Northern Illinois University
Physics 150
Lab 12
Ohm's Law
V=IR
A simple relationship among the three important electrical quantities current, voltage, and
resistance was discovered by Georg Simon Ohm. The relationship and the unit of electrical
resistance were both named for him to commemorate this contribution to physics. One statement
of Ohm’s law is that the current through a resistor is proportional to the potential difference
across the resistor. According to Ohm's law, V=IR, where: V is voltage measured in volts, I is
current measured in amperes, and R is resistance measured in ohms. In this experiment you will
test the correctness of this law.
Figure 1
Objective


Determine the mathematical relationship between current, potential difference, and
resistance in a simple circuit.
Compare the voltage vs. current behavior of a resistor to that of a light bulb.
Materials




CBL/TI
Voltage Probe System
Ammeter
wires



clips to hold wires
adjustable DC power supply
light bulb
Procedure
1. Connect the Voltage Probe.
2. Turn on the computer and start LabPro.
3. With the power supply turned off, connect the power supply, a resistor, wires,
ammeter(current probe), and clips, as shown in Figure 1. Take care that the positive lead
from the power supply and the red terminal from the Voltage Probe are connected as
shown in Figure 1. Note: Attach the red connectors electrically closer to the positive side
of the power supply. Have your instructor check the arrangement of the wires before
proceeding.
4. Zero the voltage probe with no current flowing and with no voltage applied (power
supply off), using LabPro. Voltage readings should appear on the computer screen.
5. Record the value of the resistor. Turn the control on the DC power supply to 0 V and
then turn on the power supply. Check the voltage reading and re-zero if necessary.
6. Slowly increase the potential (using the fine adjust control knob) so the smallest
measurable amount of current is flowing through the ammeter. Record the current and
voltage in data table.
7. Increase the potential (voltage) until the next readable current is obtained. Record the
current and voltage.
8. Repeat Step 7 until eight or nine different current and voltage readings are recorded.
Do not exceed five (5) volts of potential across your circuit.
9. Return the potential (voltage) to zero and replace the resistor with the light bulb.
10. Repeat steps 6 -8 for the light bulb.
Analysis
1. Prepare a graph of voltage (vertical axis) versus current (horizontal axis) for each device.
2. According to Ohm's law, V=IR. Recall that the equation for a straight line going through
the origin (the criterion for a directly proportional relationship) is y = mx. It follows then
that the slope of these graphs is equal to resistance, R. From your resistor graph,
determine the slope and mathematically compare it to the value of the resistor.
3. Resistors are manufactured such that their actual value is within a tolerance. For most
resistors the tolerance is 5% or 10%. Check with your instructor to determine the
tolerance of the resistor you are using. Calculate the range of possible values for the
resistor. Does the resistance value you determined graphically fall within the
manufacturers' specifications?
Discussion
1. Did your resistor follow Ohm’s law? Base your answer on your experimental data.
2. Describe what happened to the current through the light bulb as the potential (voltage)
increased. Was the change linear? Since the bulb gets hotter as it gets brighter, how does
the resistance vary with temperature?
3. Does your light bulb follow Ohm’s law? Base your answer on your experimental data.