# Homework Help: Second order circuit need some confirmation on the steps involved.

1. Mar 10, 2013

### berry1991

1. The problem statement, all variables and given/known data
Find v(t) for all t>0. Use second order method.

2. Relevant equations

3. The attempt at a solution
Before the switch is closed:
solving for i:
-80+160i+80i+80i=0
i=0.25A

KCL:
From node v(t):
[C dv(t)/dt] + i(t) + [V(t)/4] + [(v(t)-80i)/80]+[(v(t)-80)/160] = 0

KVL:
v(t)=L di(t)/dt

Later on solve the 1st equation for i(t), then substitute into KVL equation.

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2. Mar 10, 2013

### Staff: Mentor

Consider replacing everything before the switch with a Norton equivalent.

Have you learned about Laplace transforms yet?

3. Mar 10, 2013

### berry1991

yes, but the question ask us to use 2nd order to solve it.
Do we need to take in account for the dependent source?
we convert it to 80*0.25=20V

4. Mar 10, 2013

### Staff: Mentor

In my opinion, I don't think that using Laplace transforms to solve the differential equation would be cheating...
Yes, the dependent source makes a difference, but you may be surprised by the resulting Norton equivalent. The Norton equivalent should have a current source in parallel with a resistance; 20V is not a current (nor is the the correct Thevenin voltage).