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Unformatted text preview: properties you can m easure in the lab, to the
d istance the w eight m oves.
6. U se the principal of energy conservation to w rite an equation for the am ount of
energy d issipated in this situation, in term s of m easurable qu antities and the
efficiency. Be sure this equation is consistent w ith your d escription from step 2.
7. Sketch a graph of the d istance the w eight m oves as a function of the capacitor’s
capacitance. Assum e constant efficiency, and that the capacitor is charged to the
sam e potential d ifference for each trial. (You can check the “constant efficiency”
assum ption in the lab.) PRED ICTION
C alculate the efficiency of the electric m otor by d eterm ining the energy transferred from
the capacitor and the final energy of the lifted m ass. EXPLORATION
WARN IN G: A charged capacitor can d ischarge quickly prod ucing a painful
spark. D o not h and le the capacitors by their electrical term inals or connected
w ires by their m etal end s. A lw ays dis charge a capacitor w hen you are finished
u sing it. To d ischarge a capacitor, use an insulated w ire to briefly connect one of
t he term inals to the other.
If you have not done so, read about using the DMM in the append ix. Pay special
attention to the connections and settings that are used to m easure voltages and currents,
and w hy the DMM should be connected in the circuit d ifferently for voltage and current
m e asurem ents. Do you know w hy w e should connect them in these w ays?
N o te: Make sure you connect the + terminal of the battery 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. 60 ELECTRICAL AND MECHANICAL ENERGY – 1302Lab3Prob1
C harge the capacitor by connecting it to a battery. H ow can you use the DMM to tell if
the capacitor is fully charged ? What d o you m ean by fully charged ? Try charging it for
d ifferent am ounts of tim e. H ow long d oes it take the capacitor to fully charge?
Connect the m ass to the m otor w ith the string. Without touching the capacitor lead s to
anything else connect one lead to one term inal of the m otor and the other lead to the
other term inal of the m otor. Which d irection d oes the m otor spin? Does the d irection
that the m otor spins d ep end on how you connected the term inals to the m otor?
H ow far is the m ass lifted ? When the m ass stops lifting is the capacitor out of energy?
W hat im plications d oes this have for your m easurem ents? H ow long w ill the capacitor
support the m ass before it begins to fall? H ow can you tell w hen energy is still being
transferred to the m ass? What hap pens w hen energy is no longer being transferred to
the m ass?
Write d ow n your m easurem ent plan. M EASUREMEN T
W hat is the initial position of the m ass? W hat is the energy stored in the capacitor at
this tim e? Measure the d istance the m ass is lifted w hen the capacitor is connected to the
m otor. What w as the energy of the capacitor at the final position?
Was it necessary for the capacitor to be com p letely d ischarged at the final position? If it
w as not, w hat im plications d oes this have for your experim ent? What is m ore
im portant, the total energy the capacitor is able to store, or the am ount of energy the
Is there a w ay you can visu ally d eterm ine that the capacitor is no longer transferring
energy to the m ass? What are the obvious changes to your system w hen energy is no
longer being supplied to the m ass from the capacitor?
What are the uncertainties associated w ith your m easurem en t? Try to think of any
possible sources of uncertainty and quantify them . A N ALYSIS
G raph the d istance the w eight is pu lled versus the voltage to w hich the capacitor is
charged . Show the estim ated m easurem ent u ncertainty on your graph. 61 ELECTRICAL AND MECHANICAL ENERGY – 1302Lab3Prob1 CON CLUSION
D id your results m atch your pred ictions? Explain any d ifferences.
H ow efficient is this energy transfer? Define w hat you m ean by efficient. H ow good
w as the assum ption of constant efficiency for this situation?
You have heard that energy is alw ays conser ved . Is it appropriate to say that energy
w as conserved in this situation? Why or w hy not? 62 PROBLEM #2: SIMPLE CIRCUITS WITH CAPACITORS
You and your friend are trying to d eterm ine if you can use a cap acitor to extend the
lives of batteries in circuits. You suggest that you try a sim ple circuit w ith a capacitor,
originally uncharged , connected to a battery through a sw itch. To m onitor the output
energy, you put a light bulb in series w ith the capacitor. Your friend believes that w hen
the sw itch is closed the capacitor charges up and the bulb gets brighter and brighter
until the brightness levels off. The bulb then stays on until the sw it ch is opened . Do
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 pr...
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- Spring '14