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Circuit with switch resistors and capacitor

  1. Oct 28, 2009 #1
    1. The problem statement, all variables and given/known data
    For the circuit shown in the figure below, C = 7.50 µF, script e = 80 V, and R = 400 Ω. After having been at contact a for a long time, the switch throw is rotated to contact b.
    25-p-095-alt.gif

    a.)What is the charge on the upper plate of the capacitor just as the switch throw is moved to contact b?

    b.)What is the current just after the switch throw is rotated to contact b?

    c.)What is the time constant of this circuit?

    d.)How much charge is on the upper plate of the capacitor 7.00 ms after the switch throw is rotated to contact b?

    2. Relevant equations
    I=V/R
    Inet=V/Req
    C=Q/V

    3. The attempt at a solution
    I have no idea...at all.
     
  2. jcsd
  3. Oct 28, 2009 #2
    I really need so type of help.
     
  4. Oct 29, 2009 #3
    anyone?
     
  5. Oct 30, 2009 #4

    rl.bhat

    User Avatar
    Homework Helper

    When the switch is in the position a for a long time, potential across the capacitor is 80 V.
    Qo = CV.
    Io = Qo/RC.
    RC is the time constant.And using
    Q = Qo*e^-t/RC
    find Q.
    Don't you have any text book? Didn't you see these equations in the book? Are you not willing to open the book and go through the chapter on capacitor?
     
  6. Oct 30, 2009 #5
    It helps to remember that the charge stored on a capacitor can't jump instantaneously to a new value, and therefore the voltage across a capacitor can't jump instantaneously to a new value. Both of these have an exponential decay from their initial to their final values. This will put a constraint on the resistor current immediately after the switch is thrown.
     
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