20161202

Physics final exam question: thermal expansion of pendulum string

Physics 205A Final Exam, fall semester 2015
Cuesta College, San Luis Obispo, CA

A Physics 205A student builds a simple pendulum using a string and a mass, and at a certain temperature it has a period of 1.50 s. As the temperature increases, determine whether the period will increase, decrease, or remain the same. Ignore friction/drag, and only consider the thermal expansion of the string. Explain your reasoning using the properties of simple harmonic motion and thermal expansion.

Solution and grading rubric:
  • p:
    Correct. Discusses how the period of a simple pendulum depends on the gravitational acceleration constant (which does not change) and the string length (which expands due to the increase in temperature), such that the period will increase.
  • r:
    As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes.
  • t:
    Nearly correct, but argument has conceptual errors, or is incomplete.
  • v:
    Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Some garbled attempt at applying thermal expansion to the simple pendulum parameters.
  • x:
    Implementation/application of ideas, but credit given for effort rather than merit. Approach other than applying thermal expansion to the simple pendulum parameters.
  • y:
    Irrelevant discussion/effectively blank.
  • z:
    Blank.
Grading distribution:
Sections 70854, 70855, 73320
Exam code: final7rUk
p: 47 students
r: 1 students
t: 15 students
v: 6 students
x: 2 students
y: 0 students
z: 0 students

A sample "p" response (from student 5612):

Astronomy current events question: "nearly naked" supermassive black hole

Astronomy 210L, fall semester 2016
Cuesta College, San Luis Obispo, CA

Students are assigned to read online articles on current astronomy events, and take a short current events quiz during the first 10 minutes of lab. (This motivates students to show up promptly to lab, as the time cut-off for the quiz is strictly enforced!)
Dave Finley, "Close Galactic Encounter Leaves 'Nearly Naked' Supermassive Black Hole" (November 2, 2016)
public.nrao.edu/news/pressreleases/nearly-naked-supermassive-black-hole
The Very Long Baseline Array (VLBA) discovered that B3 1715+425 is a "nearly naked" supermassive black hole, which may be the remnant of a galaxy that:
(A) fit inside of its own event horizon.
(B) passed through a larger galaxy.
(C) was composed of antimatter.
(D) devoured all of its own stars.
(E) had dead spiral arms.

Correct answer: (B)

Student responses
Sections 70178, 70186
(A) : 2 students
(B) : 8 students
(C) : 2 students
(D) : 8 students
(E) : 1 student

Astronomy current events question: computer models of moon orbit tilt origin

Astronomy 210L, fall semester 2016
Cuesta College, San Luis Obispo, CA

Students are assigned to read online articles on current astronomy events, and take a short current events quiz during the first 10 minutes of lab. (This motivates students to show up promptly to lab, as the time cut-off for the quiz is strictly enforced!)
Matthew Wright, "New Model Explains the Moon’s Weird Orbit" (October 31, 2016)
umdrightnow.umd.edu/news/new-model-explains-moon’s-weird-orbit
Computer simulations may explain Earth's axis and the moon's orbital tilts today assuming that __________ after the large impact that formed the Earth-moon system.
(A) Earth's axis was steeply tilted.
(B) multiple moonlets were created.
(C) magnetic fields were disrupted.
(D) the sun was highly unstable.
(E) volcanic activity ceased.

Correct answer: (A)

Student responses
Sections 70178, 70186
(A) : 16 students
(B) : 0 students
(C) : 5 students
(D) : 0 students
(E) : 0 students

Astronomy current events question: proposed Bio-Indicator Lidar Instrument

Astronomy 210L, fall semester 2016
Cuesta College, San Luis Obispo, CA

Students are assigned to read online articles on current astronomy events, and take a short current events quiz during the first 10 minutes of lab. (This motivates students to show up promptly to lab, as the time cut-off for the quiz is strictly enforced!)
Lori Keesey, "New Instrument Could Search for Signatures of Life on Mars" (November 1, 2016)
nasa.gov/feature/goddard/2016/new-instrument-could-search-for-signatures-of-life-on-mars/
A proposed Bio-Indicator Lidar Instrument (BILI) would be used by future Mars rovers to remotely detect organic particles in:
(A) cloud ice crystals.
(B) underground water deposits.
(C) gases trapped in rocks.
(D) ancient fossils.
(E) dust plumes.

Correct answer: (E)

Student responses
Sections 70178, 70186
(A) : 2 students
(B) : 6 students
(C) : 2 students
(D) : 1 student
(E) : 9 students

20161130

Online reading assignment: Milky Way history, big bang clues (SLO campus)

Astronomy 210, fall semester 2016
Cuesta College, San Luis Obispo, CA

Students have a weekly online reading assignment (hosted by SurveyMonkey.com), where they answer questions based on reading their textbook, material covered in previous lectures, opinion questions, and/or asking (anonymous) questions or making (anonymous) comments. Full credit is given for completing the online reading assignment before next week's lecture, regardless if whether their answers are correct/incorrect. Selected results/questions/comments are addressed by the instructor at the start of the following lecture.

The following questions were asked on reading textbook chapters and previewing presentations on the history of the Milky Way and big bang clues, a comic strip adaptation of of Neil deGrasse Tyson's "The Most Astounding Fact" 2008 interview for TIME magazine, and Minute Physics' video explanation of Olbers' paradox.


Selected/edited responses are given below.

Describe something you found interesting from the assigned textbook reading or presentation preview, and explain why this was personally interesting for you.
"I found the process used to move the narrative of the big bang as purely conceptual to one that was based on observable evidence was really a well-thought out method. I specifically like the point about the long time exposure taken by the Hubble Space Telescope and how that was related to a concept about being lost in a werewolf ridden forest. It seems almost too simple that looking in one direction for 'gaps' would actually yield results but it is actually an extremely logical solution to questions regarding the edge of the universe."

"When it mentions that even though we can tell the universe has edges, the further out you look the further in the past you see things. I had prior knowledge of both of those concepts individualy but I never put together that when we look at the farthest point out in space it's actually the furthest in the past and since the universe is expanding the edges are actually further out than we see them. That's so cool."

"That the universe is constantly expanding in all different directions."

"That the universe was basically started with hydrogen. That is personally interesting to be because it came so complex from only one element."

"I found the explanation of what 'metals' are in astronomy helpful. I always imagined it was like regular metals we have on Earth and didn't realize it referred to atoms heavier than helium."

"From the textbook: 'When you look at a galaxy millions of light-years away, you do not see it as it is now but as it was millions of light years ago when it's light began the journey towards earth.' I thought this was so fascinating because we all look at things as they are, but we never knew that we were looking at something the way it was."

"Being able to see the past with telescopes was very interesting because everything that we have seen is only the past making the universe even more a mystery."

"The new stars have the most metals, I feel that the older stars would have more time to develop more metals."

"I found it weird that when you look at space you're essentially looking into the past."

"We are star stuff! I've always said this and believed this and I am so stoked to finally delve into it!"

"Crazy to realize that the elements that make us and the world around us are made of dead stars."

"I've always found it so interesting how we're all made out of star dust. I remember learning that in high school astronomy and I think about it all the time, super cool."

"I really enjoyed learning that we are made up of star dust because it adds a new meaning to looking at the stars."

"That the universe is progressively getting dirtier is interesting because it makes sense and from what we have learned so far I understand why and how it is happening."

"Learning about how our Milky Way was created was very interesting. It was personally interesting for me because I never new how it started. Starting out from just hydrogen then learning about how the first generation stars (type II supernova) were just exploding was interesting."

"The universe is expanding...............!!!"

"What I found interesting is how we are an expanding universe."

Describe something you found confusing from the assigned textbook reading or presentation preview, and explain why this was personally confusing for you.
"Will there ever be an end to the expanding universe? What are some theories about the end?"

"Metallification is still confusing to me, why does it help determine a star's age?"

"From the textbook: 'There is no center to the expansion of the universe, so you would not see galaxies approaching a single spot. Rather, you would see the space between galaxies disappearing, distances between all galaxies decreasing without the galaxies themselves moving, and eventually galaxies beginning to merge.' I don't know the importance of this, but whether important or not, I am still like WHAAAAT?!?!?"

"Nothing was too confusing with the reading assignments."

"That there is an edge of the universe. I understand that there has to be but my brain cannot process it."

"Seeing through telescopes are like travelling back in time?"

"The edge of the universe and the gaps. How do we know it has an edge? What is there are galaxies further than Hubble can see? This confuses me."

"Why are galaxies redshifting from us?"

Indicate how the amount of these elements in the universe have changed over time.
(Only correct responses shown.)
Hydrogen: decreased [80%]
Metals (elements heavier than hydrogen and helium): increased [84%]

The outermost layers of __________ are more abundant in metals (elements heavier than hydrogen and helium).
extremely old stars that formed a long time ago.  * [1]
young stars that formed very recently.  ********************** [22]
(There is a tie.)  [0]
(Neither, as stars cannot have metals.)  [1]
(Unsure/guessing/lost/help!)  * [1]

Indicate what produced these elements.
(Only correct responses shown.)
Helium in the sun's core: the sun [40%]
Carbon in your body: another star, in the past [76%]
Calcium in your bones: another star, in the past [64%]
Iron in your blood: another star, in the past [68%]
Gold and silver from mines: another star, in the past [60%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"What will our final exam look like? What time is it?" (The Final Exam will be Wednesday, December 14, 7:00-9:00 PM here (this classroom). The study guide for the final is now posted on the website on the last day of class announcements page.)

"When are we going to start prepping for the final? (:" (That's up to you; the topics covered on the Final Exam have already been posted.)

"Since a massive star collapses after a certain point due to the elements past iron removing energy rather than creating it, so does that mean that it may be possible to create even more elements?" (Yes, that's how we get everything else on the periodic table of elements heavier than iron.)

"How can we see Deneb as it was 1,400 years ago? That just blows my mind." (The light you see from Deneb tonight took 1,400 years to get to you, so it had to have started out from Deneb 1,400 years ago. Similar to how a postcard you receive from Europe today took on week to get to you, so it had to have started out from Europe one week ago.)

"If someone reached the edge of the universe and then kept going, would they become the new edge of the universe?" (From what we know so far, there is no physical edge in space (just an "edge" in time, but that's just from our perspective looking outwards in all directions). So you could physically move around anywhere in space, you wouldn't encounter a physical edge, but just keep moving on further and further away.)

"I don't understand what you want us to know about the big bang theory." (I don't expect you to "understand" the big bang theory (I don't anyone really does), but I would like you to describe the evidence that indicates that we live in a universe that has distances between galaxies that have been continuously expanding for approximately 14 billion years.)

"How do you think the big bang happened? where do you think we--the universe--all came from?" (There aren't any answers to this questions...yet. But there are a lot of theories right now, but not enough evidence to prove or disprove any of them.)

Online reading assignment: heat transfers

Physics 205A, fall semester 2016
Cuesta College, San Luis Obispo, CA

Students have a bi- weekly online reading assignment (hosted by SurveyMonkey.com), where they answer questions based on reading their textbook, material covered in previous lectures, opinion questions, and/or asking (anonymous) questions or making (anonymous) comments. Full credit is given for completing the online reading assignment before next week's lecture, regardless if whether their answers are correct/incorrect. Selected results/questions/comments are addressed by the instructor at the start of the following lecture.

The following questions were asked on reading textbook chapters and previewing a presentation on heat transfers.


Selected/edited responses are given below.

Describe what you understand from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically demonstrate your level of understanding.
"Heat resistance correlates to the thickness, surface area and the conductivity constant for that object type. Blackbody objects absorb and radiate heat very well, while silverbody objects reflect heat very well, while not absorbing well at all."

"Power through a wall is proportional to the temperature difference ∆T on either side, and inversely proportional to the thermal resistance R of the object, which is a measure of how difficult it is for heat to flow per time through it: R = d/(κ·A)."

"I understood most of the formulas and concepts. It is cool to see they physics behind things that I already know."

Describe what you found confusing from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically identify the concept(s) that you do not understand.
"The concept of different emissivity colors (silver vs. black) absorbing different amounts of radiation."

"I'm interested in seeing more applications of the numbers and how they can be used in the real world."

"I could use some help understanding how to apply these concepts."

"The equation for radiation is pretty confusing to me. I am sure that I could get it down with a few examples in class, but it is hard to picture solving for right now."

"Not really sure if R is like insulation in that we'd want to increase it to retain heat."

"Stefan's law is a jumbled mess and I want no part of it."

"How can an object be good at absorbing heat and emitting heat at the same time?"

"Not much, to be honest."

In order to maximize the thermal resistance of these exterior walls, should the following parameters be minimized, maximized (or has no effect)?
(Only correct responses shown.)
insulation thickness d: maximize [81%]
insulation conductivity κ: minimize [64%]
Total surface area A exposed to the outdoors: minimize [57%]

In order to minimize the amount of heat flowing per time through these exterior walls, should the following parameters be minimized, maximized (or has no effect)?
(Only correct responses shown.)
temperature difference ∆T between indoors and outdoors: minimize [64%]
thermal resistance R of the walls: maximize [71%]

For these two Leica M cameras, if they are both cooler than the surrounding environment, both will begin to heat up by absorbing radiative heat (say, from the sun). The __________ model have a faster rate of heat absorbed per time.
black.  ******************************** [32]
silver.  *** [3]
(There is a tie.)  *** [3]
(Unsure/guessing/lost/help!)  **** [4]

For these snowboarders, if they are warmer than the surrounding environment, they will begin to cool down by emitting radiative heat (say, to the overcast sky and the snowy landscape). The snowboarder wearing the __________ jacket will have a faster rate of heat radiated per time.
black.  ***************** [17]
silver.  ***************** [17]
(There is a tie.)  **** [4]
(Unsure/guessing/lost/help!)  **** [4]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Can insulation hold in all of the heat without it transferring? Is it possible for heat (or cold) to be trapped by an insulator and never transfer the energy?" (Only if the thermal conductivity κ = 0. Realistically you can try to get a very small κ value to minimize the rate of heat conducted through, but even a trickle of heat will add up over a long enough time.)

"Why is the amount of heat lost through the object referred to as power?" (The rate of heat conducted per time is in units of joules/second or watts--which is the equivalent of power, which is generally any rate of energy per time.)

"So, if I have earned all the homework points that I can, if I miss a homework assignment from now on does that mean I would go down in points? Or would I still keep the maximum amount of points that I have gotten?" (You can never go down in points. If you have already reached that maximum amount of homework points already, then you've earned the right to not do any more homework for the rest of the semester.)

"What's with the fancy cursive variable symbols?" (After going through the Roman and Greek alphabets, all we have left are script letters.)

"Would the black jacket be more effective at retaining heat because it's good at absorbing it?" (A black jacket would be efficient at absorbing heat, if it's sunny outside. But radiation is a two-way street; a black jacket would be efficient at radiating heat, in the dark. So you would go through huge temperature swings during the day and night. If you wore a white jacket, you would not be efficient at absorbing heat from the sun, but then you would also not be efficient at radiating heat at night either, so you would have less temperature variation during the day and night. Perhaps you could wear a dark and light-colored reversible jacket that you could wear as needed depending your environment and need to stay warm (or cool off).)

20161129

Online reading assignment: Milky Way history, big bang clues (NC campus)

Astronomy 210, fall semester 2016
Cuesta College, San Luis Obispo, CA

Students have a weekly online reading assignment (hosted by SurveyMonkey.com), where they answer questions based on reading their textbook, material covered in previous lectures, opinion questions, and/or asking (anonymous) questions or making (anonymous) comments. Full credit is given for completing the online reading assignment before next week's lecture, regardless if whether their answers are correct/incorrect. Selected results/questions/comments are addressed by the instructor at the start of the following lecture.

The following questions were asked on reading textbook chapters and previewing presentations on the history of the Milky Way and big bang clues, a comic strip adaptation of of Neil deGrasse Tyson's "The Most Astounding Fact" 2008 interview for TIME magazine, and Minute Physics' video explanation of Olbers' paradox.


Selected/edited responses are given below.

Describe something you found interesting from the assigned textbook reading or presentation preview, and explain why this was personally interesting for you.
"The raisin bread analogy--I never knew that the raisins would move relatively; now that I do know it, it makes sense."

"That the universe essentially began with only hydrogen, and over the course of time has fused it into heavier elements. It makes the history of the universe feel like it's working toward creating something larger."

"How we are in an expanding universe."

"That the universe is progressively getting dirtier, because it makes sense and from what we have learned so far I understand why and how it is happening."

"That we are made from 'star stuff' because we are extremely tiny and knowing we are made from stars makes us feel special."

"I really tripped out that when we look at Deneb we are seeing it 1,400 years ago."

"That we can see stars that took millions of years to reach us, which is sort of like time travel."

"How time interacts with light at great distances. Because it's something that you don't see much on Earth because the distances are too short."

Describe something you found confusing from the assigned textbook reading or presentation preview, and explain why this was personally confusing for you.
"Monolithic collapse model; I can't seem to grasp what happens or what it really does. Just one of those things I need explained in class."

"he 'gaps' and 'edges' in the universe--how do we know the universe has an end? What might the edge or end look like?"

"The edge of the universe. It makes sense that it should be there, but it's a hard concept to wrap your mind around."

"I didn't understand the difference between stars in the halo and stars in the disk. I'm not sure what those locations are referring to, in regards to the Milky Way."

"That younger stars have more metals than older stars."

"The edge of the universe and the gaps. How do we know it has an edge? What is there are galaxies further than Hubble can see? This confuses me."

"I want to do know more on the big bang theory. I don't really understand why we don't know if it happened when we have evidence/examples."

Indicate how the amount of these elements in the universe have changed over time.
(Only correct responses shown.)
Hydrogen: decreased [58%]
Metals (elements heavier than hydrogen and helium): increased [71%]

The outermost layers of __________ are more abundant in metals (elements heavier than hydrogen and helium).
extremely old stars that formed a long time ago.  ******* [7]
young stars that formed very recently.  **************** [16]
(There is a tie.)  * [1]
(Neither, as stars cannot have metals.)  [0]
(Unsure/guessing/lost/help!)  [0]

Indicate what produced these elements.
(Only correct responses shown.)
Helium in the sun's core: the sun [38%]
Carbon in your body: another star, in the past [54%]
Calcium in your bones: another star, in the past [54%]
Iron in your blood: another star, in the past [46%]
Gold and silver from mines: another star, in the past [54%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"I liked watching the video about 'Why Is It Dark at Night?' because I thought I knew the answer to that, but I actually knew nothing."

"What's your opinion of studying astronomy and not believing in the big bang theory at the same time?" (Well, the "big bang" is actually a misrepresentation of the expanding space between galaxies; but I think people can believe in certain things while scientifically studying how nature works based on evidence. Beliefs have been wrong before, but so has evidence.)

"Will the universe ever stop expanding?" (Current evidence is that the expansion rate appears to now be accelerating.)

"Is the universe expanding at an increasing rate because we are still in the big bang and apart of the expanding explosion?" (We have evidence that the expansion of space between galaxies is speeding up, but we don't know what's causing it. So for now it's called "dark energy," whatever that is. Similar to the evidence of additional gravitational forces holding galaxies together--we don't know what's causing that, but it's called "dark energy," whatever that is.)

20161128

Online reading assignment: internal energy conservation

Physics 205A, fall semester 2016
Cuesta College, San Luis Obispo, CA

Students have a bi-weekly online reading assignment (hosted by SurveyMonkey.com), where they answer questions based on reading their textbook, material covered in previous lectures, opinion questions, and/or asking (anonymous) questions or making (anonymous) comments. Full credit is given for completing the online reading assignment before next week's lecture, regardless if whether their answers are correct/incorrect. Selected results/questions/comments are addressed by the instructor at the start of the following lecture.

The following questions were asked on reading textbook chapters and previewing a presentation on internal energy conservation.


Selected/edited responses are given below.

Describe what you understand from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically demonstrate your level of understanding.
"It is not correct to say that a substance contains heat. The substance contains internal energy. Heat is the energy transfer from hot to cold."

"I somewhat understand thermal internal energy and how it is evaluated by the movement of microscopic atoms."

"I understood the equation Q = m·c·∆T as something I have done in chemistry. It is interesting now in physics."

"A low temperature object has little thermal internal energy and a high temperature object has more thermal internal energy. If temperature increases, heat from the environment is being transferred to the object and thermal internal energy increases. The opposite occurs when an object is cooled and transfers heat from the object to the environment. A system with Q = 0 is insulated from the envirnonment and there is no transfor of heat."

"I understand how to plug in things into the equation Q = m·c·∆T."

Describe what you found confusing from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically identify the concept(s) that you do not understand.
"What is our definition of the system that we are working with? I know there are many types of them, could we go over the language of how to describe what is happening in a system?"

"I don't understand how to figure out which object loses more internal energy."

"Still a little bit fuzzy how this relates to mechanical energy and work."

"I'm feeling good on this material so far."

"Not much was confusing, would maybe help just going over an example of calorimetry."

Two objects that are brought into contact with each other will reach thermal equilibrium when they have the same:
internal energy.   ****** [6]
temperature.   ********** [10]
(Both of the above choices.)   ********** [15]
(Neither of the above choices.)   * [1]
(Unsure/lost/guessing/help!)   ** [2]

Raw seafood is placed on a block of salt that has already been heated up. The energy contained in the high-temperature block of salt is then transferred to the seafood, cooking it. While it is being cooked, the internal thermal energy of the seafood __________, while the thermal internal energy of the salt block __________.
increases; decreases.   ******************************** [32]
decreases; increases.   * [1]
does not change; does not change.   [0]
(Unsure/lost/guessing/help!)   * [1]

For the seafood cooking on the salt block (ignoring heat transfers with the environment), the object that experienced the greatest amount of change (increase or decrease) in thermal internal energy was the:
seafood.   ********** [10]
salt block.   ***** [5]
(There is a tie.)   ****************** [18]
(Unsure/lost/guessing/help!)   * [1]

Frozen meat is placed in a water bath, in order to defrost it. At the very start of this defrosting process (where the frozen meat just begins to warm up from its below-freezing temperature, and the ice crystals inside have not yet reached the melting point), the internal thermal energy of the meat __________, while the thermal internal energy of the water __________.
increases; decreases.   ********************** [22]
decreases; increases.   ******* [7]
does not change; does not change.   **** [4]
(Unsure/lost/guessing/help!)   * [1]

For the frozen meat in the water bath (ignoring heat transfers with the environment), the object that experienced the greatest amount of change (increase or decrease) in thermal internal energy was the:
frozen meat.   **** [4]
water bath.   ******* [7]
(There is a tie.)   ******************** [20]
(Unsure/lost/guessing/help!)   *** [3]

A shot of whiskey is mixed with a pint of beer to make a boilermaker. Assuming that the whiskey and beer have approximately the same temperature before they are mixed together, the internal thermal energy of the whiskey __________, while the thermal internal energy of the beer __________.
increases; decreases.   * [1]
decreases; increases.   * [1]
does not change; does not change.   ****************************** [30]
(Unsure/lost/guessing/help!)   ** [2]

For the shot of whiskey being mixed with the pint of beer (ignoring heat transfers with the environment), the object that experienced the greatest amount of change (increase or decrease) in thermal internal energy was the:
shot of whiskey.   [0]
pint of beer.   ** [2]
(There is a tie.)   ***************************** [29]
(Unsure/lost/guessing/help!)   *** [3]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"I am confused with heat transfers between internal energy systems and when there may be a tie or not." (If there are only two internal energy systems that interact, then the amount they change will be a tie (one will be an increase, the other a decrease) as long as there are no third parties: one or more other internal energy systems, and/or heat losses/gains from the environment.)

"Why are we measuring temperature in kelvin?" (When we're doing chemistry, we do as chemists physicists do.)

"I observed three candles burning side by side. The candle in the center melted much faster than the two outer candles. What is the energy transfer relationship taking place?" (Radiative heat transfer!)

"Wouldn't the salt block have experienced the greatest changes in energy because it is heated up then losing heat from it transferring to the seafood?" (Perhaps, but the process of interest is after it is heated up, and the seafood is placed on it. Also, if the process is heating up the salt block, then it loses heat, the net change in internal energy wouldn't be very much.)

"Can we use the same technique for finding non-conservative energy loss for internal thermal energies as we did with mechanical energy forms?" (Yes. The transfer-balance equation, but with heat instead of work, and internal energy changes instead of mechanical energy changes.)

I'm slightly confused at the concept of heat flow. Why does the hotter object give off thermal heat to the colder object? It makes sense but I would like to know the chemical reason for this transfer of energy." (It has something to do with entropy, but even in collisions in cars we've seen how a faster car can transfer some of that translational kinetic energy to a slower car. Similarly for ideal gases, a faster (higher temperature atom) will transfer some of its internal energy to a slower (lower temperature) atom.)

"Will there be any more extra credit opportunities before the final?" (I'll look into it. But be sure to maximize the regular points remaining in this semester.)

20161127

Physics final exam question: cool aluminum block in warm water

Physics 205A Final Exam, fall semester 2015
Cuesta College, San Luis Obispo, CA

An aluminum block is placed in a container of water. The aluminum block is initially cooler than the warm water, and they are allowed to come to thermal equilibrium. The container is not thermally isolated from the environment. For the aluminum block to experience a smaller change in thermal energy than the water while they reach thermal equilibrium, determine whether heat must enter or leave the container. Explain your reasoning using the properties of heat, temperature, and thermal equilibrium.

Specific heat of aluminum is 900 J/(kg·K). Specific heat of water is 4,190 J/(kg·K).

Solution and grading rubric:
  • p:
    Correct. Discusses:
    1. how the aluminum block (initially cooler than the water) increases in thermal energy as it reaches thermal equilibrium;
    2. how the water (initially warmer than the aluminum block) decreases in thermal energy as it reaches thermal equilibrium; and
    3. from energy conservation (either explicitly shown in a transfer-balance equation, or discussed conceptually), in order for the aluminum block to undergo a smaller thermal energy change than the water, while some heat is transferred from the water to the block, some heat from the water must also be lost to the environment.
  • r:
    As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes.
  • t:
    Nearly correct, but argument has conceptual errors, or is incomplete. May discuss how heat must leave the container based on comparing just specific heats, or comparing just temperature changes of the aluminum block and water (without knowing how their masses compare, or their respective initial temperatures).
  • v:
    Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Some garbled attempt at applying energy conservation to thermal energy changes and heat transfers with the environment.
  • x:
    Implementation/application of ideas, but credit given for effort rather than merit. Approach other than applying energy conservation to thermal energy changes and heat transfers with the environment.
  • y:
    Irrelevant discussion/effectively blank.
  • z:
    Blank.
Grading distribution:
Sections 70854, 70855, 73320
Exam code: final7rUk
p: 29 students
r: 3 students
t: 26 students
v: 6 students
x: 3 students
y: 2 students
z: 1 student

A sample "p" response (from student 3179):

Physics final exam problem: more dangerous collision type

Physics 205A Final Exam, fall semester 2015
Cuesta College, San Luis Obispo, CA

A 2016 Fiat 500X car[*] (mass 1.2×103 kg) driving at 2.0 m/s collides with a stationary Ford F-150 pick-up truck[**] (mass 1.8×103 kg). It is claimed that an elastic collision is more dangerous for the passengers than a completely inelastic collision[***].
  1. For an elastic collision, the car would rebound with a speed of 0.40 m/s in the reverse direction (while the truck would move forward after the collision).
  2. For a completely inelastic collision between the car and truck, they would stick and both move together in the forward direction.
Solve for (a) the final speed of the truck after the elastic collision, (b) the final speed of the truck after the completely inelastic collision, and (c) discuss whether this claim is plausible or implausible. Ignore friction, drag, and external forces. Show your work and explain your reasoning using properties of collisions, energy (non-)conservation, and momentum conservation.

[*] caranddriver.com/fiat/500.
[**] buyersguide.caranddriver.com/ford/f-150/specs#features.
[***] James Cunningham, Norman Herr, Hands-On Physics Activities with Real-Life Applications: Easy-to-Use Labs and Demonstrations for Grades 8-12, Wiley (1994), p. 323.

Solution and grading rubric:
  • p:
    Correct. Determines/discusses:
    1. the final velocity of the truck in the elastic collision from applying momentum conservation;
    2. the final velocity of the truck in the completely inelastic collision from applying momentum conservation; and
    3. makes some reasonable interpretation that the claim of the elastic collision being more dangerous than a completely inelastic collision is plausible, given that the initial-to-final velocity changes for the truck (and the car) are greater for the elastic collision than for the inelastic collision.
  • r:
    Nearly correct, but includes minor math errors.
  • t:
    Nearly correct, but argument has conceptual errors, or is incomplete. At least has one correct final velocity for the truck, the other result is problematic but at least momentum conservation was applied.
  • v:
    Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Some garbled attempt at applying momentum conservation for to find the final velocity of the truck for one collision, but the other collision is incomplete or conceptually problematic (such as applying kinetic energy conservation for the completely inelastic collision, claiming zero final velocities, etc.).
  • x:
    Implementation of ideas, but credit given for effort rather than merit. Approach involving methods other than momentum conservation.
  • y:
    Irrelevant discussion/effectively blank.
  • z:
    Blank.
Grading distribution:
Sections 70854, 70855, 73320
Exam code: final7rUk
p: 24 students
r: 3 students
t: 14 students
v: 16 students
x: 6 students
y: 5 students
z: 2 students

A sample "p" response (from student 1793):