MIDTERM 2
Quiz Instructions
Before beginning this quiz, you must join the Zoom session for your section:
Section 101 Zoom Session:
Section 102 Zoom Session:
Section 103 Zoom Session:
You need to remain connected to Zoom and leave your webcam turned on throughout the exam.
Once you start the quiz, please do not refresh your browser, use browser buttons, or navigate away from the exam screen until the quiz has
been submitted.
During the exam, you are not permitted to ask questions, except in cases of supposed errors or ambiguities in examination questions. In this case,
use the "Ask for Help" button in Zoom. You will then be moved to a different breakout room to ask your question.
If you need to leave your desk during the exam (e.g., for a bathroom break), also use the "Ask for Help" button
After 1 hour and 15 minutes, the quiz will end and you will be expected to stop working. At this point, you should scan your written questions as you
would for the written homework/tutorials, and submit to Gradescope. Your exam PDF must be submitted by 8:35pm PST. If you have any technical
issues with uploading, you must e-mail your solutions to your lecture prof before this time.
EXAM RULES:
This is a closed book exam. You may only refer to the formula sheet provided. You may not communicate with any other person. You may not share
the questions during or after the exam with any other person, website, etc... . Please keep in mind that some students will be writing the midterm at a
later time.
Any calculator is allowed; memory must be cleared.
MIDTERM CONFLICT PEOPLE: If you filled in the midterm conflict form and are planning to take the exam at the alternate times, you must not
begin the quiz now. There will be a separate quiz assigned to you, to be taken at the time you selected. If you start the quiz, you are committing to
finishing it at this time, and no further attempts will be considered.
Formula sheet:
If the formulae don'e show up below, you can get a PDF with formulae and images for the questions here: MT2formpics.pdf
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Question 1
2 pts
A spacecraft travelling to Mars has a solar panel oriented perpendicular to the Sun’s rays that produces electrical power by
converting some fixed fraction of the energy of the incident solar radiation into electrical energy. If the solar panel produces 100W
of electrical power when it is near the Earth, what electrical power will it produce when the spacecraft reaches Mars, whose
distance from the Sun is 1.5 times the Earth-Sun distance?
338W
150W
67W
225W
100W
44W
30W
Question 2
2 pts
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For the engine cycle shown, the efficiency is 0.15. How much heat must be supplied to the gas per cycle?
100J
1333J
15J
115J
667J
Question 3
2 pts
Star A has one quarter the radius and gives off light with half the peak wavelength as compared with star B. The total power of
radiation produced by star A (assuming both have emissivity e=1) is approximately
2 times that of star B
16 times that of star B
1/4 times that of star B
1/2 times that of star B
4 times that of star B
8 times that of star B
1/8 times that of star B
the same as star B
1/16 times that of star B
Question 4
2 pts
Two identical cylinders each contain 1 mole of helium gas with an initial temperature 500K and initial volume of 5L. For cylinder 1,
the piston is pulled out rapidly so that the gas expands adiabatically to 10L. For cylinder 2, a piston is pulled out slowly so that the
gas expands isothermally to 10L. We can say that the work done by the gas during the expansion is
the same in both cases but not equal to zero
larger for cylinder 1
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larger for cylinder 2
zero in both cases
Question 5
2 pts
A planet orbiting a nearby star lies at a distance D from that star. The star has radius R and the planet has radius r, emissivity
e=1/2, and albedo 0. If the intensity of radiation from the star at the location of the planet is IS, the equilibrium surface temperature
T of the planet (assumed to be uniform over the surface) satisfies
π r2 IS = 2 π r2 σ T4
π R2 IS = 2 π r2 σ T4
2π R2 IS = π r2 σ T4
π r2 IS = π r2 σ T4
4 π r2 IS = 2 π r2 σ T4
2 π D2 IS = π r2 σ T4
For the following two written questions, please input your answers into Canvas but upload your written solutions to Gradescope.
Each question is worth 10 points.
Your submitted answer here should agree with the answer on your Gradescope submission.
Question 6
0 pts
Fjóla lives in Iceland on the top of a medium-sized cliff above rocks heated by geothermal activity. Tired of climbing the rope to get
to her house, Fjóla decides to build a “gas elevator” that uses heat from the rocks to warm the air (CV = 5/2 R) in a large cylinder
with a freely movable piston on top. To use the elevator, Fjóla steps onto the piston, and then the cylinder is moved on top of the
hot rocks, so that heat flows into the gas, causing the piston to rise.
The cylinder has a cross-sectional area of 0.2m2 and the piston has an initial height of 1m and initial gas temperature of 300K after
Fjóla steps onto it. The piston and Fjóla together have a mass of 250kg and the outside air pressure can be taken to be 100kPa. If
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the hot rocks provide heat to the gas at a rate of 2000W, how long does it take for Fjóla to reach her house at a piston height of
4m? Answer in seconds, without units.
Assume that no heat enters or leaves the cylinder except at the bottom.
Question 7
0 pts
You are working on the design for a TV remote control based on the adiabatic compression of a gas.
The remote works by rapidly compressing gas in a cylinder. The end of the cylinder away from the piston is made of a very thin
disk of copper that quickly comes to the same temperature as the gas without cooling the gas significantly. You have equipped
your television with an IR sensor that receives the thermal radiation from the copper disk (emissivity 0.3). In order to trigger the
sensor and turn on the television, you need the IR radiation from the copper disk to have a peak wavelength of 3000nm and a
power of 5W.
a) If the gas in the cylinder is helium (CV = 3/2R) with an initial temperature of 20 degrees Celsius, and an initial pressure equal to
the ambient pressure of 100kPa, what compression ratio and cylinder diameter do you need for your design?
b) If the initial distance between the piston and the copper disk is 0.1m, how much work must you do to compress the piston in
addition to the work done by the outside air, assuming that the piston is stationary after the compression?
Put your answer for the compression ratio below, and your other answers in the next questions.
Question 8
0 pts
Put in your answer for the diameter of the cylinder, in centimeters, without units.
Question 9
0 pts
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10/24/21, 8:41 AM
Quiz: MIDTERM 2
Put in your answer for the work done by you, in Joules, without units.
Not saved
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