- #1

darkfeffy

- 17

- 0

Hi everyone,

I know this should be obvious, but there's something I am just NOT getting.

Imagine a simple series RC circuit with a DC source as shown in the attachment. As can be seen from the picture, I have solved the differential equation in capacitor current in the time domain. In order to be able to solve the problem, I have assumed that dE/dt = 0 as this is a dc source.

I wish to know why I can't take the Laplace transform of both sides of the equation (*). I know that Laplace(0) = 0, so this would give a bogus equation (i.e. I(s) = 0, which is wrong). But if my equation (*) is right, then why can't I use the laplace transform of both sides at this point?

From textbooks, I read that the DC source is considered as a step input, thus in the Laplace domain, this would be E/s (in other words, the switch is closed at t=0). So, what is wrong with equation (*)?

Please don't hesitate to point out trivialities.

Thanks for your understanding.

e.

I know this should be obvious, but there's something I am just NOT getting.

Imagine a simple series RC circuit with a DC source as shown in the attachment. As can be seen from the picture, I have solved the differential equation in capacitor current in the time domain. In order to be able to solve the problem, I have assumed that dE/dt = 0 as this is a dc source.

I wish to know why I can't take the Laplace transform of both sides of the equation (*). I know that Laplace(0) = 0, so this would give a bogus equation (i.e. I(s) = 0, which is wrong). But if my equation (*) is right, then why can't I use the laplace transform of both sides at this point?

From textbooks, I read that the DC source is considered as a step input, thus in the Laplace domain, this would be E/s (in other words, the switch is closed at t=0). So, what is wrong with equation (*)?

Please don't hesitate to point out trivialities.

Thanks for your understanding.

e.