# Calculating V(0+) in an RLC circuit

Discussion in 'Homework Help' started by saruji, Sep 23, 2012.

1. ### saruji Thread Starter New Member

Oct 18, 2011
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0
We have just gone over 2nd order RLC circuits in class and I am having a difficult time under standing the 0- and the 0+ of the circuit posted v(0-) is easy its just a voltage divider, but what are they looking for when they ask for v0+? So I assume right after the switch flips the voltage is whatever has built up in the capacitor so in my case 8V as it is parallel with the 4k resistor. Or does v0+ mean that they are looking for v as it changes from 0 to infinity ?

Problem from my dropbox http://db.tt/DeQ3ODzd

Thank you

2. ### Ron H AAC Fanatic!

Apr 14, 2005
7,012
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You have four nodes: The battery (+), a, b, and the one at the bottom, which isn't named. Where is the measurement being made, i.e., across which nodes?

3. ### saruji Thread Starter New Member

Oct 18, 2011
14
0
The measurement is being made at the B node

4. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
790
You have gotten off to a poor start. Va(0-) is not 8V. Re-think part (a) of the problem.

5. ### saruji Thread Starter New Member

Oct 18, 2011
14
0
Is it 4 volts?

6. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
790
That's correct.

7. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
790
The capacitor voltage at t=0+ will be the same as at t=0-