Ohm’s Law II
Ohm’s Law is perhaps the most essential equation to analyzing circuits.
Named after German physicist Georg Ohm, this law states that current is proportional to
voltage divided by resistance, and that resistance is an independent variable.
The purpose of this experiment is to first prove Ohm’s law with a graph and then
apply it in order to find unknown resistance and examine the resistance of a light bulb.
Materials Needed:
Computer with Capstone Software
Light Bulb
Pasco 850 Interface
10Ω resistor
100Ω resistor
Two banana cables ( ͡° ͜ʖ ͡°)
Jumper Wires
Part 1: Proving Ohm’s Law
1. Using the listed materials, construct the circuit
displayed in figures. 1 and 2. Connect one end
Fig. 1
of the banana cables into the ports labeled “to
Power Amp.” in figure 1, and the other end into
the “output 1” ports of the interface.
2. Start the Capstone program and create a new
a) Start by going to hardware setup and
clicking the output ports. Select the
voltage sensor option.
b) Drag in a graph from one of the side
tabs. Select output voltage as the
measurement for the y-axis and output
current as the measurement for the xaxis. On the bottom, set the frequency
detection to 500Hz.
c) Open the signal generator. Set it to
60Hz and 3V. Select the triangle AC
waveform and check the “auto” box.
Your set up should look similar to
figure 3.
2. Record for three seconds then stop. The graph should look like a linear equation.
Fig. 2
3. Select two points on the line and calculate the slope. Since I = V/R, R =V/I, which
is the slope we found. The slope should equal 10Ω. If it does not, reassess the lab
and try again.
4. Select a random resistor that you do not know the value of, but has a black third
band. Use it to replace the 10Ω resistor.
5. Repeat steps 2 and 3 to find the value of the new resistor. Then, use the resistor
bands to calculate the actual value of the resistor and compare it to the value you
Fig. 3
Part 2: Using Ohm’s law
1. Rewire the circuit from the resistor to a light bulb,
as shown in figures 4 and 5.
Fig. 4
2. Change the setting on signal generator from 60Hz to
0.3Hz, and set the amplitude to 2.5V. On the
bottom, change the frequency detection from 500Hz
to 4Khz.
Start recording. Note the light bulb turning on and
off, and what the graph does during the times the
light bulb is either on or off. Remember that before
the light bulb lights, it must heat up, and afterwards
it must cool down.
Use the tools to calculate the slope
of the graph before the light bulb
turns on (not while it is on) and the
slope after it turns on. Place them
in the appropriate boxes below.
Rbefore on=________________Ω
Rlafterturning on= ________________Ω
(Optional) Part 3: The Change in Resistance
Fig. 5
The following is performed with an identical set-up to that of part two. You can
simply modify that experiment as described in the following.
1. Drag in a new table and graph from the side tab.
2. On the top left of the table should be an option to insert a new column. Click it
once, so that the table has three columns in total.
3. In the first and second columns, click the box labeled “select measurement” and
then select Output Voltage and Output Current respectively.
4. In the third column, press “select measurement” and move the mouse over “create
new” in the drop down menu. Then select the option “calculation.”
5. Towards the top of the table should be a line with red text labeled Calc. If this is
not visible, left click anywhere on the table. Now, right click the box in the third
column where you initially inserted the calculation.
6. Once the line on the top is available, right click the line with red text and press
“insert data,” then insert Output Voltage. Press the slash on the keyboard, and
then right click the line again, this time choosing to insert Output Current.
7. Set the graph to record “Calc” on the Y-axis and time on the X-Axis.
8. Record, and observe your results. Note what the graph does when the light bulb
lights and turns off.
1. Why do you think the graph of V/I does not have a constant slope in the light
bulb? Does it obey Ohm’s law?
2. As you begin recording, there is a loop on the graph that differentiates from the
curves that the light bulb repeatedly produces. In terms of temperature, why does
this happen? (Optional, find the slope of this curve on the segment the light bulb
is off)
3. What are some potential sources of error?