COMPOUND CIRCUITS
LOGGING ON
 Go to www.ExploreLearning.com and log in using your Username and Password.
 Select “Browse Gizmos” icon at the upper left.
Select “Grades 9 – 12 Physics”
Select “Electricity and Magnetism”
Select “Advanced Circuits” and Launch the Gizmo.
GETTING STARTED
 To build a simple circuit with a single bulb and battery, use the cursor to drag a battery, two
wires, and a bulb. Notice the light bulb glows. The “slider” at the bottom of the circuit board
adjusts the voltage.
 Remove one of the wires and replace it with a switch. Turn the switch on and off.
MEASURING VOLTAGE AND CURRENT
1. Build a simple circuit with one bulb, one battery, a switch and a wire. Set the voltage to 10 volts.
2. Using the cursor, drag the positive terminal of the volt meter to the
positive side of the bulb and the negative terminal of the meter to the
negative side of the bulb. The voltage in the circuit is displayed at the
bottom left of the Gizmo. The voltmeter must be placed “across” the
bulb. (see diagram to the right)
3. Using the cursor, drag the ammeter and place it near one of the
connections in the circuit. The current (in amperes) is displayed below
the voltage. The ammeter must be placed “in series” with the bulb. (see diagram to the right)
PART I: SERIES CIRCUITS
Construct a circuit with two bulbs in series. Set the voltage to 10.0 volts. Measure the overall voltage
in the circuit by placing the appropriate terminal of the voltmeter at the terminals of the battery.
Measure the voltage across each bulb. Next, place the ammeter in the circuit to determine the current
at the designated points within the circuit. Record your data in the tables.
Voltage (volts)
A
Across
Bulb A
Across
Bulb B
Across the
Battery
Between
Battery & A
Current (amperes)
Between
A&B
Between
Battery & B
B
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Construct a circuit with three bulbs in series and measure the voltage and current as you did above.
Voltage (volts)
Across
Across
Bulb B
Bulb C
Across
Bulb A
A
B
Across the
Battery
C
Current (amperes)
Between
Battery & A
Between
A&B
Between
B&C
Between
Battery & C
Compare the voltage across the battery (total voltage) and the voltage at each bulb for the two
circuits. Write a generalized statement that describes the relationship between the total voltage
and the voltage at each bulb for a series circuit.
Compare the current at the battery (total current) and the current at each bulb. Write a generalized
statement that describes the relationship between the total current and the current at each bulb.
CALCULATING RESISTANCE
Using Ohm’s Law ( V voltage = I current x R resistance ), calculate the resistance in Ohms at the various
points within the circuit. The unit for resistance, Ohm, is symbolized by the Greek letter omega (Ω).
Express your answers to 3 significant figures.
Resistance (Ohms)
Resistance (Ohms)
Bulb A
Bulb B
Battery
Bulb A
Bulb B
Bulb C
Battery
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MEASURING RESISTANCE
Reconstruct the series circuit with two bulbs without a battery. The Ohmmeter will only function when
there is no current running through the circuit. To check the total resistance in the circuit, place the
Ohmmeter where the battery would normally be located. Then, place the Ohmmeter at opposite
terminals for each bulb to measure the resistance of the bulb. Then, reconstruct the series circuit with
three bulbs and check the total resistance and the resistance of each bulb.
Resistance (Ohms)
Bulb A
Bulb B
Resistance (Ohms)
Total
Bulb A
Bulb B
Bulb C
Total
Write a generalized statement that describes the relationship between the total resistance in the
circuit (as measured where the battery would be located) and the resistance at each bulb.
How does the total resistance in a series circuit change when increasing the number of bulbs?
Does the resistance of each bulb change when increasing the number of bulbs in the circuit?
PART II: PARALLEL CIRCUITS
Construct a circuit with two bulbs in parallel and set the voltage to 10.0 volts. Measure the overall
voltage in the circuit by placing the appropriate terminal of the voltmeter at the terminals of the
battery. Measure the voltage across each bulb. Next, place the ammeter in the circuit to determine
the current at the designated points within the circuit. Record your data in the tables.
Voltage (volts)
A
Across
Bulb A
Across
Bulb B
Across the
Battery
Between
Battery & A
Current (amperes)
Between
A&B
Between
Battery & B
B
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Construct a circuit with three bulbs in parallel and measure the voltage and current as you did above.
Across
Bulb A
B
A
Voltage (volts)
Across
Across
Bulb B
Bulb C
Across the
Battery
C
Current (amperes)
Between
Battery & A
Between
A&B
Between
B&C
Between
Battery & C
Compare the voltage across the battery (total voltage) and the voltage at each bulb for the two
circuits. Write a generalized statement that describes the relationship between the total voltage
and the voltage at each bulb.
Compare the current at the battery (total current) and the current at each bulb. Write a generalized
statement that describes the relationship between the total current and the current at each bulb.
CALCULATING RESISTANCE
Using Ohm’s Law ( V = I R ), calculate the resistance for each bulb in the two circuits.
Resistance (Ohms)
Bulb A
Bulb B
Resistance (Ohms)
Bulb A
Bulb B
Bulb C
CALCULATING TOTAL RESISTANCE FOR PARALLEL CIRCUITS
The total resistance of a parallel circuit can also be calculated using the equivalent resistance equation,
where the total resistance in a parallel circuit RTOTAL can be calculated from the following formula:
1
Rt
1
RA
1
RB
1
RC
...
As you have seen, the total resistance in a series circuit
increases as the number of bulbs increases, for a parallel
circuit, the total resistance decreases as more bulbs are
added.
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Using the equivalent resistance equation, substitute the values for the resistances of each bulb and
calculate the total resistance in each parallel circuit. Show your work below.
(Hint: Reduce the fraction rather than calculate.)
Total Resistance for Two Parallel Bulbs
Total Resistance for Three Parallel Bulbs
How does the total resistance in a parallel circuit change when increasing the number of bulbs?
PART III: BUILDING COMPOUND CIRCUITS
In this part, you will build several examples of compound circuits that combine parallel and series
components.
Build the circuit shown in the diagram to the right. This circuit
should look familiar since it is the compound circuit that was
constructed in the previous laboratory. Set the voltage of the
battery to 10.0 V.
A
C
Notice that bulbs A & B are in series, and that bulbs A & B are
parallel with bulb C. As you have concluded from the previous
experiments:
B
FOR SERIES CIRCUITS
The total voltage in the circuit is the sum of the voltages across each bulb.
The current is the same anywhere in the circuit.
The total resistance in a circuit is the sum of the resistances at each bulb.
FOR PARALLEL CIRCUITS
The voltage is the same anywhere in the circuit.
The total current is the sum of the currents in each circuit.
The total resistance of a circuit follows the relationship:
1
Rt
1
RA
1
RB
1
RC
...
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Use the voltmeter and ammeter to measure the
voltage and current at each bulb, A, B & C. Record
your values in the table.
Use Ohm’s Law to calculate the resistance for each
bulb.
Bulb
Resistance
(Ohms)
Current
(amperes)
Voltage
(volts)
A
B
C
Total
DETERMINING TOTAL RESISTANCE, CURRENT & VOLTAGE
Using the information from the table above, you will next determine the total current, voltage, and
resistance in the compound circuit.
To determine the total current:
1. Use the series rule for current for the right-hand circuit (bulbs A & B).
Current is the same in both bulbs.
2. Use the parallel rule for current for the entire circuit (right and left hand):
Total current is the sum of the currents in each circuit.
Total Current = ______________ + ________________
(bulbs A & B)
(bulb C)
To determine the total voltage:
1. Use the series rule for voltage for the right-hand circuit.
Voltage in circuit is the sum of voltages in the bulbs.
2. Use the parallel rule for voltage for the entire circuit (right and left hand).
Total voltage is the same as the voltage in each circuit.
Total Voltage = ________________
To determine the total resistance:
1. Use the series rule for the resistance for the right-hand circuit:
The resistance in this circuit is the sum of the resistances for bulbs A & B
Right-hand circuit resistance = _________________
2. Use the Equivalent Resistance Rule for the entire circuit (right and left hand).
1 / R total = 1 / right hand + 1 / left hand
R TOTAL = _____________________
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CALCULATING POWER
The power of a light bulb, measured in Watts, is determined by the following equation:
Power (Watts) = Current (Amperes) x Voltage (Volts) or P = I V
Calculate the power (in Watts) for each of the bulbs in the circuit diagram
Bulb A
Bulb B
Bulb C
Construct a statement that relates the power of the bulb to its apparent brightness.
BUILDING COMPOUND CIRCUITS (CONTINUED)
Examine the circuit shown in the diagram to the right.
How many different pathways are there for the current
A
C
B
D
in the compound circuit? __________
Which bulbs does each circuit contain?
Left Side: ___________ Right Side: _____________
Are the bulbs in each circuit in series or parallel with each other? _______________
Are the two pathways (Left and Right Side) in series or in parallel with each other? _____________
Notice that some of the values have been included in the table below. The first step in determining the
missing values is to assess which bulbs in the circuit are identical to each other. Then, information
which is known concerning one bulb can added to the information known about similar bulbs.
Using this information fill in the missing data without constructing the circuit. Then, calculate the total
current, voltage, and resistance in the compound
Voltage
Current
Resistance
circuit.
Bulb
(Volts)
Now, construct and test the circuit using the voltmeter
and ammeter to determine the voltage and current at
each bulb to confirm your solution.
B
Bulb B
Bulb C
D
5.0
0.33
15
Total
Power (Watts)
Bulb A
(Ohms)
A
C
Finally, calculate the power (in Watts) at each light
bulb.
(Amperes)
Bulb D
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POST LABORATORY PROBLEMS
Without using the computer simulation, determine the missing values in the data tables for each of the
following circuits. Afterwards, build the circuit and test your answers using the ammeter, voltmeter,
and Ohmmeter. Remember to first assess which bulbs in the circuit are identical to each other, then
the missing information can be determined from similar bulbs.
Circuit 1
Bulb
Circuit 1
Voltage
Current
A
Resistance
A
0.27 A
2.0 V
B
C
15 Ω
D
Total
B
10.0 V
C
Circuit 2
D
A
Bulb
Voltage
A
B
B
C
D
Circuit 2
Current
Resistance
Power
0.29 A
1.43 V
C
15 Ω
D
E
4.28 V
Total
E
-----
Circuit 3
Bulb
A
B
Voltage
A
B
D
Resistance
Power
0.13 A
2.0 V
C
C
Circuit 3
Current
15 Ω
6.0 V
Total
-----
D
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