Resistance & Ohm's Law

Physics Lab 212P-5

Resistance & Ohm's Law

NAME:______

LAB PARTNERS: ______

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LAB SECTION:______

LAB INSTRUCTOR:______

DATE:______

EMAIL ADDRESS:______

Physics Lab 212P-5

Software list

Microsoft Excel

Equipment List (all items marked with * are in the student kit, others are supplied at the time of the lab)

Science Workshop Interface + voltage probes with alligator clips

Digital multimeter + probes

Carbon resistor

Diode

Measuring scale

*Two 1.5 V batteries + a battery holder

*Flashlight bulbs (one round, one long) + bulb holders

*Hookup wires with alligator clips

*Thick nichrome wire (bare wire with no insulation)

*Thin nichrome wire (bare wire with no insulation)

*Compass

Lab Activity 1: Resistance and Resistivity of Nichrome

Find the two nichrome wires in your kit and use some tape to anchor a straight length of each wire flat onto the lab table (a straight segment of 20 - 25 cm should suffice).
Set the digital multimeter (DMM) provided to measure "resistance." Note that a DMM measures "resistance" by sending a known current I through the object to be measured and then measuring the voltage V across the probes (i.e. R = V/I). This measuring current is typically quite small.
Measure the lead and internal resistance of the DMM. 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 DMM.

Rlead,DMM = ______

Using a scale to measure length and the DMM to measure resistance, determine the resistance of each wire for each of 10 different lengths (e.g. from 1 cm to 20 cm in 2 cm intervals).
Enter these values in an Excel spreadsheet.
Use Excel to plot a graph of resistance versus length for each wire and fit each graph to a straight line. Include these graphs with your lab report.
Note: if you are still not too comfortable with Excel, make sure you ask for your instructor's help!
Answer the following questions:

Q1. From the two graphs and the linear fits, determine the ratio of the diameters of the two wires. Explain how you do this.

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Discuss the relationship between the y-intercept of your linear fit and Rlead,DMM measured previously. Is the lead and DMM resistance a systematic or a random error? Should your measured value of Rlead,DMM be subtracted off any subsequent resistance measurements?

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Q2. You are told that the thin wire is "30-gauge" in the American Wire Gauge (AWG) system. This means that it has a diameter = 0.0125 inches. (1 inch = 2.54 cm.) Using the graph of resistance versus length for the thin wire, calculate the resistivity of Nichrome.

Q3. 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 Q2, what is the current density in the wire?

Q4. If the density of conduction electrons in Nichrome is around 1028 electrons/m3, what is the drift speed of the electrons in the wire under the conditions described in Q3? How long does it take an electron to travel from one end of the wire to the other?

Lab Activity 2: Ohm's Law

Ohm's law states that -- under certain conditions -- the voltage across an object varies linearly with the current through the object: i.e. V = IR.

Another way to say this is that the resistance of the object is independent of the voltage applied across it. A device that obeys this law is said to show "ohmic" behavior and a graph of voltage versus current shows a straight line whose slope is equal to the resistance of the device. However, by convention, current versus voltage (an I-V plot) is usually graphed. In this case the slope is equal to the inverse of the resistance. Devices that deviate from this law, i.e. the voltage versus current curves exhibits a change in slope as the current is changed are said to show "non-ohmic" behavior. The resistance is still equal to the slope of the voltage versus current graph, but in contrast to the ohmic case, the resistance changes as the current is changed.

The circuit needed to determine whether or not a given device obeys Ohm's Law is shown in the figure below. In this circuit, a variable voltage is applied across the device S, and the current and voltage through the device is measured. The voltage and current are measured using the Science Workshop interface. The ammeter and voltmeter for this experiment are built into the Science Workshop interface. A detailed understanding of the circuit is not necessary at this stage: it will be obvious to you how this circuit works once we get to circuits.

Set up the circuit shown above.

Plug the banana plug into the Output of the Science Workshop interface.

Start Science Workshop
Click the "Sample V" button.
Set the Amplitude for 5.000 V.
Set the frequency for 0.4 Hz.
Click on the triangle waveform.
Click on the "Auto" button.
Click the "Sample I" button.
Click & drag the "Graph" icon to Output I.
Change the input on the x-axis to AnalogOutput.

Click the "sampling options" icon.
The default rate at which the program measures data is 10 Hz (i.e. 10 readings per second). Change this to 100 Hz.

Now, connect the carbon resistor (brown, black, brown) in the position S in the circuit.
Produce a graph that displays I versus VAB.
Click on the "REC" icon on your program screen.
After the current makes a couple of sweeps, stop recording data by clicking on the "STOP" icon.
Produce a linear fit to the graph.
Click on the "" icon. This should open a frame on the right side of your graph.
Click on the "" icon and select curve fit  linear fit from the pop-up menu.
Note the value of the slope of the linear fit
Include all relevant plots with your report.

Q5. 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 value of the resistor using the DMM. How do these values compare to the stated resistance value (brown, black, brown) of 100?

Rgraph = ______RDMM = ______

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Replace the resistor with the long light bulb included in your lab kit.
Produce a graph that displays I versus VAB. Include this graph with your report.
Change the voltage sweep to 0.015 V and produce a second graph. Use the linear fit function to estimate the low voltage resistance of the light bulb. Include this graph with your report.

Q6. Does the light bulb have a constant resistance? Is the light bulb an "ohmic" or "non-ohmic" device? Explain your answer.

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Q7. Why does the slope of the light bulb graph change? Explain (qualitatively) why the slope changed when a current was flowing through it.

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Q8. 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?

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Q9. Measure the resistance of the light bulb using the DMM. How does this value compare to the value obtained from the low voltage sweep above. Is this what you expect and why?

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Replace the light bulb with the diode, reset the voltage sweep to 5.000 V, and produce a graph that displays I versus VAB. Please include this plot with your report.

Q10. 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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