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

In summary, the conversation discusses finding v(t) and i(t) for a given circuit. The student is confused about the application of the principle of continuous current for capacitors at t=0 and the answer of 0 mA for i(t) at t<0. The expert clarifies that the principle does not apply to capacitors and the answer is expected to assume i(t) as the capacitor current.
  • #1
zr95
25
1

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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  • #2
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.
 

1. What is an RC circuit?

An RC circuit is a circuit that contains a resistor (R) and a capacitor (C) connected in series. The resistor limits the flow of current while the capacitor stores and releases charge.

2. How do you find v(t) and i(t) in an RC circuit?

To find v(t) and i(t) in an RC circuit, you can use Kirchhoff's laws and Ohm's law to create a differential equation. This equation can then be solved using various methods, such as substitution or Laplace transforms, to obtain the functions for v(t) and i(t).

3. What is the time constant in an RC circuit?

The time constant (τ) in an RC circuit is the time it takes for the capacitor to charge up to approximately 63% of its maximum charge. It is calculated by multiplying the resistance (R) and capacitance (C) values in the circuit: τ = RC.

4. How does changing the capacitance or resistance affect v(t) and i(t) in an RC circuit?

Changing the capacitance or resistance in an RC circuit will affect the time constant (τ), which in turn affects how quickly the capacitor charges and discharges. A larger capacitance or smaller resistance will result in a longer time constant and slower changes in v(t) and i(t), while a smaller capacitance or larger resistance will result in a shorter time constant and faster changes.

5. What are some real-life applications of RC circuits?

RC circuits have many practical applications, such as in filters for signal processing, timing circuits in electronic devices, and energy storage in power supplies. They are also commonly used in electronic communication systems, such as radio and television receivers, to filter out unwanted frequencies and improve signal quality.

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