Ohm’s Law and the Measurement of Resistance
Today you will determine the resistance of unknown resistors using Ohm’s Law. Then, you will design an
experiment to measure the resistivity of Play-Doh.
Remember, your lab report should contain all information someone would need to recreate your
experiment. This includes a detailed description of your procedure, any figures or circuit diagrams
needed, and a careful analysis of the data. For more information on how you should prepare your report
see the lab guidelines sheet you received at the beginning of the semester.
Experiment 1: Testing Ohm’s Law for a resistor
Your instructor or TA will show you how to set up the circuit if it is not already set up for you. (Make sure
your lab report contains a circuit diagram which accurately describes how everything is connected). By
adjusting the voltage on the power supply, you can change the voltage across the resistor. As you do this the
current should also change. You should have two resistors of unknown resistance at your lab station. For each
resistor, take about 8 or 9 readings of voltage and current. Keep the voltage under 5 volts. Record your data in
a table. Then, make a plot of voltage vs. current for each resistor. Determine how the plot of voltage vs.
current can be used to find resistance.
Question: Do the resistors you tested in this experiment follow Ohm’s Law? Why or why not?
Experiment 2: Testing Ohm’s Law for a light bulb
Repeat experiment 1, instead using a light bulb as your resistor. This time, take readings of the current for the
following voltages: 0.1 V, 0.2 V, 0.3 V, 0.4 V, 0.5 V, 0.75 V, 1 V, 1.5 V, 2.0 V, 2.5 V, 3.5 V, 5.0 V.
Question: Does the light bulb you tested in this experiment follow Ohm’s Law? Why or why not?
Experiment 3: Play-Doh Resistor
Design and conduct an experiment to accurately measure the resistivity ρ of Play-Doh! (Remember to include
all the details of your experiment in your lab report).
Please do not plug the banana plug or alligator clips directly into the Play-Doh since it does not come off
very easily. Instead, rest your Play-Doh resistor on the conducting “v” shaped trays. Connect the
alligator clips to the stands to connect the Play-Doh resistor to your circuit.
Some Hints:
1. Shape your Play-Doh into a cylinder with length L and cross-sectional area A.
2. The quality of your experimental results depends strongly on the uniformity of the cylinders. Avoid cracks or
voids by kneading thoroughly before rolling.
Questions:
1. Based on your measurement of the resistivity of Play-Doh, calculate how long a piece of copper wire needs
to be to have the same resistance as a 10-cm long cylinder of Play-Doh. Assume the copper wire and the PlayDoh have the same diameter.
2. Explain the difference between resistance and resistivity.
3. Using the ohmmeter, NOT the powered circuit, measure your resistance. What is the value? Measure your
resistance across several body parts. Does the resistance vary? If so, what seems to affect it?
4. For your resistance, what amount of current would go through your body if you came in contact with a 120 V
potential difference?
Note: High currents through your body can cause serious injury or death. Here are a few of the typical
consequences of different currents:
1 mA
5 mA
10 mA
15 mA
70 mA
Can be felt
Is painful
Causes spasms
Causes loss of muscle control
Probably fatal