Ohm’s Law II

Abstract:

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.

I=V/R

Experiment:

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

PASCO EM AC/DC

o Light Bulb

Pasco 850 Interface

10Ω resistor

100Ω resistor

Two banana cables ( ͡° ͜ʖ ͡°) 

Jumper Wires

Procedure:

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 lab.

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

x-axis. 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.

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.

Fig. 2

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

found.

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. 3

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.

3. 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.

4. 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.

Analysis:

Rbefore on=________________Ω

Rlafterturning on= ________________Ω

(Optional) Part 3: The Change in Resistance

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.

Fig. 5

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.

Questions:

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?