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!

Capacitors and energy

  1. Feb 27, 2007 #1
    1. The problem statement, all variables and given/known data
    hey


    A 2.21 uF capacitor is charged by a 20.0 V battery. It is disconnected from the battery and then connected to an uncharged 3.60 uF capacitor (Fig. 17-29).
    http://www.webassign.net/giancoli5/17_28.gif

    Determine the total stored energy at the following points in time.
    (b) after they are connected


    2. Relevant equations
    C= Q/V
    E=.5 QV

    3. The attempt at a solution
    I've tried to use the equations but I keep getting them wrong...
    The answers I've gotten are
    .00116
    7.2e-4
    .001279
    .001162
    4.42e-4
    2.905e-4
    884e-6

    Thanks
     
  2. jcsd
  3. Feb 27, 2007 #2

    mjsd

    User Avatar
    Homework Helper

    what points in time? also at which capacitor? or or both?
     
  4. Feb 27, 2007 #3
    oh
    after the two capacitors are connected
    the first time was before they are connected.. which was 4.42e-4
    I'm not sure .. but I assume it's both.. cuz that's all it says .. there's only one blank.. so I guess one answer.. >_<
     
    Last edited: Feb 27, 2007
  5. Feb 27, 2007 #4

    mjsd

    User Avatar
    Homework Helper

    ok the question is not that clear to me. but a few things for you to think about: conservation of charge/energy, compare the voltage across C1 and C2think about what would happen upon connecting C2 to the system note C1 not eq to C2
     
  6. Feb 27, 2007 #5
    I keep getting 4.42x10-6 ... but that's not the answer... TT________TT
     
  7. Feb 27, 2007 #6

    mjsd

    User Avatar
    Homework Helper

    if it is not clear what the question is really asking, it is very hard for me to help. what 's the correct answer?
     
  8. Feb 27, 2007 #7
    I don't know.. It only tells me if I'm wrong..

    the whole problem looks like this, exactly

    "A 2.21 µF capacitor is charged by a 20.0 V battery. It is disconnected from the battery and then connected to an uncharged 3.60 µF capacitor (Fig. 17-29).

    http://www.webassign.net/giancoli5/17_28.gif
    Figure 17-29

    Determine the total stored energy at the following points in time.
    (a) before the two capacitors are connected
    _____J
    (b) after they are connected
    _____J
    (c) What is the change in energy?
    _____J"

    I think the V will change...
     
    Last edited: Feb 27, 2007
  9. Feb 27, 2007 #8

    mjsd

    User Avatar
    Homework Helper

    my guess is that since it is asking for "total" stored energy .... assuming no loss in the wires, there should be no change at all. NB: 3.6uF capacitor is not charged, ie. does n't store any energy initially
     
  10. Feb 27, 2007 #9
    thanks for helping... it for some reason.. it's not the answer..

    can anyone else please try and help!! TT_TT
     
    Last edited: Feb 27, 2007
  11. Feb 27, 2007 #10
    hey guys..
    the answer is 0.000168
    but i dunno how.. >_<
    can someone explain? thanks
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Capacitors and energy
  1. Energy In a Capacitor (Replies: 1)

  2. Energy and Capacitors (Replies: 10)

  3. Capacitors and energy (Replies: 20)

  4. Energy and Capacitors (Replies: 3)

  5. Capacitor and energy (Replies: 1)

Loading...