PHYS 1420: College Physics II Summer 2018

Lab 03:
Resistance and
R es i st i v i t y
INTRODUCTION
Resistivity is the tendency of a material to
behave as a resistor. You already know that
not everything conducts electricity equally
well, and that some materials (like copper)
resist very little, while others (like rubber)
provide enough resistance to effectively
prevent the flow of current. Resistivity is a
fundamental material property (like density or
melting point), while the total resistance (R)
depends on the material, the geometry, and
the temperature. The dependence on
geometry (really, a volume dependence: cross-
sectional area (A) and length(L)) can be
quantified:
L
R=ρ
A
where ρ is the resistivity. As you might expect,
a longer wire (of any material) creates greater
resistance, and a fatter wire (greater cross-
section)
€reduces total resistance. We can
easily test the length dependence, and
simultaneously find the resistivity of an unknown wire.
OBJECTIVES
๏ Use Ohm’s Law to establish the dependence of resistance on length
๏ Distinguish between resistance and resistivity
๏ Find the resistivity of a given wire and from this determine the material
EQUIPMENT
๏ DC power supply ◉ Electrical wires
๏ Vernier LabQuest ◉ Current and voltage probes
๏ Potentiometer board ◉ Vernier calipers This is the same circuit you built last week!

EXPERIMENTAL PROCEDURE
๏ The current and voltage probes are extremely
sensitive, and can be permanently damaged by
overloading them. The current probe will be literally
destroyed by currents greater than 0.600A. The
voltage probe is limited to 6V. It is slightly more
forgiving, but will also be rendered permanently
inoperable by exposure to voltages greater than 6V.
๏ It is necessary to set the power supply to limit the
maximum current output. We will always use the
power supply in constant voltage (CV) mode. The
power supply has both coarse and fine adjustments
for both current and voltage. It also has a HI–LO range
selector.
๏ When the selected range is LO, the maximum current
in CV mode will be 0.200A. With the power supply off,
zero all of the control knobs to the left (anti–
clockwise). Now adjust the fine control of the current
to maximum (clockwise). The coarse control remains
This is the same circuit you built last week! The adjustable
at zero. When you switch the power supply on, you
potentiometer replaces the resistor on the circuit board.
will adjust only the voltage to power your circuit. The

Course Web: http://faculty.uca.edu/njaustin/PHYS1420/Laboratory page 01/02

current will never exceed 0.200A. Note that you are not voltage–limited, so you must still be careful not to exceed 6V.
๏ Use the circuit diagram and the photograph above to set up the potentiometer as the resistor in the DC circuit.
๏ The sliding scale can be adjusted to create a complete circuit with a specific length of resisting wire. Each of the four buttons shown
can be depressed (one at a time!) to complete the circuit. Pay careful attention to which contact you are using, and the corresponding
length of wire!
๏ For ten different length of potentiometer wire, collect voltage and current data using the same method as last week. Make sure to very
carefully keep track of the length of wire. Save each data set (tap the File Cabinet icon on the graph tab) before proceeding to the next
length.
QUESTIONS
1. Measure and record the diameter (d) of the
wire using the vernier calipers. Use this to
calculate the cross-sectional area of the wire.
2. If you have not already, graph the voltage and
current data for each length of wire. The slope
of each line is the resistance for that specific
length of wire. Keep track by making a table of
values for length of wire and corresponding
resistance.
3. Because the resistivity (ρ) and area (A) of the
wire are constant, the resistance is directly
proportional to the length:
⎛ ρ ⎞
R = ⎜ ⎟L
⎝ A ⎠
where (ρ/A) is the proportionality constant, or
slope of the R vs. L curve. Graph this data, fit
the curve, and extract the slope. It should be
quick
€ and simple to use a spreadsheet to make
this graph.
4. Calculate the resistivity of your wire:
ρ ρ
Your graph may (or may not...depends on the material) slope = = 2 ρ = ( slope)( πr 2 )
have a slope resembling this example. A πr OR
where slope is the slope of your R vs. L curve
and r is the radius of the wire (r = ½d). Compare this value to Table 27.1 at the top of the page, and determine the
material of the wire. How close did you get to the actual value? Calculate the percent error in your experimental value.
€ €
QUIZ 03
With your lab partners, prepare a data sheet and summary of this experiment. You should include:
๏ A table with your data neatly presented.
Include the appropriate units! This does not need to include each V vs I table and graph generated for each individual trial completed
using the LabQuest.
๏ The graph of resistance vs. length.
You should have the axes properly labeled and scaled, and include the best-fit line. Make it easy to read and interpret!
๏ The calculated resitivity.
Show all your calculations. This includes the area, resisitivity, and percent error based on your identification of the wire’s most
probable composition.
๏ A brief comment on the success of your experiment.
How close did you get to the expected value? Name at least two sources of experimental uncertainty.
๏ You may submit electronically.
If you are not able to send your work to the printer in CCCS 101. Upload directly to the shared folder on the Google drive for which you
have been sent a specific link, share a Google doc or sheet directly with me, or e-mail your file.
๏ Submit your work no later than 9:40 AM Tuesday morning, 17 July.
Please be sure to include the names of all group participants. All group members will receive the same score for this submission.

Course Web: http://faculty.uca.edu/njaustin/PHYS1420/Laboratory page 02/02