# Homework Help: [rcl circuits] is this a contradiction?

1. Sep 20, 2014

### Vishera

Problem:

Attempt:

But here is what happens when I apply nodal analysis to the same node at t>0:

Is this a contradiction? Is the voltage of the resistor at t=0+ equal to 16V or 0V?

2. Sep 20, 2014

### Vishera

I think I figured it out. At $t=0^-$, the capacitors are replaced with open circuits and the inductors are replaced with wires but this does not apply at $t=0^+$.

3. Sep 20, 2014

### iRaid

When t<0, an inductor acts as a short circuit and a capacitor acts as an open circuit. At the instant t=0+, the circuit goes through a transient phase. Once t>>0, the circuit becomes steady state and once again the inductor acts as a short and the capacitor as an open.

4. Sep 21, 2014

### Vishera

Thank you for this. Argh, I wish my textbook explained this to me. Why does the inductor and capacitor act as closed/open circuits in the steady state but not in the transient phase?

5. Sep 21, 2014

### iRaid

Basically an inductor goes from some high voltage to 0, which would cause all the current to flow through it (which is a short) when time goes to infinity. I'm not sure if you've seen the graphs of these, but that should help you, so I'd say look it up.