LAB 3
Ohm’s Law & Play-Doh Resistance
OBJECTIVES
1. Apply Ohm’s Law to determine resistance.
2. Determine the functional dependence of resistance in terms of length & area
EQUIPMENT
DC Power Supply, DMM, Play-Doh, resistor
PROCEDURE
Part 1: Ohm’s Law
a. Draw a circuit diagram of a resistor R = 100 connected across a power supply, with
a voltmeter to measure the resistor voltage VR and an ammeter to measure the
resistor current iR. Now build the circuit on a breadboard.
b. Adjust the power supply until VR = 1.0 V. Record VR and iR in a table. Continue
recording data in 1.0 V increments up to VR = 5.0 V. Record all of your data into a
table.
c. Reverse the polarity of the voltage applied to the resistor by switching the leads at
the power supply. Measure iR at 3 different values of VR and compare with your
previous results. Does the polarity of the voltage across the resistor make any
difference in its behavior?
d. Use Excel to plot current vs. voltage. Fit a straight line to the data and determine the
resistance Rthy from the slope of the line? Use a percent difference to compare Rthy to
Rexpt where Rexpt is the resistance measured by a DMM.
e. Answer the following questions:
 Does the resistor have constant resistance? Explain.
 Does the resistor obey Ohm’s law? Explain.
Part 2: Play-Doh Resistance dependence on Length & Area
Length Dependence
a. Roll the Play-Doh out into a long flat rectangular shape about one to two centimeters
in thickness evenly across the dough. This is your resistor. Without cutting your PlayDoh resistor, divide it (lightly mark it with a plastic knife) into four even sections that
are uniform and all the same length. Measure the length and cross-sectional area of
the resistor and record in the data table. Setup an Excel table that reads Length
(cm), R = ∆V/i (Ω).
b. Set up the Play-Doh resistor in a simple circuit with the ammeter and voltmeter (the
exact same setup as Part 1). Use aluminum foil to connect the Play-Doh resistors
with the power supply.
c. Turn on the power supply and turn the voltage dial up until the current reads about
20 mA. Take a voltage and current reading from the meters immediately and then
turn off the power. Now that you have set this voltage, DO NOT change the voltage
again for the rest of this experiment.
d. Now cut off one of the even sections of the of Play-Doh resistor, making a shorter
resistor. Take data for the voltage, current, and length and record in the data table.
e. Continue cutting length and taking readings until the Play-Doh resistor has been
divided into four strips.
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Area Dependence
Set up your circuit with only one strip of the Play-Doh resistor (from Part 1e).
Measure the cross-sectional area of the resistor and record in the data table. Setup
an Excel table that reads Area (cm2), R = ∆V/i (Ω).
h. Decrease the voltage in the circuit until the ammeter reads around 20 mA. Now that
you have set this voltage, DO NOT change the voltage again for the rest of this
experiment. Take a voltage, current and area reading and record them into the Excel
table.
g. Now add the next strip on top of the first strip. Continue adding strips on top and
taking readings until all four strips have been added.
Data Analysis
 Use Excel to plot R vs. L and R vs. A
f.
Answer the following questions using short concise sentences:
 How does resistance depend on length L? How does resistance depend on area A?
 Explain physically how the resistance changed with length and area.
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