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Homework Help: Circuit with an uncharged capacitor - desperate

  1. Apr 14, 2009 #1
    circuit with an uncharged capacitor -- desperate

    1. The problem statement, all variables and given/known data
    http://img91.imageshack.us/img91/7533/physicsimage.gif [Broken]
    Switch S has been open for a long time. It is closed suddenly at t=0.
    A long time later, it is opened.
    Determine the potential across and the current through each component at the following times.

    Immediately after S closes. <-- one i dont understand...
    A long time after S closes.
    Immediately after S is reopened.

    2. Relevant equations

    3. The attempt at a solution
    So immediately after S closes, the capacitor charge is neutral, right? (Since it is uncharged.) The solution says that the current will not go through Resister 2.
    So I do not understand why the current will not go through Resistor 2 and go through Capacitor instead.
    I thought current charges cannot jump to the other side of the capacitor...

    There is potential difference between battery, so I thought the current will try to go through resister 2 due to the emf of the battery...
    But that is not the case.

    I am wondering why all currents going to R2 and C is used to charge the capacitor...
    and i also do not understand why that results in no current through R2....

    Thank you SO much in advance.

    I am sso stuck and very desperate....

    If you cannot answer all my questions, my biggest most important question is... Why does current NOT go through R2?

    Again, thanks so much for your time.
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Apr 14, 2009 #2


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    Homework Helper

    Re: circuit with an uncharged capacitor -- desperate

    For the time being forget R2.
    Close the switch .What happens?
  4. Apr 14, 2009 #3
    Re: circuit with an uncharged capacitor -- desperate

    You need another equation for the capacitor.

    [tex]i = C \frac{dv}{dt}[/tex]
  5. Apr 14, 2009 #4
    Re: circuit with an uncharged capacitor -- desperate

    If there is no R2, then the battery charges the capacitor.
    Btw, that raises another question.
    Since electrons are the charge carriers and electrons cannot ever travel to positive side of the capacitor, does the resistor 1 connected to the positive plate of the capacitor play a role? I am so confused. Thanks for your help.

    Thank you so much for your response. I am wondering if you could elaborate a little more. ^^
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