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Name: __________________________
ECET 231 – Circuit Analysis I
Problem Set 3
Due: September 24, 2009
Solve each of the following problems (taken from the course text). Express all answers in
engineering notation. Show your work.
Refer to the circuit below to respond to problems 1 – 3.
1.
Determine the current, I, for each of the following.
a. E = 40 V, R = 20 Ω
b. E = 35 mV, R = 5 mΩ
c.
V = 200 V, R = 40 kΩ
d. E = 10 V, R = 2.5 MΩ
e. E = 7.5 V, R = 2.5 x 103 Ω
f.
2.
V = 12 kV, R = 2 MΩ
Determine R for each of the following.
a. E = 50 V, I = 2.5 A
b. E = 37.5 mV, I = 1 mA
c.
E = 2 kV, I = 0.1 kA
d. E = 4 kV, I = 8 x 10-4 A
3.
Compute V for each of the following.
a. I = 1 mA, R = 40 kΩ
b. I = 10 μA, R = 30 kΩ
c.
I = 10 mA, R = 4 x 104 Ω
d. I = 12 A, R = 3 x 10-2 Ω
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4.
A 48-Ω hot water heater is connected to a 120 V source. What current does it draw?
5.
What is the potential difference between the two ends of a 20 kΩ resistor when the current
is 3 x 10-3 A?
6.
A relay with a coil resistance of 240 Ω requires a minimum of 50 mA to operate. What is
the minimum voltage that will cause it to operate?
7.
A resistor dissipates 723 joules of energy in 3 minutes and 47 seconds. Calculate the rate
at which energy is being transferred to this resistor in joules per second. What is its power
dissipation in watts?
8.
How long does it take a 100 W soldering iron to dissipate 1470 J?
9.
A 120 V electric coffee maker is rated at 960 W. Determine its resistance and rated
current.
10.
A warning light draws 125 mA when dissipating 15 W. What is its resistance?
11.
What is the voltage drop across a 10 Ω resistor that is dissipating 90 W?
12.
A 2.2 kΩ resistor with a tolerance of 5% is connected to a 12 V DC source. What is the
possible range of power dissipated by the resistor (assuming it is within tolerance)?
13.
For a resistive circuit, E = 12 V.
a. If the load dissipates 8 W, what is the current in the circuit?
b. If the load dissipates 36 W, what is the load resistance?
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14.
The load on a 120 V circuit consists of six 100 W lamps, a 1.2 kW electric heater and an
electric motor drawing 1500 W. If the circuit is fused at 30 A, will the circuit continue to
operate when a 900 W toaster is plugged in? Justify your answer.
15.
An indicator light on a control panel operates continuously, drawing 20 mA from a 120 V
supply. At $0.09 per kilowatt-hour, how much does it cost per year to operate the light?
16.
An electric device with a cycle time of 1 hour operates at full power (400 W) for 15 minutes,
at half power for 30 minutes, then cuts off for the remainder of the hour. If the cycle
repeats continuously and power costs $0.10/kWh, determine the yearly cost of operating
the device.
17.
The power input to a motor with an efficiency of 85% is 690 W. What is its power output?
a. in watts
b. in hp
18.
The power output of a transformer with η = 97% is 50 kW. What is its power in?
19.
For a certain device, η = 94%. If losses are 18 W, what are Pin and Pout?
20.
A 240 V, 4.5 A water heater produces heat energy at a rate of 3.6 MJ per hour. Calculate:
a. the efficiency (η) of the water heater
b. the annual cost of operation if power costs $0.09/kWh.
21.
A 120 V DC motor develops an output of 4.2 hp. If its efficiency is 92%, how much current
does it draw?