Lab 5 Resistor Circuits!
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Objective!
The purpose of this lab is to examine the current through and the voltage across resistors in series
and parallel circuits.!
Introduction!
According to Ohm's Law, the voltage across a ohmic resistor drives a current through the resistor
that is inversely proportional to its resistance.!
I =
V
!
R
We will look at three different types of circuits to verify Ohm’s Law and to observe the current and
voltage characteristics of each type of circuit. !
!
Experiment 1: Series Circuit!
Pick three resistors and measure their resistances and record these values in your lab notebook.
Use resistors whose resistances are between 500 Ω and 5000 Ω.!
Diagram a series circuit with the resistors you picked. Place the three resistors in series. Place
the DMM in series with the circuit, between the power supply and the first resistor. Set your DMM
to read current. Double check to see that you have indeed built the following circuit. Turn on the
power supply and set it to 10 V. Record the current that flows out of the power supply.!
Note that the current flowing through all three resistors are the same since there is only one path
for the current to flow.!
!
Now, set the DMM to measure voltage. Measure the total voltage across all three resistors. This
is the same as the voltage from the power supply. Calculate the total resistance due to all three
resistors using ohm’s law. How is the total resistance related to the resistance of each resistor?!
For series resistor circuits, the equivalent resistance is!
Req = R1 + R2 + R3 !
The next step is to measure the voltage across each resistor. Record these voltages. Calculate
the resistances using the ohm’s law and compare the it to the stated values. Does ohm’s law give
you the correct resistances?!
!
Rev 7/14/14!
page 1
Experiment 2: Parallel Circuit!
Diagram a parallel circuit with two of your resistors that have different resistances. Wire up this
circuit. Double check it before you turn on the power.!
!
Measure the total current and the current through each resistor. This will involve setting the DMM
to current and putting it in series with what you are measuring. Measure the voltage across the
power supply and each of the resistors. Record all of these values.!
Note that the voltages across both resistors are the same. Calculate the total resistance of both
resistors. For parallel resistor circuits, the equivalent resistance is!
−1
⎛ 1
1 ⎞⎟
⎟⎟ !
Req = ⎜⎜⎜ +
⎝ R1 R2 ⎟⎠
Does you data show this? How is the total current related to the two current that actually goes
through the resistors?!
Experiment 3: Mixed Circuit!
Build a circuit that features two of your resistors in parallel, and the third resistor is series with this
pair. Diagram the circuit. Measure the total current through and the total voltage across the entre
circuit. Calculate the equivalent resistance of the resistors using ohm’s law?!
!
Calculate the equivalent resistance of the resistors using the two rules above as well. Doe sohm’s
law give you the correct result?
Rev 7/14/14!
page 2
Experiment 4: Light Bulb!
Make a series circuit with a 200 to 500 Ω resistor and a light bulb. Incrementally increase the
supply voltage to the circuit and measure the voltage across the light bulb as well as the current
through the light bulb. Measure both the voltage across and the current through the light bulb with
the supply voltage from 0 up to about 15 V in step of 1 V.!
Plot the bulb voltage as a function of the bulb current. The slope of this plot is the resistance of
the lightbulb. What does the data tell you about the resistance of the light bulb? Does the
lightbulb follow ohm’s law?!
Conclusion!
In your conclusion, you should compare your measured results (and calculation using ohm’s law)
to the expected values derived from the rules for adding resistors in series and parallel and the
color bands of the resistors.!
Rev 7/14/14!
page 3