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theatrical prod uction, the play’s d irector w ants a light bulb to d im very slow ly for
d ram atic effect. You d esign a sim ple, inexpensive circuit to autom atically accom plish
this task: a battery, a sw itch, a light bu lb, and a capacitor in series. You have been asked
to d em onstrate d ifferent rates of d im m ing for the light bulb so the d irector can select
the one that best fits the perform ance. You need to d eterm ine how to ad just the am ount
of tim e it takes for the light bulb to go out by varying the capacitance. To m ake a
proper com parison you m ake sure that the capacitor is initially uncharged .
Instructions: Before lab, read the laboratory in its entirety as well as the required reading in the
t extbook. In your lab notebook, respond to the warm up questions and derive a specific prediction
for the outcome of the lab. During lab, compare your warm up responses and prediction in your
group. Then, work through the exploration, measurement, analysis, and conclusion sections in
sequence, keeping a record of your findings in your lab notebook. It is often useful to use Excel to
perform data analysis, rather than doing it by hand. EQUIPMEN T
Build the circuit s how n; y ou
h ave a choice of capacitors.
You w ill also have a
stopw atch and a d igital
m ultim eter (DMM). Legend: A + - light bulb
capacitor U se the accom panying legend
t o help you build the circuit.
C ircuit I switch
wire Read the section T he Digital M ultimeter (DM M ) in the Equipment a ppend ix.
If equipment i s missing or broken, submit a problem report by sending an email to
l email@example.com . Include the room number and brief description of the
p roblem. PRED ICTION
Restate the problem . What is the quantity you w ill be m easuring and w hat is the
quantity you w ill be controlling? H ow d o you think they w ill d epend on one another?
Give your reasoning. Explain w hat is going on in each com ponent of the circuit.
Sketch a graph of the tim e it takes for the light bulb to turn com pletely off as a fu nction
of the capacitor’s capacitance. 67 CAPACITANCE – 1302Lab3Prob3 EXPLORATION
WARN IN G: A charged capacitor can d ischarge quickly prod ucing a painful spark. D o
n ot h and le the capacitors by their electrical term inals or connected w ires by th eir m etal
e nd s. A lw ays discharge a capacitor before you use it and w hen you are finished using
i t. To d ischarge a capacitor, use an insulated w ire to briefly connect one of the term inals
t o the other.
Exam ine each elem ent of the circuit b efore y ou build it. H ow d o you know if the
battery is "good "? Be sure the capacitors are uncharged .
After you are convinced that all of the circuit elem ents are w orking and that the
capacitor is uncharged , connect the circuit bu t d o not close it yet.
N ote: Make sure the + terminal of the battery is connected to the + terminal of the
capacitor! These capacitors are only d esigned to charge one w ay. If you connect the
capacitors up the w rong w ay, the capacitance w ill change in an unpred ictable m anner.
N ow , close the circuit and observe how the brightness of the bulb changes over tim e.
H ow long d oes it take for the bulb to turn off?
From w hat you know about a battery, how d oes the potential d ifference (voltage) across
the battery change over tim e? Check this using the DMM set for potential d ifference
(Volts). From your observations of the brightness of the bulb, how d oes the potential
d ifference across the bulb change over tim e? Check this using the DMM. What can you
infer about the change of voltage across the capacitor over tim e? Can you check w ith a
DMM? Use the concept of potential d ifference to explain w hat you observe.
Develop a m easurem ent plan that w ill allow you to d eterm ine the tim e it takes a bulb to
turn off as a function of capacitance. You w ill w ant to d ecid e how m any d ifferent
capacitors you need to use, how m any tim e m easurem ents to take for each capacitor,
w hat you m ean by the light bulb being “off,” and how to ensure that the capacitor is
uncharged before you m ake each m easurem ent. M EASUREMEN T
Follow your m easurem ent plan. Be sure to record your estim ated uncertainty for each
m easurem ent. 68 CAPACITANCE – 1302Lab3Prob3 A N ALYSIS
G raph the tim e it takes for the light bulb to turn off, as a function of capacitance, w ith
the capacitor initially uncharged . CON CLUSION
H ow d id your m easu rem ent com pare w ith your pred iction? Using conservation of
charge, conservation of energy, and the m od el of a capacitor as tw o plates separated by
a short d istance, explain how the capacitance affects the tim e it takes for the bulb to turn
off. 69 CAPACITANCE – 1302Lab3Prob3 70 PROBLEM #4: CIRCUITS WITH TWO CAPACITORS
You recently purchased a used cam era w ith an electric flash. After taking a roll of
pictures you are d isappointed that the flash isn’t bright enough. You look in the cam era
and notice that the flash w orks by allow ing a battery to...
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This document was uploaded on 02/23/2014 for the course MANAGMENT 2201 at University of Michigan.
- Spring '14