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Phy101
Resistors in Series and Parallel
(revised by S Dey)
Name: _______________________________
Date: _____________
Lab Partners: ________________________________________
Introduction: As you have learned so far, any device placed in a circuit will have some resistance.
Devices such as hair dryers and toasters use resistance to produce heat. Others, like your computer, have a
resistance that produces heat as an unwanted by product. If you have ever felt the bottom of your laptop
when it is on or were annoyed by the buzz of a desktop cooling fan you have experienced the negative side
of resistance. There are two basic ways to connect resistors (keep in mind when we say resistors we mean
any device with resistance): in series or parallel.
In the diagram below is a circuit set up with three resistors in series. Notice there is only one path for the
current to flow. What does this tell you about the current through each resistor? What does this mean for the
voltage drop across each resistor? What is the overall resistance of the circuit? These are some of the
questions we will explore for each of the circuits in this lab.
R1
R2
R3
V
Figure 1
In the next diagram three resistors are connected in parallel. Notice that when the current from the battery
reaches point A in the circuit the current splits off into three separate paths. At point B these three currents
come back together to again equal the current coming from the battery. Points A and B are called junctions.
Any point in the circuit where the current splits up or comes back together is called a junction. Again we will
discover answers to questions similar to those above for parallel circuits.
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R1
Junction
R2
R3
Junction
V
Figure 2
These simple circuits will be set up using a circuit breadboard shown in figure 1. The breadboard is an array
of metal spring contacts that are embedded in a plastic block. It is used to construct temporary circuits for the
purpose of testing them. Because the contacts connect component leads with mechanical pressure and not by
soldering, circuits that are built on breadboards are easy to construct, modify, and dismantle.
Figure 1, The Breadboard
To use a breadboard you will need to know which contacts are internally joined. Each horizontal row of five
contacts along the top and bottom are joined together separated only at the center of the board. We will call
this row of contacts the Power Supply Contacts. Thus, along the top there are 25 contacts connected together
directly below each of the red lines and another 25 contacts connected together directly above each of the
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blue lines. Similarly, along the bottom there are two more rows of 25 contacts below the red lines, and two
more rows of 25 contacts above the blue lines. These 8 rows are often used as a way to distribute power to
the circuit. For example, if your power supply is set to 6.0 V or you have a 6.0 V battery, you could connect
the positive terminal to one of the top two rows and the negative terminal to one of the bottom two rows.
You can now supply power to the remaining contacts in the center labeled A to J.
The remaining contacts on the breadboard are joined in vertical columns of five contacts each. From the
figure you might be able to see that there are 64 columns above the central groove and 64 columns below. By
placing one lead of a resistor into one of the columns and a lead of another resistor into that same column
you have connected the two resistors in series. There are still three more contacts in that column allowing
you to connect more components to those two resistors.
Part I: Resistors in Series.
Measure the resistance of your resistors using your ohmmeter.
R1 =
R2 =
R3 =
Set up the circuit shown below. (R1 from the Power Supply Contacts (PSC) to A 64, wire from E 64 to F 64,
R2 from J 64 to PSC, do not connect the power supply to the breadboard yet)
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Figure 1
Measure the resistance across the R1 + R 2 combination.
R1 + R2 =
Replace the connecting wire with R3. Then measure the resistance across the following combinations.
R1 + R3 =
R3 + R2 =
R1 + R3 + R2 =
What can you conclude about resistances in series?
Write an equation for finding the Total Resistance in Series. (This is called the Equivalent Resistance, Req.)
Plug in your power supply and hook your voltmeter up to its terminals. Set the voltage as close as possible to
6.0 V as possible and record the voltage below. Always include a switch in your circuit. Only close the
switch long enough to take your measurements. If you leave the switch closed too long it will heat up
your resistors causing a significant change in resistance.
Vpower supply =
Now hook-up the power supply to the breadboard as shown below.
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Figure 2
Measure the voltage across each resistor and record the results below. Remember voltages are measured in
parallel. You do not need to disconnect the circuit, just connect one test lead to each side of your resistors.
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Figure 3
V1 =
V2 =
V3 =
VTotal (V1 + V2 + V3) =
Compare VTotal to the Voltage across the power supply. What can you conclude about the voltage across the
each resistors in series?
Write an equation relating the voltage of the power supply to the voltage across each resistor.
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Disconnect R1 from the PSC and place a connecting wire into the empty hole in the breadboard. You have
now created a break in the circuit so that you can measure the current flowing into R1.
Figure 4
Repeat for R2 and R3.
I1 =
I2 =
I3 =
Now break the circuit between the power supply and the breadboard and connect an ammeter to measure the
current flowing from the power supply.
IPower Supply =
Compare the current through each resistor to IPower Supply. What can you conclude about the current through
each resistor in a series circuit?
Write an equation relating the current flowing from the power supply to the current flowing through each
resistor.
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Part II: Resistors in Parallel.
Set up the circuit shown below. (R1 from PSC to A 64, R2 from PSC to B 64)
Figure 5
Measure the following resistances.
R1 =
R2 =
Connect a wire into the PSC section with R1 and R2 and a second wire into D 64 to measure the following:
R1 and R2 in Parallel =
= RTotal 1
How does the total resistance in parallel compare to the total resistance in series?
How does the total resistance in parallel compare to each of the individual resistors before they are placed in
the parallel circuit?
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Connect R3 from PSC to C 64 . Measure the total resistance.
RTotal 2 =
What happened to the total resistance when you added R3 ?
On your calculator find
1/R1 =
1/R2 =
1/R3 =
1/RTotal1 =
1/RTotal2 =
1/R1 + 1/R2 =
1/R1 + 1/R2 + 1/R3 =
What do you notice?
Write an equation for finding the Total Resistance in Parallel. (This is called the Equivalent Resistance, Req.)
Measure the voltage across each resistor and record the results below. Remember voltages are measured in
parallel. You do not need to disconnect the circuit, just connect one test lead to each side of your resistors.
V1 =
V2 =
V3 =
Compare the Voltage across each resistor to the Voltage across the power supply. What can you conclude
about the voltage across each resistors in parallel?
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Write an equation relating the voltage of the power supply to the voltage across each resistor.
Disconnect R1 from the PSC and place a connecting wire into the empty hole in the breadboard. You have
now created a break in the circuit so that you can measure the current flowing into R1.
Figure 6
Repeat for R2 and R3.
I1 =
I2 =
I3 =
ITotal (I1 + I2 + I3) =
Now break the circuit between the power supply and the breadboard and connect an ammeter to measure the
current flowing from the power supply.
IPower Supply =
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Compare ITotal to IPower Supply. What can you conclude about the current through each resistor in a parallel
circuit?
Write an equation relating the current flowing from the power supply to the current flowing through each
resistor.
Part III: Resistors in Series and Parallel.
Set up the circuit shown below. (R1 from PSC to A 64, R2 from B 64 to B 59, R3 from D 64 to D 59, wire
into E59)
Figure 7
Measure the following:
VPower Supply =
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Req =
IPower Supply =
V1 =
V2 =
V3 =
I1 =
I2 =
I3 =
Now using the equations you found relating resistances, currents, and voltages in series and parallel circuits,
calculate the quantities you measured above.
Req =
V1 =
V2 =
V3 =
I1 =
I2 =
I3 =
Show Work Below
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Question
1. Consider the following if you set up the circuits in the lab using light bulbs as your resistors. Would the
bulbs continue to be lit for the following conditions? If so, would they be brighter (draw more current) or
dimmer (draw less current)?
a) Light bulb R2 burns out in the series circuit?
b) Light bulb R1 burns out in the parallel circuit?
c) Then light bulb R3 burns out in the parallel circuit?
d) Light bulb R3 burns out in the series and parallel circuit?
e) Then Light bulb R1 burns out in the series and parallel circuit?
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2. For the series and parallel circuit, V = 9.0V, R1 = 5Ω, R2 = 10Ω, and R3 = 15Ω. Show that the power
supplied by the battery is equal to that dissipated in the resistors. What principle does this illustrate? Use the
table below.
Circuit
Element
Current
I
Power Dissipated
R1 = 5 Ω
R2 = 10 Ω
R3 = 15 Ω
X
X
X
X
Power
Supplied
Battery
V = 9.0V
Show Work Below
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