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lab 19

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  4.  “Determining the Resistance of a Wire using Ohm’s Law”
  5. Lab # 19
  6. March 16, 2010
  7. Tuesday Lab
  8. Lab Period 9
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  19. Purpose: In this laboratory exercise, we will use “Ohm’s Law” to determine the resistance of a Nichrome wire. We will explore how the resistance of a wire depends upon its length. Finally, we will examine the relationship between voltage and current graphically for various lengths of the Nichrome wire.
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  21. Introduction: In this experiment, we will create a simple electrical circuit by connecting specific lengths of the Nichrome wire to a variable DC power supply. An ammeter will be connected in a series arrangement with the Nichrome wire to ascertain how length of a wire affects its resistance. Finally, for each length of wire, the voltage, will be plotted as a function of current and the slope of these graphs calculated in order to determine the resistance’s of the wire.
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  23. Procedure: Connect the black connecting cable of the lab's DC power supply to the power supply's (-) black output. Connect the other end of the cable to the Left Fastening Post of the Nichrome wire meter stick. Connect the red connecting cable of the lab's DC power supply to the power supply's (+) red output. Connect the other end of the red connecting cable to a smaller red connecting wire containing two alligator clips, one on either end. Connect the other end of the smaller red connecting wire to the positive terminal of the Ammeter which reads 0-5 amps. Connect a small black connecting wire containing two alligator clips one on either end to the negative end of the ammeter. Connect the other end of the black connecting wire to the Right Fastening Post of the Nichrome wire meter stick. Set the Radio Shack digital multimeter to the DC volts position. Insert the red probe wire into the red socket which reads. Connect the black wire from the multimeter to the Left Fastening Post and then place the red Fastening Post. Make sure the lab’s DC power supply control knob is set to the zero position counter clockwise. Turn on the switch labeled “Main AC” and the one to the left of it labeled “0-30 DC”. If the power supply is working two red lights should come on. Very carefully and slowly, rotate the control knob counterclockwise such that the ammeter reads 0.25 amps. Record the voltage for this value of the current and record your data in Table 1. Then move the red prove from the Multimeter to Nichrome wire positions of 50 cm, 70 cm, and 90 cm and measure these respective voltages. Record these voltages in the appropriate places on Table 1. After you have taken your data, immediately turn off the “0-30 DC”  power switch to prevent overheating of the wire. Repeat the procedure again for the currents of 0.5 A, 0.75 A, 1.25 A, 1.5 A, and 1.75 A. Again, after you have taken your data, immediately turn off the “0-30 DC” power switch to prevent the wire from overheating.
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  26. Data:                                                   Table 1
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  28. Current (Amperes)       Voltage (Volts) Nichrome Wire’s Length (Meters)
  29. 0..25 A 0.138 volts     0.3 meters
  30. 0.50 A  0.224 volts     0.3 meters
  31. 0.75 A  0.326 volts     0.3 meters
  32. 1.00 A  0.476 volts     0.3 meters
  33. 1.25 A  0.548 volts     0.3 meters
  34. 1.5 A   0.684 volts     0.3 meters
  35. 1.75 A  0.800 volts     0.3 meters
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  38. 0.25 A  0.223 volts     0.5 meters
  39. 0.50 A  0.361 volts     0.5 meters
  40. 0.75 A  0.522 volts     0.5 meters
  41. 1.00 A  0.758 volts     0.5 meters
  42. 1.25 A  0.880 volts     0.5 meters
  43. 1.50 A  1.103 volts     0.5 meters
  44. 1.75 A  1.228 volts     0.5 meters
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  47. 0.25 A  0.309 volts     0.7 meters
  48. 0.50 A  0.496 volts     0.7 meters
  49. 0.75 A  0.718 volts     0.7 meters
  50. 1.00 A  1.038 volts     0.7 meters
  51. 1.25 A  1.225 volts     0.7 meters
  52. 1.50 A  1.518 volts     0.7 meters
  53. 1.75 A  1.778 volts     0.7 meters
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  56. 0.25 A  0.209 volts     0.9 meters
  57. 0.50 A  0.632 volts     0.9 meters
  58. 0.75 A  0.913 volts     0.9 meters
  59. 1.00 A  1.332 volts     0.9 meters
  60. 1.25 A  1.560 volts     0.9 meters
  61. 1.50 A  1.988 volts     0.9 meters
  62. 1.75 A  2.240 volts     0.9 meters
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  70. Questions:
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  72. 1) Plot a graph of Voltage (Y-axis) vs. Current (X-axis) for each of your four lengths of Nichrome wire. You should plot all four graphs on the same graph axis
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  74. -On graph paper
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  76. 2) For each of the graphs you made above, determine the slope of the each graph. What information is obtained from the slope of each graph?
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  78. -Resistance is obtained from the slope of each graph.
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  87. 3) From the graphs you made in question 1, what effect does the length of Nichrome wires have on its resistance? Explain why this is the case.
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  89. - The longer the wire, the greater the resistance because of the equation R=ρL/A
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  91. 4) Knowing R = ρ x L / A
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  93. And using the graphs from above and the four lengths of Nichrome wired used, calculate the diameter of the Nichrome wire for each length used.
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  103. 5) Why does Nichrome make an excellent heating element in toasters? Explain why Nichrome heats up when a current is passed through it.
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  105. -Nichrome has high resistivity, and the greater the resistance, the greater the heat.
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  107. 6) Did your data and graphs confirm Ohm’s law? Why or why not?
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  109. -Yes, because the ration of V to I was constant in every case.
  110. 7) Name two ways you can increase the current in your Nichrome wire.
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  112. -(1) Increase the voltage or (2) lower the resistivity as seen to increase I in the equation I = V/R.
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  114. 8) Why would it be a poor idea to make household wiring out of Nichrome wire? Using you reference table, what type of wire would be used for household wiring?
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  116. -Nichrome would be a poor choice to use because since it has a high resistivity, it will cause fires, but copper should be used for household wiring since it has a lower resistivity and is cheap.
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  119. Conclusion
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  121.         In this lab, we explored how the resistance of a wire depends upon its length. Also we examined the relationship between voltage and current. From this lab, Ohm's law was confirmed whereas the current in the circuit was directly proportional to the applied voltage if the resistance remained constant. The lab, though using only one type of wire, did a pretty decent job of demonstrating the patterns expected. There was very little hardship doing the lab correctly because the lab gave a very good procedure, producing expected results and good best fit lines. Not many sources of error were human related in this lab, since assuming the directions were followed properly setting up the system, simple errors such as wrong reading or pushing the cable too hard on the wired stick were mainly possible. However, many mechanical and technological problems could have occurred such as different table charges, malfunctioning multimeters or ammeters, wires of wrong material, etc. This lab was extremely straightforward and easy to grasp, so the lab did not really need any improvements to make it better.
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