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Stevens High School Physics Laboratory Manual
11
Ohm’s Law – Part 2
Students: This will be an in-class report. Ideally, you will turn in the analysis questions today at the end of the
lab, but you may have an extra day if you need it.
Purpose: The objective of this laboratory is to test the basic formulas used when determining equivalent
resistance by measure the voltage and current in simple series and parallel circuits of two resistors.
Concept:
The resistance of a circuit depends on many factors, none more important than the arrangement
of individual resistors. If electrons (or other current-carrying particles) are forced to go through multiple
consecutive resistors (in series, say), the effective resistance increases:
𝑅𝑠 = ∑ 𝑅𝑖
(1)
𝑖
If, on the other hand, electrons are presented with a choice of multiple paths to travel between two points in a
circuit (as in a parallel circuit), then the equivalent resistance decreases:
𝑅𝑝 = ( ∑
𝑖
1
)
𝑅𝑖
(2)
All the while, conservation of charge and energy dictate the voltage drop and current through each resistor. The
equation which relates current, electrical potential (voltage), and resistance is Ohm’s Law (V = IR), and the
validity of this equation will be assessed in this lab.
Materials:
 Multimeter
 6 to 8 alligator-clip wires
 2 resistors
 DC power supply
Procedure:
1. Wire together a circuit (either series or parallel) with your two resistors and connect it to the power
supply. Double check all of your connections.
2. Record your expected total resistance (using the equations for series and parallel resistors) in a data
table similar to Table 2.
3. Set the multimeter probes to measure voltage and connect the meter to your circuit so that you are
measuring the total voltage drop across both resistors. This entails having the black probe in the center
slot and the red probe in the right slot. If you look closely, you will see that the meter is labeled. NOTE:
If the meter beeps, turn off the power supply and double check your setup.
4. Set the power supply control knob to the minimum level, then turn it on.
5. Record the voltage reading on the multimeter – note if the display is reading mV or V.
6. Turn off the power supply.
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Stevens High School Physics Laboratory Manual
7. Set the multimeter to measure current – you will also have to rewire your circuit slightly. DO NOT
change the power supply setting.
8. Turn the power supply on and record the current. Immediately turn off the power supply.
9. Repeat steps 1-8 for at least 4 more power supply settings.
10. Repeat steps 1-9 for the other circuit type (i.e., series or parallel)
Data: Record your measurements and observations in tables like those on the following pages. Note that you
will
Data Analysis:
1. For each row of each table, compute the resistance using Ohm’s Law. Then, average all of the series
values to obtain the average equivalent series resistance.
Series
I [A]
V [V]
Measured Average
Resistance []:
Calculated Equivalent
Resistance []:
Percent Error:
Parallel
R []
I [A]
V [V]
R []
Measured Average
Resistance []:
Calculated Equivalent
Resistance []:
Percent Error:
Table 1: Reduced data showing average measured resistance.
2. Repeat step 1 for the parallel circuit.
3. Next, calculate the expected resistance for each circuit using the resistor labels and the equations for
equivalent resistance we used in class.
4. Compute the percent difference between the measured average equivalent resistance and the
calculated expected equivalent resistance.
5. Discuss, using at least two complete sentences, the sources of error in your lab.
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