Find v(t) and i(t) in the RC circuit

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SUMMARY

The discussion focuses on finding the voltage v(t) and current i(t) in an RC circuit using principles such as voltage division and Kirchhoff's Current Law (KCL). The user expresses confusion regarding the behavior of current through capacitors at t = 0 and the assumption of zero current for t < 0. It is established that while inductor current must be continuous, capacitor current can change instantaneously, leading to the conclusion that i(t) is zero for t < 0 due to the capacitor acting as an open circuit in a DC regime.

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  • Understanding of RC circuit analysis
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  • Knowledge of capacitor and inductor behavior in circuits
  • Basic principles of circuit analysis under DC conditions
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Homework Statement


upload_2016-11-1_19-30-45.png


Homework Equations


Voltage Division
v2 = vs * (r2/(r2+r1))
KCL - sum of all currents at a given node is 0
V= i * r

The Attempt at a Solution


[/B]
This is my solution:
https://scontent-atl3-1.xx.fbcdn.net/v/t34.0-12/14962850_1306124986078874_1009698561_n.jpg?oh=f9056525f1013bcfa49c664feeff07e2&oe=581B4244

This is my professor's solution:
upload_2016-11-1_19-34-19.png


I understand finding v(t) but when it comes to i(t) I'm confused.
First conceptual question:
I know the current across an inductor must be continuous at t = 0 but does this also apply to the current through a capacitor at t = 0? I imagine this isn't the case since neither me or the professor's solution point to this.

Next question, how is 0 mA the answer for t<0 ? If the switch is closed during this time then there should still be a current going down that path.
Was my approach for finding i(t) when t<0 incorrect? I drew the circuit for t<0, under DC regime a capacitor can be treated as an open circuit, I found Rth and found the current of that circuit. Wouldn't the current of that circuit be i(t) for t<0?

I know I asked a lot of questions so answering any part of it, if not all, would still be extremely helpful.
Thanks in advance!
 
Last edited:
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zr95 said:
does this also apply to the current through a capacitor at t = 0?
No. Capacitor current can change instantaneously, which prevents the voltage across the capacitor to change instantaneously. Capacitor resists sudden changes in the voltage across it.
zr95 said:
how is 0 mA the answer for t<0 ?
I believe you are expected to assume i(t) as the capacitor current. Hence for t<0, it is 0.
 

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