Tension on a capacitor from Laplace domain to time domain.

[maCrobo]

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?

 

[rude man]

If you could provide the entire problem and not just some of it?

 

[CEL]

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?

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)).

 

[rude man]

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)).

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.

 

[maCrobo]

Thank you, guys for the answer, but I have already figured it out.
Bye.