Physics Online
Ohm's Law
Introduction
The purpose of this lab is to test Ohm'slaw. As with the previous lab, we will bemeasuring current, potential difference, and resistance.Ohm's law says that the potential difference equals the current timesthe resistance, or V = IR. If we graph the potential differenceas a function of currentfor an electrical component, then the slope of the line will be thecomponent's resistance.
One can also use multiple resistors in various configurations. Resistors in series (connected at one end with no other branches to that connection) can be treated as a single resistor with equivalent resistance of Req = R1 + R2. Resistors in parallel (connected at both ends) can be treated as a single resistor with equivalent resistance of Req = (1/R1 + 1/R2)-1. Testing these formulas is a secondary purpose of this lab.
Equipment You Procure
- digital camera
- 5 AA batteries
Equipment from Kits
- 2 digital multi-meters (DMM)
- 5 AA battery holders
- Two resistors between 400 Ω and 5000 Ω
- Black wires with clips
- Alligator clips
Experimental Procedures
1)Use the color code to findtwo resistors between 400 Ω and 5000 Ω. Using the digitalmulti-meter (DMM) as an ohmmeter, measure and record their resistances.
2)Connect one lead of an empty AA battery holder to one end of a resistor. Connect the other end of the resistor to one probe of a DMM configured as an ammeterat the 20 mA setting (see instructions in the Introduction to Electronics lab). Connect the other probe of the ammeter to the unconnected lead of the battery holder. You may choose to use the black wires to connect one device to another.
3)Double check your configuration. You may choose to take a photograph and have your instructor check it for you before you proceed. Place a AA battery in the holder. Your ammeter should now register the current through the resistor in mA. If the current is negative, simply record the absolute value.
4)To simultaneously measure the potential difference across the resistor, configure your second DMM as a voltmeter at the 20 V setting (see instructions in the Introduction to Electronics lab) and place the probes at opposite ends of the resistor. If the potential difference is negative, simply record the absolute value. See figure 1.
5)You might take a photograph of your circuit in action at this point. Include a total of about 3 photos in your report.
6)Remove the battery from the battery holder.
7)Connect an additional battery holder in series (connect the black to the red!) with the existing battery holder(s).
8)Repeat steps 3 through 8 until you have done the experiment with a total of 5 batteries in series.
9)Using the data, plot the potential differenceon the y-axis and the current on the x-axis. Insert a trendline and observe the slopeof the line to compute the experimental resistance of the circuit. You do not need to compute an error in the slope, but you should check if the line goes through most of the error bars. Compare the experimental value from the graph to the value obtained with the ohmmeter.Do not calculate R = ΔV/I.
10)Connect the two resistors in series (connect at one end).Repeat steps 2 through 9.Measure the potential difference across the pair of resistors.Compare the experimental value on the graph to the theoretical value (calculated equivalent resistance).Do not calculate R = ΔV/I.
11)Connect the two resistors in parallel (connect at both ends). Repeat steps 2 through 9. Measure the potential difference across either resistor. Compare the experimental value on the graph to the theoretical value (calculated equivalent resistance).Do not calculate R = ΔV/I.
12)Do not calculate R = ΔV/I.
Figure 1: The digital multi-meter in the upper right corner is configured as an ammeter at the 20 mA setting. The digital multi-meter in the lower right corner is configured as a voltmeter at the 20 V setting. Connect the battery last so that you can reduce the risk of damaging your ammeter.