# Problems with First order RC circuit

1. Sep 24, 2016

### thuc

• Member warned to use the homework template for posts in the homework sections of PF.
I have problems with question b. I don't know where to start. If the switch is on steady state, the current through capacitor would be 0 and i can't have K. This makes me confused.
This is the Total response function of a): V0(t) = K^(-1/2t)

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2. Sep 24, 2016

### Simon Bridge

The image shows a 12V power source with 2$\Omega$ internal resistance connected, in parallel, with a parallel RC circuit (3F and 4$\Omega$) via a switch.
The voltage across the capacitor is labelled $v_0$.
Part (a) says the switch, open for a long time, is closed at t=0, find $v_0(t)$.
Part (b) says the switch has been closed for a long time, and is opened at t=0, find $v_0(t)$.
These two parts normally have a similar method of solution, so please show your method for part (a) so we can see how you are thinking.

For part (b) ... what is physically happening? What is the initial voltage on the capactor, what happens to the charge on the capacitor for t>0?

3. Sep 25, 2016

### thuc

Ok, this is part a)

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4. Sep 25, 2016

### cnh1995

Are you familiar with the response of a series RC circuit excited with a step input?
Are you familiar with the Thevenin's theorem?