Lab 7: Complex Elec. Circuits, Part II
Introduction
This week’s laboratory exercise is a continuation of last week’s. You might want to look
over last week’s for review purposes.
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-parallel circuits
Introduction and
prediction
Consider the circuit shown in Figure 4. Assuming it is built with three identical bulbs,
predict the order of brightness of the bulbs. Explain your reasoning.
+
A
–
B
C
Figure 4 A series-parallel combination
Procedure
Problem and
prediction
Procedure
•
Construct the circuit in Figure 4, using three identical bulbs. Since this is the first
time you have constructed a circuit from a schematic diagram, you might want to
have the instructor check it.
•
Is it really working? Try unscrewing either bulb B or C. Explain how this
demonstrates the current actually flows through these bulbs.
•
Why are these bulbs acting the way they are? How do the currents in these bulbs
compare with that in A?
The last time you worked with bulbs in parallel, unscrewing one bulb produced no
change in the brightness of the second. Why is this no longer the case? To understand
this you must consider the voltage across bulb A and the voltage across bulbs B and C.
First predict the relative voltages. That is, is the voltage across A less than, equal to, or
greater than, the voltage across B and C? Explain your prediction.
•
Measure the voltages across the bulbs.
•
Predict what will happen to the voltage across bulb B if you unscrew bulb C.
•
Try it. Did you predict correctly? Explain why the observed action occurred.
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Complex Electrical Circuits
2.
a.
Applications
Short circuits
Introduction and
prediction
Procedure
A “short circuit” is a frequent problem in electrical circuits. A short circuit is simply a
wire, or other good conductor, connected across a circuit element, like a bulb. Thus,
you can think of a short circuit as a resistor with R = 0. Predict what will happen to the
brightness of the bulbs if you connect a wire or clip lead across bulb C.
•
Try it! To make sure it really is a short circuit, measure the potential drop across
the wire or clip lead. Comment on any discrepancies from the predictions you
made.
•
Predict what will happen to the brightnesses if a short circuit were connected
across bulb A instead of C. Try it. Comment.
b. Open circuits
Introduction
Procedure
An “open circuit” is a break in a circuit through which no current can pass. In the
series-parallel circuit you created an open circuit when you unscrewed the bulb.
Another common open circuit is a switch in the “off” position. You might think of an
open circuit as a resistor with R = .
•
Create an open circuit by unscrewing bulb A from its socket.
Predict the voltage across the socket that held bulb A.
•
c.
Measure the voltage and compare with your prediction.
Multi-bulb circuits
Introduction and
prediction
Consider the series-parallel circuit in Figure 5. Predict the relative brightness of the
identical bulbs.
+
A
B
–
C
Figure 5. Three-bulb circuit
How do you make such a prediction? Bulbs B and C are in series, and together the unit,
(B+C) is in parallel with A. Because they are in parallel the potential drop across bulb
A is the same as that across (B+C). Now, current is V divided by resistance. Since
(B+C) is a series combination, its resistance is larger than A. So the current through
the unit (B+C) is less than that through A.
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Complex Electrical Circuits
Because bulbs B and C are in series, each has the same current, and thus will be equally
bright. So, bulb A must have the largest current and be the brightest. B and C have
equal but smaller currents, so they are equal in brightness and dimmer than A. Thus
A>B=C
Make predictions, using the same reasoning, for the four-bulb circuits shown below.
You don’t have to write down your explanations, but do discuss them with your lab
partner before actually building the circuit. Be sure to spend time understanding what
went wrong with your incorrect predictions. Talk to your lab partner or to your
instructor.
+
C
A
+
A
D
–
–
D
B
C
B
b)
a)
A
A
+
+
B
C
–
B
C
–
D
D
c)
d)
Figure 6. Four-bulb circuits
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Complex Electrical Circuits
Names
Section
Complex Electrical Circuits, Part II
1.
Series-parallel circuits
Prediction of brightnesses of A, B, and C:
Observations and comparisons:
What happened when you unscrewed either bulb B or C?
How does this show that current is actually flowing?
Why are these bulbs acting the way they are? Compare currents in them to current in A.
Prediction of potential difference across A in comparison to that across B and C.
Explain:
Measurements:
Voltages with bulbs screwed in:
Voltages with bulb unscrewed:
Comment on any discrepancies from the predictions you made.
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Complex Electrical Circuits
2.
Applications
a.
Short circuits
Prediction of brightness of the three bulbs when short circuit is created.
Observations and comments:
Prediction of effect of short circuit across A.
Observations and comments:
b. Open circuits
Prediction of voltage across socket that held A (include reason)
Measurement of voltage across socket:
Comment on any disagreement:
c.
Multi-bulb circuits
Prediction of brightnesses in Figure 5.
Reasoning used to make prediction:
Predictions for brightnesses of circuits in Figure 6
a)
b)
c)
d)
Observed brightnesses
a)
b)
c)
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Complex Electrical Circuits
d)
6