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Circuit Analysis of driven RC Circuit with short circuits

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i82muchpi
#1
Oct22-12, 11:10 AM
P: 4
1. The problem statement, all variables and given/known data
Determine the value of the current labeled i and the voltage labeled v at t=0+, t= 1.5 ms, and t = 3.0 ms.



2. Relevant equations
Ohm's law: v = i *r
Decay in source free RC Circuit v(t) = v@0 * e^(-t/RC)

3. The attempt at a solution
i(0-) = 0.1 A.
Therefore,
v across 200 ohm = 200* 0.1 = 20 V
Since no current flows through capacitor with constant dc current,
v across capacitor = 20 V @ t < 0

v across capacitor cannot change immediately as it would require infinite power so v across capacitor @ t = 0 = 20 V

Here's where the confusion is: Should I include the 200 ohm resistor in my RC circuit? Why or why not? Also, will my circuit be a source free RC circuit?

Also, what will be the current i through the 200 ohm resistor when t>0?

My last question is a theoretical one: Can a current through a resistor change immediately at one instant of time?
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lewando
#2
Oct22-12, 11:23 AM
PF Gold
P: 1,053
200Ω in parallel with a short circuit (0Ω) is 0Ω (a short circuit).

Theoretically, an ideal resistor can experience an immediate change of voltage across it or current through it. However, all real resistors will have tiny capacitive/inductive characteristics.


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