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ENGR&204 Spring 2012 Homework 6 Due the last class of the term (before the final test)
Use the cookbook!
Find i for t > 0 if the circuit is in the steady state at t = 0-.
Find v for t > 0 if the circuit is in the steady state at t = 0-.
Find v1 for t > 0 if there is no initial stored energy.
Find i for t > 0 if the circuit is in the steady state at t = 0-.
A laser pulse power circuit is shown in the figure below. It is required that v(0) = 7.0 v and i(0) = 3.5 A.
Find the R, the resistance required for these conditions. Assume that the switch has been closed for a
long time before it is opened at t = 0.
R
t=0
2
20 V
1H
8
v(t)
3F
i
To = 0 the coil acts like a short and the capacitor acts like an open (it is fully charged). At t = 0 the circuit
will reduce to:
The current in the 8 ohm resistor is (7 v/8  = 0.875 A. KCL at the top node gives the current in the
resistor R to be 3.5 A + 0.875 A = 4.375 A. Then KVL clockwise around the loop containing R is -20 +
4.375(R) +7 = 0. Solve for R gives 2.97 .
The circuit is in the steady state before the switch closes at t = 0 s. The switch stays closed for 1.5 s and
then opens. Find i(t) for t > 0 s.
Use step functions to create the voltage shown.
Find the step response i. (This means vg = u(t) volts)