Q: 1
st
Order Circuits: Find τ, Initial Value
v
0
or
i
0
, Final Value
v
∞
or
i
∞
1. Find τ
Find the
R
Th
for the 1
st
order circuit at t > 0
●
Turn off all the Sources
Voltage Source → Short Circuit (Wire)
Current Source → Open Circuit (Removed Wire)
●
Remove the Energy Storing Device (Capacitor or Inductor)
Replace by a pair of Terminals
●
Determine the Resistance at the Terminals (Start with inner most resistors)
Resistors in Series → Current can only flow one path between them
Resistors in Parallel → End Terminals connect to the same nodes
Calculate
τ
●
For a 1
st
order RC circuit:
τ =
R
Th
C
●
For a 1
st
order RL circuit:
τ =
L
R
Th
Note:
Any resistor in parallel with a “Wire” has a resistance of 0
. (“Shorted” Resistor)
Any resistor in series with an Open Circuit (Removed Wire) has a resistance of 0
. (“No Effect”)
2. Find Initial Value
v
0
or
i
0
Step 1
First thing you need to determine is the Capacitor Voltage
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 Spring '08
 FOWLER
 Steady State, Volt, Inductor, Electrical resistance, Series and parallel circuits

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