Download Complex Circuits I 0.8

Lab 6: Complex Electrical Circuits, Part I
Introduction
In this laboratory you will connect electric lamps together in a variety of circuits. The
purpose of these exercises is to extend your understanding of the “physics” of simple
electrical circuits. That is, the purpose is for you to learn qualitative properties of
circuits. There is an additional purpose, to help you learn the skills of being able to
predict what you expect to happen, observe what happens, and correct the original
misunderstanding, if necessary. What you learn in assembling, observing, and measuring
the properties of these circuits will be applicable to the real-world electrical circuits you
will meet on the job and in everyday life.
Note that, as in the last lab, you will be asked to make predictions, then measurements,
and then to discuss how the predictions and measurements agreed. You will not be
graded on the predictions, but rather on your measurements, observations, and
discussion.
1.
Series connections
a. The effect of an ammeter
Introduction
Procedure
In an ideal world, a meter should have no effect on the circuit it’s measuring. Before
making measurements, then we should find out if our DMM used as an ammeter changes
the circuit in which it is used. To make this test, you will need two meters, so find
another pair of students and do this part together.
•
Make a single-bulb circuit with a socket using the #502 bulb.
•
Set the power supply at 5 volts. Measure the current through the bulb first with the
meter on the 300 mA range, and then with it on the 10 A range (remember that you
have to plug the red lead into a different terminal). Are the currents the same? Is
the bulb the same brightness? What changed?
•
Use the second DMM as a voltmeter to measure the potential drop across the
ammeter. An ideal ammeter would have no voltage across it. Is the DMM ideal?
Does the voltage drop depend on whether it is used on the 300 mA or the 10 A
range?
•
Explain why the current through the bulb depends on the ammeter range.
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Complex Electrical Circuits
b. The second type of bulb
Introduction
Procedure
This week you will use two different types of bulbs. One has the number 425 on the
base, the other 502. The first task is to find out how these two bulbs differ.
•
Return to the single-bulb circuit with the power supply set at 5 volts. Screw first
one bulb into the socket and then the second type. Compare their brightnesses.
•
Predict which bulb will draw more current. Predict which will have the higher
resistance.
•
Now let’s make a measurement and see if you were correct. Use the DMM to
measure the voltage of the power supply and adjust it to be exactly 5 volts.
•
Measure the current in the circuit and record your result on the worksheet. Hint: in
view of your results in part a, can you measure the current through one bulb on the
300 mA range and the other on the 10 A range?
•
Measure the current through the second bulb.
•
Calculate the resistances of the two bulbs when each has 5 volts across it.
c. Two different bulbs in series
Introduction and
prediction
You now know which bulb is brighter, which draws more current, and which has a higher
resistance. Predict which bulb will be brighter when the two lamps are connected in
series. Write your prediction on the worksheet now!
#2
A
B
C
+
Power Supply
F
–
#1
E
D
Figure 1. Series circuit
Procedure
•
Now try the experiment. Wire the circuit shown in Figure 1. Again, set the power
supply at 5 volts. Remember to put a 425 bulb in one socket and a 502 into the
other.
•
Discuss any discrepancies with your predictions.
•
There is a mystery here. Why doesn’t the one bulb glow? Perhaps no current is
going through it. See what happens if you unscrew the bulb. Explain as fully as you
can why it doesn’t glow when it is in series with the other bulb.
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Complex Electrical Circuits
d. The equivalent resistance of a series combination
Introduction
Procedure
2.
Each bulb acts as an obstacle to the flow of charge and develops a potential drop across
it. You can also consider the two bulbs in series as a single obstacle. There is a current
through “it” and a potential difference across “it,” so “it” has a resistance. How does the
resistance of this series combination compare with the resistances of each bulb?
•
With the two bulbs in series, determine the resistance of each bulb. Remember that
because the resistance of a bulb depends on the current through it, you can’t use the
ohmmeter or the measurements you made above to find the resistances. Make one
measurement of the current through the bulbs (Question: Which ammeter range
should you use so that it has the least effect on your results?) Measure the potential
differences across each bulb. The worksheet will guide you.
•
Do you need to make further measurements to find the resistance of the two bulbs
in series? If not, which previously tested “law” of circuit analysis do you need to
use? If you want to make another measurement, go ahead. Then calculate the
resistance.
•
Compare the resistance of the series combination to that of the individual bulbs.
Parallel Connections
a. Two bulbs in parallel
Introduction and
prediction
Imagine the two circuits shown in Figure 2. Bulb C in the circuit on the right is “in
parallel” with the bulb B. Now, make a prediction. What will be the order of brightness
of the three bulbs, A, B, and C? In other words, which will be brightest, second
brightest, and least brightest. You may also say that you think two or more bulbs will
have the same brightness. Write down your prediction on the worksheet.
B
A
+
Power Supply
–
+
Power Supply
–
Figure 2. Single bulb and parallel bulb circuits
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C
Complex Electrical Circuits
Procedure
•
Build the circuit, using #502 bulbs, and see. A good way to compare brightnesses is
to watch the brigtness of B as you quickly connect and disconnect C. Any
surprises? What can you conclude about the current in each of the parallel bulbs in
comparison with the current through the single bulb?
•
Let’s check this conclusion by making some current measurements. To help you
make these measurements, change the wiring slightly to that shown in Figure 3
below. Use one clip lead or wire between points B and C and another between B
and G. Then use a third one to connect to the power supply at A. Do the same for
DE, EH, and EF. Make sure both bulbs light.
#2
C
A
+
D
B
Power Supply
F
–
E
#1
G
H
Figure 3. Parallel Bulbs
•
Before making a measurement, make a quantitative prediction. Suppose the current
through a single bulb is i1 On the circuit diagram on the worksheet, write your
predictions in the boxes for the current in every wire in terms of the current i1.
Discuss the predictions carefully with your partner.
•
Measure the actual currents in the various branches of the circuit, and write down the
results in the boxes on the answer sheet in terms of the current through the #502 bulb
that you measured in part 1 of this laboratory. Note that you will have to think
carefully about making this measurement. To measure the current in wire AB you
have to replace the wire with the ammeter. The same is true for the wire BC. Be
sure to use the ammeter range that will have the least effect on your results.
•
Discuss any discrepancies between your predictions and your measurements.
•
There is another important fact about parallel combinations. Measure the potential
difference across bulb #1 and then across bulb #2. How do they compare? How do
they compare with the EMF produced by the power supply?
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Complex Electrical Circuits
b. Current in the branches
Introduction and
prediction
Procedure
Prediction
In the case of two identical bulbs the currents through the two bulbs were equal. The
sum of these currents was equal to the current from the power supply. How does the
brightness of one bulb depend on the second bulb? What does that mean about the
current? Predict what would happen to the current through #1 if you were to remove
bulb #2. Predict what would happen if you replaced #2 with a 425 bulb. Write these
predictions on the worksheet.
•
Try it. Unscrew the bulb #2. What do you see? Does what you see agree with
your prediction?
•
Measure the current through bulb #1 now.
•
While measuring the current through #1 screw a 425 bulb into the #2 socket. What
happens to the current through bulb #1?
•
Measure the current through bulb #2 and the current from the power supply (wire
AB). Are the currents #1 and #2 equal? Is their sum equal to the current from the
supply? In other words, is Kirchhoff’s node law satisfied?
What would happen if a third bulb were put in parallel with the other two? Pat, Leslie,
and Robin disagreed again. Pat said that the first two bulbs would get dimmer, because
the current through them would decrease in order to satisfy Kirchhoff’s node law. Leslie
said that one-third of the current would go through the new bulb, so the currents through
the other bulbs would adjust. Robin said that the brightness of the first two bulbs
wouldn’t change, because the power supply would supply whatever current was needed.
With whom do you agree? Use the knowledge you gained in the experiment above to
write brief explanations to the three students.
c. The equivalent resistance of a parallel combination
Introduction
Procedure
Each bulb acts as an obstacle to the flow of charge and develops a potential drop across
it. You can also consider the two bulbs in parallel as a single obstacle. There is a
current through “it” and a potential drop across “it,“ so “it” has a resistance. How does
the resistance of this parallel combination compare with the resistances of each bulb?
•
Determine the resistance of each bulb. You have measured the currents in the
preceding part. In case you have changed the power supply setting, you should
probably recheck the EMF.
•
Do you need to make further measurements to find the resistance of the two bulbs
in parallel? If not, go ahead and calculate the resistance.
•
Compare the resistance of the parallel combination to that of the individual bulbs.
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Complex Electrical Circuits
Names
Section
Complex Electrical Circuits, Part I
1. Series connections
a. The effect of ammeters
Range
300 mA
10 A
Current (A)
Potential drop across
ammeter (V)
Why does current through the bulb depend on the ammeter range? Use Kirchhoff’s loop law in your explanation.
b. The second type of bulb
Prediction: which bulb will draw more current: 425 or 502? Justify your answer please.
Prediction: which bulb will have more resistance: 425 or 502? Justify your answer please.
V=
(volts)
425 bulb
I=
(amps)
R=

V=
(volts)
502 bulb
I=
(amps)
R=

Comments on the correctness of your predictions.


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Complex Electrical Circuits
c. Two different bulbs in series
Prediction: When the 425 and 502 are connected in series, how bright will each be compared with their brightness
when each is the only bulb in the circuit.
Explain your reasoning.
Observed brightnesses:
Comment on agreement or disagreement:
Why doesn’t the one bulb glow yet the other one does? Explanation.
d. The equivalent resistance of a series combination
Resistance of 425 when in series circuit: R = _____ (V)/_____(A) =
()
Resistance of 502 when in series circuit: R =_____ (V)/_____(A) =
()
Resistance of two in series: R = _____ (V)/_____(A) =
()
Compare resistance of series combination to resistance of any one of the two devices. (i.e. it is smaller, equal to, or
larger than a single bulb.)
2. Parallel Connections
a. Two bulbs in parallel
Predict the order of brightness of A, B, and C.
Observed order of brightness:
Comments:
How do currents in parallel bulbs compare with the current in a single bulb?
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Complex Electrical Circuits
Predicted currents in terms of I1.
#2
C
A
+
B
Power Supply
–
D
E
#1
F
G
H
Measured currents
#2
C
A
+
B
Power Supply
–
D
E
#1
F
G
H
Comments on agreement or disagreement:
Bulb #1 V =
Bulb #2 V =
b. Current in the branches
Prediction: What would happen to current through #1 if Bulb #2 were removed? Justify your reasoning:
Prediction: What would happen to current through #1 if second type of bulb were put into #2 socket? Justify your reasoning:
Observation: What happened when #2 was removed:
Measurement: current through #1. Compare with previous measurement:
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Complex Electrical Circuits
Observation: What happened when second type of bulb was used in #2 socket:
Measurement: current through #1. Compare with previous measurement:
Is current through #1 equal to current through #2?
Measurement: current from power supply:
Does current from supply equal to sum of currents in bulbs? Show the work that supports your conclusion.
Summarize what you have learned about parallel combinations:
Describe how the currents in the branches compare with the total current.
Describe how the current in one branch depends on the current in the other branch.
Describe how the voltages across the two branches compare with each other and with the voltage produced by
the power supply.
Regarding the question of putting a third bulb were put in parallel with the other two. Whose prediction do
you support?
Comments to
Pat
Leslie
Robin
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Complex Electrical Circuits
c. The equivalent resistance of a parallel combination
502 bulb R =
()
425 bulb R =
()
Parallel combination (make sure you show your method)
R=
()
Compare resistance of parallel combination to that of any of the individual bulbs.
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