nov 6, 2017 #4

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    What does that give for t=0?
    But that's different from change in voltage and change in energy.
     
  6. Nov 6, 2017 #5
    "what does that give for t=0?"

    80 volts. what are you implying?

    "but that's different from change in voltage and change in energy"

    what are you implying? the expressions for v(t) and w(t) i have provided are the capacitor voltage and energy stored in a capacitor, respectively, at a particular instance for a time. i dont think the answer is w(870 microseconds) - w(0 seconds). please let me know if i am wrong. thank you for your reply.
     
  7. Nov 6, 2017 #6

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    But:
    Yes, you're right, I misread the question.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted