Silicon, Circuits, and the Digital Revolution

SCEN103 Midterm Exam             NAME:______________________

April 26, 2000

For full credit, work must be shown, including formulas used and algebraic and numerical steps. Answers must include units.

1. (2 pts.)
Why is household wiring constructed so that appliances are connected in parallel to the source of voltage, as in the figure below?

2. (3 pts.)

The lamp dims in the circuit above when the vacuum cleaner is running. Explain this observation using circuit concepts.
HINT: the observation cannot be explained without explicitly considering the resistance of the house wiring.

3. (2 pts.)
Estimate the monthly cost for keeping a 100 W light burning overnight on my front porch.

4. (4 pts.)
MacGyver is in another tight spot. He is pinned down in the hardware supply area of a military complex (big mistake for his captors!). Pete is about two-tenths of a mile away in a deserted mining camp waiting for Mac to return with a gas-powered chainsaw to cut down a solar-powered satellite communications tower.

Looking through the supplies, Mac finds an old 120V ac, 2400 W electrical power saw. “That won’t work,” he thinks to himself. “There is no ac where Pete’s at. Wait! What’s this over here. A 240 V ac switch box! My lucky day…”

What’s the plan? Mac is preparing to run wiring from the 240 V source in the military complex to the mining camp 0.2 miles away. When he reaches Pete he hopes that the 240 V will drop to 120 V because of the resistance in the “extension cord” so that he can use the 120 V saw as designed. What wire gauge should he select from the spools available?

 Wire Gauge Resistance per hundred feet (ohm) 10 0.12 12 0.19 14 0.29 16 0.47

5. (3 pts.)
When a 200 ohm resistor is attached to a battery as shown, the ammeter reads 40 mA. Is the source of voltage a 1.5 V cell or a 9.0 V battery? What is the value of its internal resistance?

6. (2 pts.)
Your roommate will be starting an undergraduate research project this summer in the College of Marine Studies. Chris is designing a circuit that will be attached to small buoys to monitor salinity at 100 test locations. The prototype circuit draws about 80 mA. The experimental buoys are already overloaded so each gram counts. What is the smallest cell that Chris can use to power the circuit for at least 24 hours? For at least a week?

 Cell Size Rated Capacity (mAh) D 15000 C 7800 AA 2850 AAA 1150 Borrowed from Duracell OEM site.

7. (2 pts.)
Write a single sentence that expresses Ohm’s Law in words; required words are resistor, voltage, current, through, and across.

8. (2 pts.)
Why is electrical power transported at high voltage over long distances? What is the advantage of ac over dc in the distribution of electrical power?

 Comments, suggestions, or requests to ghw@udel.edu. "http://www.physics.udel.edu/~watson/scen103/colloq2000/exams/midl-00s.html" Last updated May 24, 2000. © George Watson, Univ. of Delaware, 2000.