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Tension on a capacitor from Laplace domain to time domain.

  1. Jan 21, 2012 #1
    The problem ask me to find the tension on a capacitor after a switch has been opened.

    I have everything in terms of equations in s-domain and I'm sure they aren't wrong because I checked on the book. My unique problem is to understand a certain passage necessary to find the voltage knowing the current.

    I found I(s) and I know v(0)/s of the capacitor. Then I just used this relation to find V(s) and compute its value in time: I(s)=C (sV(s)-v(0)). Here I solved for V(s) and I got the wrong answer. In fact, the book uses a different equation, actually just I sign changes, the following: V(s)= (1/sC) I(s) - v(0)/s. The first thing I thought was it was a Typing error, but checking other exercises showed it wasn't.

    So, my question is: why do we use it?
  2. jcsd
  3. Jan 22, 2012 #2

    rude man

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    If you could provide the entire problem and not just some of it?
  4. Jan 23, 2012 #3


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    Your equation is dimensionally wrong!
    You wrote sV(s)-v(0).
    You have a voltage (v(0)) subtracted from the derivative of a voltage (sV(s)).
  5. Jan 23, 2012 #4

    rude man

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    His equation is not dimensionally wrong.

    V(s) has units of volt-sec since V = ∫v*exp(-st)*dt, s has units of sec-1, so sV(s) has units of volts, just like v(0).

    However, since the OP won't disclose the problem there is little else we can do for the chap or lass.
  6. Jan 23, 2012 #5
    Thank you, guys for the answer, but I have already figured it out.
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