PHYS245 Electricity and Electronics
Syllabus - Classes - Assignments - Laboratory - Resources
First Examination, Spring 2005

PHYS245 First Midterm Exam March 18, 2005

This is a closed book exam.
Programmable calculators and graphing calculators may be used during this exam.
Show ALL work on problem sheet and only on that sheet.
Please read questions carefully.
Credit may be lost inadvertently if solutions are not neat and orderly.
Be careful with units, signs, and significant figures.

1. (18 points)

Justify your answers in all cases!

  1. Why does a lamp dim when a vacuum cleaner starts? Sketch a circuit that will demonstrate the effect, and explain using simple circuit concepts.
 
  1. Calculate an estimate of the cost of operating a standard hairdryer for a year, assuming a typical daily usage.
 

  1. The circuit at the right has three identical resistors and an ideal current source. After the switch is closed, does the voltage across the current source increase or decrease?

    By what percentage?

2. (22 points)

  1. Consider three AAA cells connected in parallel, as shown schematically below, each with source voltage 1.5 V and internal resistance 0.6 ohm. Construct the equivalent Thévenin generator that represents this combination.


  2. Consider the effect of flipping the orientation of one of the cells, as shown below. Describe the resulting Thévenin generator for this circuit.

3. (30 points)

The motor that was designed for the previous year’s sophomore design project was wound with an inductance of 61 mH and a resistance of 30 ohm. The power factor was nearly corrected for 60 Hz by adding a capacitance of 45 microfarad capacitor in parallel with the motor. The capacitance was added to the circuit in the form of two capacitors in parallel, one of value 30 microfarad and the other 15 microfarad.

Unfortunately the 30 microfarad capacitor has become disconnected because of a broken lead, leaving only 15 microfarad in the following circuit:

Errant motor circuit

  1. What is the value of the resulting power factor? Lagging or leading?

  2. What is the value of the average power delivered by the source? And the reactive power?

  3. Compare the current through the source in the above circuit with the current that would flow in the intended circuit (with the power factor corrected). Be quantitative.

4.  (30 points)

  1. Find the current through each voltage source and the voltage across the current source.
 
  1. Check the power associated with each element.
  Calculation Value (mW)
12 V source:    
10 mA source:    
6 V source:    
1000 ohm:
    
330 ohm:    
3000 ohm:    
  Checksum:
(six entries should sum to zero!) 		 

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Last updated May 23, 2005.
Copyright George Watson, Univ. of Delaware, 2005.