Resistance and Ohm's Law

By Dustin Freeman,2014-05-09 22:07
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Resistance and Ohm's Law

    Resistance and Ohm’s Law

Name: ____________________________ Date: ______________

    Name: ____________________________ Lab Sect.: __________

    Name: ____________________________ Lab Instructor: ______________________

Lab Activity 1: Resistance and Resistivity of Nichrome

    ; Measure the lead and internal resistance of the multimeter. On the lowest resistance scale,

    connect the red and black voltage leads together. Allow the reading to stabilize and note

    the value of the lead and internal resistance of the multimeter.

    R = _______________________ lead,DMM

    ; Use Excel to plot a graph of resistance R versus length L for each wire and add a Linear-

    Fit trendline to each graph. Include these graphs with your lab report.

Answer the following questions:

    Q1. You have created two graphs of resistance versus length, and found that data follows a linear trend:

    ; What do the y-intercepts of the linear fits mean physically?

    ; Are they a random error or indicative of something systematic?

    ; Is there any relationship between these y-intercepts, and the lead/internal resistance of the


    Q2. From the two graphs and the linear fits, determine the ratio of the diameters of the two wires. Explain how you do this.

    Q3. You are told that the thin wire is "32-gauge" in the American Wire Gauge (AWG) system. This means that it has a diameter d = 0.0080 inches. (1 in = 2.54 cm.) Using the graph of

    resistance versus length for the thin wire, calculate the resistivity of nichrome.

    Q4. Suppose a 5-cm length of the thin wire is connected to two (ideal) 1.5-V batteries in series. Assuming that the resistivity of the wire is the same as in your measurements in Q3, what is the

    magnitude of the current density J in the wire?

     283Q5. If the density of conduction electrons n in nichrome is around 10 electrons/m, what is the

    drift speed v of the electrons in the wire under the conditions described in Q4? How long does it d

    take an electron to travel from one end of the wire to the other?

Lab Activity 2: Ohm's Law

    ; Include all relevant plots with your report.

Q6. Is the resistor an "ohmic" or a "non-ohmic" device? From the I-V plot for the resistor,

    determine the value of the resistance. Measure the resistance of the resistor using the multimeter. How do these values compare to the stated resistance value (brown/black/brown) of 100 ?

    R = ___________ R = ___________ DMMgraph

    ; Disconnect the output wires from the resistor, and connect them to the "long" light bulb.

    ; Click the Start button, and again, record data for a few sweeps to produce a graph that

    displays I versus V across the DUT. Include this graph with your report.

    Q7. Does the light bulb have a constant resistance? Is the light bulb an "ohmic" or "non-ohmic" device? Explain your answer.

    Q8. Why does the slope of the light bulb graph change? Explain (qualitatively) why the slope changed when a current was flowing through it.

    Q9. The slope of the graph for the light bulb is not symmetrical. Why is the slope of the current trace different when the filament is heating up compared to the trace of the current when the filament is cooling down?

    ; Now, decrease the voltage sweep (in the Signal Generator window) to 0.015 V and

    produce a second graph. Use the linear fit function to estimate this low-voltage resistance

    of the light bulb. Include this graph with your report.

    Q10. Measure the resistance of the light bulb using the multimeter. How does this value compare to the value obtained from the low voltage sweep above. Is this what you expect and why?

    ; Disconnect the output leads from the light bulb and connect them to the diode. The diode

    is the smallest component on the LabKit module, and has a specific polarity, indicated by

    a thin colored band near one end of the device. The colored band marks the negative (or

    low-potential) end of the diode.

    ; Reset the voltage sweep back to 5.000 V, and produce an I-V plot for the diode. Include

    this plot with your report.

    Q11. Is the diode an "ohmic" or "non-ohmic" device? Explain your answer. Suppose someone asked you: "What is the resistance of that diode?" How would you respond?

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