PH  262 / 11 Fall 2001
Engineering Physics II
Professor: Alvandi
Submitted By:
Andrew Buettner
Partner: Peter Fetterer
Lab #4: Electrical Resistance
Table Of Contents
1) Cover Page...............................1
2) Table of Contents.....................2
3) Objective..................................3
4) Components Used....................3
5) Procedures................................3
6) Lab Data /
Results.....................4
1) Table 1...........................................4
2) Diagram 1......................................4
3) Diagram 2......................................5
4) Diagram 3......................................5
5) Diagram 4......................................5
6) Diagram 5......................................6
7) Diagram 6......................................6
7) Answers to Lab Questions........6
8) Conclusions..............................8
9) Attachments..............................8
Objective
The objective of this lab is to investigate the relationship between current and voltage for several common electrical circuit elements. Then use this data to explain the behavior and effectiveness of resistance.
Components Used
1) Variable voltage power supply
2) 300W resistor
3) 500W resistor
4) Multimeter
5) Light bulb
6) GE Diode
Procedures
1) Assemble the following circuit:
2) Using V=1v  V=10v in 1v increments measure the current through the resistor.
3) Repeat step 2 using the 500W resistor.
4) Repeat step 2 using both resistors in series.
5) Repeat step 2 with both resistors in parallel
6) Replace the resistor with the light bulb and repeat step 2
7) Replace the bulb with the diode in a forward bias configuration.
8) Using voltages of .125V  .5V in .125V increments, find the current through the diode
9) Reverse the diode bias and repeat step 8.
10) Plot all information on graphs.
Lab Data / Results
1) Table 1: Voltage  Current Relationships

240W 
470W 
Parallel 
Series 
Light 

1.26V 
4.99mA 
2.62mA 
7.34mA 
1.74mA 
345mA 
2V 
7.97mA 
4.19mA 
11.86mA 
2.80mA 
420mA 
3V 
11.97mA 
6.28mA 
17.7mA 
4.19mA 
492mA 
4V 
15.91mA 
8.36mA 
23.7mA 
5.55mA 
563mA 
5V 
20.1mA 
10.52mA 
31.1mA 
6.99mA 
626mA 
6V 
23.9mA 
12.5mA 
37.6mA 
8.38mA 
681mA 
7V 
28.3mA 
14.8mA 
44.0mA 
9.75mA 
735mA 
8V 
31.9mA 
16.8mA 
50.1mA 
11.1mA 
800mA 
9V 
36.4mA 
18.9mA 
56.6mA 
12.6mA 
840mA 
10V 
41.6mA 
21.1mA 
63.1mA 
13.95mA 
890mA 
2) Diagram 1: Graph of V / I for 240W Resistor
3) Diagram 2: Graph of V / I for 470W Resistor
4) Diagram 3: Graph of V / I for Parallel Resistors
5) Diagram 4: Schematic 1, Enlarged View
6) Diagram 5: Graph of V / I for Series Resistors
7) Diagram 6: Graph of V / I for Light Bulb
Answers to Lab Questions
1) Q: Given a curved line and a straight line illustrating the relationship between voltage and current, which one is ohmic?
A: The one with the straight line.
2) Q: Given the lab illustration, what is the resistance of the ohmic material?
A: 1W
3) Q: For the same image, find the resistance at 2V, 4V, and 5V.
A: 2V > .8W; 4V > 1.33W; 5V > 1.67W
4) Q: How does the resistivity of the material increase as the voltage increases?
A: The resistance increases.
5) Q:What is the equivalent resistance of two resistors in series?
A: R_{T} = R_{1} + R_{2}
6) Q: Is R_{T} greater or less than the individual resistors?
A: Greater
7) Q: If R1 and R2 are both non ohmic, and their equivalent resistances were found independently, then they were put together, would their resistance remain the same, and explain why.
A: No, if they are non ohmic, then there is no linear relationship between the voltage and current, and the voltage would be divided between the two devices.
8) Q: What assumptions are made for ohmic materials?
A: That their resistivity is not dependent on temperature, pressure, or any other physical change.
9) Q: What are the units of aV/T, and are they unitless?
A: There are no units, the result is a scalar number with no properties assigned.
10) Q: What is the value of a/T at room temperature (Rounded Whole)?
A: a @ 55
11) Q: Find the equivalent resistance of a diode as a function of voltage.
A: dI/dV = e^^{aV/T}*a/T
12) Q: What is r at .125V, .25V, and .5V, why are the vlues so different? (Note: no current values were taken so R can not be found)
A: V = .125V : R = 5.13x10^{4}W; V = .25V : R = .481W; V = .5V : R = 422KW There is a small current that still flows causing the resistance to change dramatically at low voltages.
13) Q: Does the Current / Voltage data from the diode support the equation I = I_{0}e^^{aV/T} equation, explain.
A: No data was taken that could support or refute this question.
Conclusions
From this lab Ohm's Law can be proven. For ohmic elements, the relationship between current and voltage is linear, and easily predictable. However, there were inaccurate values taken for both of the nonohmic elements. The light produces inaccurate results because it's current draw was beyond the capabilities of the supply powering it. Also, no readings were made for the diode because there is no available power supply capable of generating safe voltages for the diode (Below 1V). The lowest voltage that can be produced with a conventional power supply (one designed around the LM317 or equivalent) has a minimum voltage of approximately 1.25V. Due to the constant voltage nature of the diode, there was no way of producing a suitable voltage, therefore readings were not taken.
Attachments
Original lab data
Calculations