Please help...I've spent hours on this one

Discussion in 'Homework Help' started by syasse, May 3, 2012.

  1. syasse

    Thread Starter New Member

    Feb 24, 2012
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    I was given a circuit where the two capacitors were charged to some value. At the point where they are grounded, one capacitor is grounded at the neg. terminal and the other is grounded at the positive terminal. (essentially, the look as two batteries connected pos to neg w/ a ground in between them. At t= ∞, I need to find voltage a the top left and voltage at the bottom left of the circuit.

    If anyone could help me get this thing started in the right direction i would be truely grateful.
     
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  2. strantor

    AAC Fanatic!

    Oct 3, 2010
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    0v.

    (no supply)
     
  3. WBahn

    Moderator

    Mar 31, 2012
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    But note that the problem starts off with the capacitors initially charged.

    1) Before the switch is closed, what is the voltage at the top left and the bottom left?

    2) Can the voltage across a capacitor change instantaneously?

    3) In the moment after the switch is closed, what is the voltage at the top left and the bottom left?
     
  4. strantor

    AAC Fanatic!

    Oct 3, 2010
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    He's asking for voltage @ t = ∞. I assume that's why he didn't give us a voltage for t = 0; because it doesn't matter - whatever it was, it's gone now, bled through the caps. That's my take on it.
     
  5. WBahn

    Moderator

    Mar 31, 2012
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    My bad. You are correct. My vision is blurry enough that I had to look about three times before I could see that it was an infinity symbol. It looks like a fuzzy zero to me, and since everything looks fuzzy to me... (actually, my prblem is that I almost always see fairly strong ghost images slightly offset to the right, so a o usually looks like a oo with varying degrees of space/overlap).
     
  6. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    The OP's question is still somewhat ambiguous. The final value is 0V only if the capacitors are charged to the same voltage. The OP stated they are charged to "some value" rather than the "same [equal] value".

    An interesting question to ask is - what is the result for the two measurement locations if C3 was charged to 15V and C4 to 5V?
     
    Last edited: May 3, 2012
  7. WBahn

    Moderator

    Mar 31, 2012
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    Very true. In fact, I wouldn't be surprised if catching this subtle point wasn't a big part of the question (and, I have to admit, I missed it).

    Since the capacitors are the same size, whatever voltage the top node is decreased by, the bottom node is increased by. For your example, the initial difference is 20V and so the top has to lose 10V and the bottom has to gain 10V (in order to place 0V across the resistor). So both nodes would end up at +5V.

    Here's another interesting set of questions that could be asked, with some unintuitive answers.

    1) How much total energy is stored on both capacitors before the switch is closed?

    2) How much energy is therefore dissipated in the resistor?
     
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