ENEE 204 Spring 2002
HW#1
th
Due: February 5 at Lecture
Read: Chapter 1 in Basic Electric Circuit Theory
Problem 1
Use PSpice to simulate this simple circuit consisting of a DC voltage source, a 20Ωresistor and a
current probe. Save the circuit and run Analysis/Simulate.
20
V
a. Vary the voltage from 0 to +10 volts. Run simulation and record the current for each voltage.
Plot the current vs voltage. The plot should be a straight line. What is the slope of the
straight line?
b. Not all electrical devices obey Ohms law. For example, replace the resistor with a component
D1N4002. Again vary the voltage from 0 to +10 volts. Plot the current vs voltage.
What is this component?
c. For voltages very close to zero, this device can be approximated by a resistor ( a straight line
on the i-v plot). For what range of values of voltage this approximation is reasonable, and what is
the value of the resistance?
Problem 2
a. Consider the simple circuit shown below. What is the current flowing
through each resistor?
10
+
100V

20
40
30
b. Construct above circuit with Pspice. Save the circuit and run Analysis/Simulate.
Measure the voltages at all nodes and calculate the voltage drop across each resistor.
Plot the voltage drop vs. the resistor value. Explain the dependence..
Problem 3
Voltage across a 1μF capacitor is varying in time as shown below.
100
0
80
V 0
o 60
l 0
t 40
a 0
g
20
e
0
(V)
0
0
200
1
2
3
4
5
6
7
8
9
1
0
time
(sec)
a. Using the appropriate terminal relation
calculate and plot the current flowing into the capacitor
as a function of time. Clearly label the axis on your graph.
b. Solve this problem with PSpice (hint: use appropriate voltage source)
Problem 4
a. Voltage across 10mH inductor varies as follows:
1
0
V
5
o
l
0
t
a
g -5
e
-10
0
2
4
6
time (sec)
8
10
12
At t = 0 the current is zero. Plot the current flowing through the inductor as a function of time
b. Solve the problem using PSpice.
c. Now consider the same inductor with a more complicated voltage waveform, as shown below.
1
0
V
5
o
l
0
t
a
g -5
e
-10
0
2
4
6
time (sec)
8
10
12
Again, at t = 0 the current is zero. Plot the current flowing through the inductor as a function of
time
d. Solve the problem using PSpice. Hint: It may take a more complicated voltage source.
Consider placing voltage sources in series to produce the desired waveform.
Problem 5
A 100Watt light bulb is connected to 120Volt power supply (Let’s assume it’s DC).
a. What is the current through the filament?
b. What is the resistance of the filament?
c. The resistance of metals increases with temperature. In case of tungsten filament of a
light bulb, the white hot filament has the resistance which is about a factor of two higher
than at the room temperature.
When does a light bulb dissipate more power - when it is just turned on or after it was on for a
long time?
Problem 6
For t >0 current flowing into 1 μF capacitor is given by the expression
I  20 e-t/5
where t is the time in seconds. The voltage on the capacitor was zero a t = 0.
What is the energy stored in the capacitor for t >> 5 seconds.
Problem 7
The voltage across a 7mH inductor is given by v(t) =0 for t< 0, and v(t) = 2 t + 3t2 volts for t  0
Assume i(0) = 0
a. Calculate the current through the inductor at t = 2 s.
b. Calculate the magnetic flux linkage through the inductor at t = 2 s.
c.
Calculate the instantaneous power flowing into the inductor at t = 2 s
d. Calculate the total energy stored in the inductor at t = 2 s