# SCEN103 Midterm Exam

October 17, 1996

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

1. An 800 W toaster, a 1000 W iron, and a 700 W food processor are all connected to a 115 V circuit with a fuse at 15 A. If these appliances are all turned on will the fuse blow? Support your answer with numerical evaluations. (3 pts)

2. Draw a schematic of the circuit that we used to measure the resistance of Play-Doh, indicating the correct placement of an ammeter and voltmeter. (2 pts)

Describe the procedure and analysis that we performed to establish that the resistance of a cylinder is inversely proportional to its cross-sectional area. (3 pts)

Background story for 3: On Tuesday May 14, 1996 most of the Delmarva peninsula south of Smyrna lost its electrical service because of inadvertent tripping of high voltage circuit breakers at the Keeney Extra High Voltage Substation near Glasgow. These breakers follow the transformers that lower the voltage from 500 kV, the level at which the electrical power is brought across the Delaware river from the Hope Creek power plant, to the 230 kV level used to distribute the power throughout the peninsula.

3. Assume that service was disrupted to 300,000 residents, each with 1200 W average demand. Calculate the minimum current rating needed for breakers operating at 230 kV. (2 pts)

4. What are the chief differences between a AA battery and a D battery? (1 pt)

5. Explain how the Copper-Top Tester for the AA battery works by using simple circuit concepts and Ohm's law. (3 pts)

6. A 5.0 ohm resistor is connected to a real AA battery (1.5 V). The current flow that results is 0.25 A. What must the internal resistance of the battery be? (2 pts)

What fraction of the delivered power is lost to the internal resistance? (1 pt)

7. Consider these circuits constructed from resistors and a real battery (that is, with an internal resistance as depicted). Do not assign numerical values to the resistances! -- answer the questions in words through careful reasoning.

In circuit above, the resistor R1 is dissipating a certain amount of power...

When resistor R3 is added to the circuit as shown above, in parallel with R1, does the power dissipated by R1 increase, decrease, or stay the same? Explain thoroughly! (3 pts)