PHYS345 First Midterm Exam October 8, 1998

1.

1. Does the emf lead or lag the current?   -   lead

1. Does the voltage across the current source increase or decrease?   -   decrease

2. Construct the Thevenin generator that would represent this circuit.   -   VTH = 12 V; RTH = 6 ohm

2.

1. What is the current through the lamp?   -   0.20 A

2. What is the terminal voltage of the AAA cell?   -   1.38 V
3. What is the current through the lamp now?   -   0.23 A

4. What is the terminal voltage of the AAA cell now?   -   1.61 V

5. Was adding the 9 V battery worth it?   -   No, of the 3.7 W delivered by the additional battery, 3.0 W is wasted by its internal resistance.

3.

1. Operated at 120 V, what is the current through the motor when operated at 60 Hz?   -   17.9 A \/ -63.4 deg

2. What power is delivered by the voltage source?   -   960 W

3. What power is dissipated by the motor?   -   960 W

4. What is the power factor of this circuit at 50 Hz?   -   0.93

5. What capacitance do you need to correct the power factor back to unity?   -   470 uF

4.

1. What power should the resistor be able to dissipate adequately to conduct this test successfully?   -   around 14W

2. A current of 1.10 A is measured with the DMM. What is the internal resistance of the battery?   -   0.91 ohm

What is the resistance of the starter motor?   -   0.10 ohm

"http://www.physics.udel.edu/~watson/phys345/exams/mid1-98f.html"
Last updated Oct. 15, 1998.
Copyright George Watson, Univ. of Delaware, 1998.