Experiment
9
Factors Affecting Electrical Resistance
INTRODUCTION
C
op
y
Metals are generally excellent conductors of heat and electricity. Because of this, we tend to
neglect any resistance in wires when we examine the behavior of ordinary loads (bulbs, resistors,
motors, etc.) in electrical circuits. But metals are not perfect conductors, otherwise the wires in
our toasters would not get hot. In this experiment, you will examine the factors that affect the
electrical resistance of metals.
OBJECTIVES
In this experiment, you will



Collect potential difference and current data for a number of metal rods.
Determine the electrical resistance for each of the readings.
Determine how length, cross-sectional area, and type of material affect the resistance of a
metal rod.
n
MATERIALS
Vernier Resistivity Rod Set or
similar metal rods having different diameters
dissimilar metal rods with the same diameter
Vernier Current Probe (if using a conventional
power supply)
Vernier Power Amplifier computer program1 (if
using LabPro or LabQuest Mini without
Logger Pro)
ua
tio
Vernier data-collection interface
Logger Pro or LabQuest App
Vernier Power Amplifier or
conventional power supply
Vernier Instrumentation Amplifier
clip leads
PRE-LAB INVESTIGATION
Ev
al
The wires of a toaster carry electrical current between the electrical outlet and the toaster. The
wires inside the toaster also carry electrical current. During its operation, the wires inside the
toaster get hot enough to glow, yet the wires to the outlet do not. List some factors that might
determine how hot a wire will get when it conducts electricity.
1
Vernier Power Amplifier computer program can be downloaded from www.vernier.com/downloads.
Advanced Physics with Vernier – Beyond Mechanics
©Vernier Software & Technology
9-1
Experiment 9
PROCEDURE–EQUIPMENT SETUP
How you set up the apparatus depends on the interface (LabQuest, LabQuest Mini, or LabPro)
and whether you are using the Vernier Power Amplifier or a conventional power supply. Follow
the set-up instructions for the equipment combination you are using. Figures 1 and 2 apply to a
setup that uses a Power Amplifier.
Figure 1
Figure 2
Power Amplifier and LabQuest
1. Turn on the power amplifier. Use the Current Sense cable that came with the power amplifier
to connect an analog channel on LabQuest to the Current Monitor LQ/LP port on the power
amplifier. Use the mini stereo cable that came with the power amplifier to connect the
Speaker Out port on LabQuest and the Audio In port on the amplifier. Select a brass rod with
the same diameter as the other materials (aluminum, copper, etc.) you will test. If you are
using the Vernier rod set, use the 3.18 mm (1/8 inch) rod. Connect clip leads to each end of
the rod. Attach the other ends of the clip leads to the terminals on the power amplifier (see
Figure 1).
2. Connect an Instrumentation Amplifier to the interface. Set the range to 0–20 mV. Start the
data-collection program.
3. Connect clip leads to the red and black terminals of the instrumentation amplifier; these will
act as the probe leads for your measurements of potential. Connect the black lead to a
position just inside the lead carrying current to the rod. Note this as the zero position. Touch
the other lead (hereafter referred to as the red lead) to the black lead on the rod and zero both
sensors (see Figure 2).
4. Adjust the voltage until the current through the rod is approximately 0.5 A using the
appropriate instructions.
LabQuest with Logger Pro and the Power Amplifier
a. Choose Set Up Sensors ► Show All Interfaces from the Experiment menu.
b. From the Sensor Setup dialog box, click Power Amplifier; the Power Amplifier dialog
will be displayed. Close the Sensor Setup dialog box and move the Power Amplifier
dialog box to a convenient place on your screen.
c. In the Waveform list, click DC Mode and set the potential to 0.6 VDC. Click Start. Adjust
the voltage as necessary to produce the desired current.
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Advanced Physics with Vernier – Beyond Mechanics
Factors Affecting Electrical Resistance
LabQuest as a standalone device and the Power Amplifier
a. Launch Power Amplifier from the Home screen.
b. In the mode list, select DC and set the potential to 0.6 VDC. Tap Start.
c. Launch LabQuest App from the Home screen. Check the current; if necessary, return to
the Power Amplifier app to adjust the voltage to produce the desired current.
Power Amplifier and LabPro
1. Use the Current sense cable that came with the power amplifier to connect Channel 4 on
LabPro to the Current Monitor LQ/LP port on the power amplifier. Select a brass rod with
the same diameter as the other materials (aluminum, copper, etc.) you will test. If you are
using the Vernier rod set, use the 3.18 mm (1/8 inch) rod. Connect clip leads to each end of
the rod. Attach the other ends of the clip leads to the terminals on the power amplifier.
2. Connect an Instrumentation Amplifier to the interface. Set the range to 0–20 mV. Start the
data-collection program.
3. Connect clip leads to the red and black terminals of the instrumentation amplifier; these will
act as the probe leads for your measurements of potential. Connect the black lead to a
position just inside the lead carrying current to the rod. Note this as the zero position. Touch
the other lead (hereafter referred to as the red lead) to the black lead on the rod and zero both
sensors.
4. The following steps will enable you to run a DC current of approximately 0.5 A through the
rod.
a. Choose Set Up Sensors ► Show All Interfaces from the Experiment menu.
b. From the Sensor Setup dialog box, click the picture of the Power Amplifier in CH4 and
choose Analog Out. This brings up the Analog Out dialog box. In the Waveform list, click
DC Output and set the Amplitude to 0.6 V. Click OK and check the current. Return to the
Analog Out dialog box to adjust the voltage as necessary to produce the desired current.
c. You can close the Sensor Setup dialog box until you want to turn off the power.
Power Amplifier with Vernier Power Amplifier computer program and LabPro or LabQuest Mini
1. Turn on the Power Amplifier. Use the Current Sense cable that came with the power
amplifier to connect an analog channel on LabQuest to the Current Monitor LQ/LP port on
the power amplifier. Use the mini stereo cable that came with the power amplifier to connect
the Speaker Out port on your computer and the Audio In port on the amplifier.
2. Select a brass rod with the same diameter as the other materials (aluminum, copper, etc.) you
will test. If you are using the Vernier rod set, use the 3.18 mm (1/8 inch) rod. Connect clip
leads to each end of the rod. Attach the other ends of the clip leads to the terminals on the
power amplifier (see Figure 1).
3. Connect an Instrumentation Amplifier to the interface. Set the range to 0–20 mV. Start the
data-collection program.
4. Connect clip leads to the red and black terminals of the instrumentation amplifier; these will
act as the probe leads for your measurements of potential. Connect the black lead to a
position just inside the lead carrying current to the rod. Note this as the zero position. Touch
the other lead (hereafter referred to as the red lead) to the black lead on the rod and zero both
sensors (see Figure 2).
Advanced Physics with Vernier – Beyond Mechanics
9-3
Experiment 9
5. Adjust the voltage until the current through the rod is approximately 0.5 A.
a. Launch the Vernier Power Amplifier computer program.
b. In the Waveform list, click DC Mode; set the potential to 0.6 VDC. Click Start. Adjust the
voltage as necessary to produce the desired current.
Conventional Power Supply and LabQuest, LabQuest Mini, or LabPro
1. Connect a Vernier Current Probe to the interface. Connect the current probe in series to the
negative terminal on the power supply and to one end of the brass rod (see Figure 3).
Figure 3
2. Connect an Instrumentation Amplifier to the interface. Set the range to 0–20 mV. Use a lead
to connect the black post on the instrumentation amplifier to ground (usually the black
terminal on the power supply). Start the data-collection program.
3. Connect clip leads to the red and black terminals of the instrumentation amplifier; these will
act as the probe leads for your measurements of potential. Connect the black lead to a
position just inside the lead carrying current to the rod. Note this as the zero position. Touch
the other lead (hereafter referred to as the red lead) to the black lead on the rod and zero both
sensors (see Figure 2).
4. Your instructor will provide specific instructions that will enable you to run a DC current of
approximately 0.5 A through the rod.
PROCEDURE–DATA COLLECTION
Part 1 Effect of length
1. Change the data-collection mode to Events with Entry. Enter length as the Name and m as
the Units.
2. Connect the red lead right next to the black lead at the zero position. Click or tap Keep. Enter
0 for the length.
3. Move the red lead 5 cm from the black lead at the zero position. Click or tap Keep. Enter 5
for the length.
4. Move the red lead 5 cm farther from the zero position. Click or tap Keep and enter the new
length.
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Advanced Physics with Vernier – Beyond Mechanics
Factors Affecting Electrical Resistance
5. Repeat Step 4 until you have five more readings. Store this run, then turn off the power to the
apparatus.
Part 2 Effect of material
1. Replace the brass rod with one made of a different metal. It should have the same diameter as
the rod you used in Part 1. (Why?)
2. As you did for Part 1, set up a current of approximately 0.5 A in the rod. Repeat Steps 2–4
from Part 1. Store this run, and then turn off the power to the apparatus.
3. Repeat Step 2 using additional metal rods. Store these runs.
Part 3 Effect of diameter
1. Obtain one of the other brass rods; record the diameter. Connect the apparatus as you did for
Part 1.
2. Turn on the power and adjust the current to approximately 0.5 A. Connect the red lead at the
25 cm mark. Rather than open a new experiment file with diameter as the independent
variable, you can continue to use your file and simply record the value of the current and
potential from the sensors in your notebook.
3. Repeat Steps 2 and 3 with the rest of the brass rods in the set of materials for this lab. Save
the file.
EVALUATION OF DATA
Part 1
1. Disconnect your sensors, then view your first run.
2. Choose New Calculated Column from the Data menu in Logger Pro. In LabQuest App,
create this calculated column by choosing New Calculated Column from the Table menu on
the Table tab. Enter resistance as the Column Name and ohms as the Units. Using Ohm’s
law, determine the equation to use in the equation field. Keep in mind that the potential was
measured in mV.
3. Choose to view only one graph. Select resistance as the label of your vertical axis. If the
relationship between resistance and length appears to be linear, fit a straight line to your data.
Part 2
1. Repeat the evaluation you did in Part 1 for the runs you performed with different types of
metal.
2. What does the slope of the best-fit line for a metal tell you about its relative ability to
conduct electricity?
Part 3
1. Calculate the resistance from your recorded values of current and potential difference.
Advanced Physics with Vernier – Beyond Mechanics
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Experiment 9
2. Choose New from the File menu.
3. Manually enter data for diameter and resistance for the brass rods you examined.
4. Create a new calculated column showing the cross-sectional area of the rods.
5. Change the horizontal axis of your graph to cross-sectional area. What relationship appears
to exist between resistance and the cross-sectional area of the rods?
6. Test your conclusion by creating another calculated column and modifying the crosssectional area so as to produce a linear plot. If successful, write a statement describing the
relationship you have found.
Part 4 Consolidation of your findings
1. At this point, you have two proportional relationships. In your class discussion you will
discuss how these can be combined to form a single proportionality relating resistance,
length, and the cross-sectional area of the rod.
2. The name given to the constant of proportionality for the relationship you have determined is
resistivity, . What are the units of this constant?
3. Using known values for resistance, length, and area, determine the value of  for your
particular brass. How does this value compare to the table value for this metal?
EXTENSION
If you are trying to power your electric lawnmower using a long extension cord, explain why it
would be better to use 12-gauge rather than 16-gauge wire. Do a web search for American wire
gauge to find the diameters of these two wires. Use what you have learned about resistance and
the equation for electrical power to calculate the ratio of the power lost due to heating in these
two wires. Assume the current in both wires is the same.
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Advanced Physics with Vernier – Beyond Mechanics
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THIS IS AN EVALUATION COPY OF THE VERNIER STUDENT LAB.
This copy does not include:

Essential instructor information including discussions on how to lead students
to a successful activity

Sample data

Important tips for successfully doing these labs

Answers to questions and extensions
The complete Advanced Physics with Vernier – Beyond Mechanics lab manual includes 22
experiments, as well as essential teacher information. The full lab book is available for
purchase at: www.vernier.com/phys-abm
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