Physics quiz question: changing emissivity of a brick

Physics 205A Quiz 7, fall semester 2019
Cuesta College, San Luis Obispo, CA

"Red bricks"
Ramesh NG
flic.kr/p/8TmkRf

A red brick has an emissivity of 0.93[*], and has a surface temperature of 270 K. Ignore conduction and convection heat transfers to/from the environment. If the brick were coated with aluminum paint such that its emissivity was lowered to 0.45, the rate of heat per time the brick _________ would decrease.
(A) radiates to the environment.
(B) absorbs from the environment.
(C) (Both of the above choices.)
(D) (Neither of the above choices.)

[*] thermoworks.com/emissivity-table.

Correct answer (highlight to unhide): (C)

The net power (rate of heat per time) radiated is given by:

Power = –e·σ·A·((T_obj)⁴ + (T_env)⁴) = –e·σ·A·(T_obj)⁴ – e·σ·A·(T_env)⁴,

where the first (negative) term corresponds to the rate of heat being removed (radiated) from the object, while the second (positive) term corresponds to the rate of heat being put into (absorbed) by the object (in order to be consistent with the ±Q convention for removing heat from (–) or putting heat into (+) a thermodynamic system).

As it is a factor common to both the radiation and absorption terms, lowering the emissivity value from 0.93 to 0.45 would then decrease both the rate of heat per time radiated to the environment and the rate of heat per time absorbed from the environment.

Sections 70854, 70855
Exam code: quiz07VlnC
(A) : 10 students
(B) : 5 students
(C) : 35 students
(D) : 1 student

Success level: 69%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.52

Physics quiz archive: temperature, thermal equilibrium, heat transfers

Physics 205A Quiz 7, fall semester 2019
Cuesta College, San Luis Obispo, CA
Sections 70854, 70855
Exam code: quiz07VlnC



Sections 70854, 70855 results

0- 6 :	* [low = 3]
7-12 :	**********
13-18 :	**************
19-24 :	******************* [mean = 18.9 +/- 6.2]
25-30 :	******* [high = 30]

Online reading assignment: heat transfers

Physics 205A, fall semester 2019
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 can transfer in three different ways: conduction, convection, and radiation. Conduction is the transfer of heat energy by direct contact; convection is the movement of heat by actual motion of matter; radiation is the transfer of energy via electromagnetic waves."

"Conduction is a transfer of heat through an item, mainly thorough metals since they can conduct heat much better than wood or plastics. Radiation is another form of heat transfer, but from light, not through bulk movement or necessarily through objects."

"Thermal resistance of an object is related equal to the thickness divided by exposed surface area and the material-dependent conductivity. Heat can be transferred via conduction, convection, or radiation."

"In order to maximize thermal resistance, the wall/object needs to be as thick as possible, in order to reduce the amount of heat that passes through."

"Forced convection is the transport of thermal energy by a force like blowing, it does not just naturally circulate."

"Convection is heat transferred from bulk movement of fluids. Conduction where heat is directed through a material."

"Convection occurs when part of a fluid is warmed, it expands and its density decreases such that the cooler surrounding fluid, which now has a greater density, will push the warmer fluid upward because the cooler, denser fluid exerts a buoyant force on the warmer, less dense fluid. I also understand how light-colored objects reflect more radioactive waves and are less susceptible to absorbing heat through radiation than dark-colored objects, which absorb energy as heat through radiation of electromagnetic waves much better."

"One major understanding I have grasped from this presentation is that radiation is a two-way street indicating that an object good at absorbing heat will also be good at emitting heat; moreover, an object that is not good at absorbing heat would likewise be bad at emitting heat."

"The flow of heat moves differently based on certain aspects of an object or environment. For example, something that is the color black will absorb more heat quickly than something that is not the color black."

"I didn't get to it."

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.

"I think I understood the concepts from the presentation preview. Just need to see more examples."

"I found it confusing that anyone would choose to use Q as a symbol for heat. I also don't know why the units are what they are for the Stefan-Boltzmann constant."

"I'm just having a bit of trouble uncovering the equations and their parts."

"The topics that seemed confusing from the reading were exactly how to apply the equations and laws to everything, and exactly what each variable means, for example Stefan’s law quantitatively describing power."

"Why do different materials absorb heat at different rates?"

"I do not understand what Stefan's law is and what the variables mean exactly. Why are blackbodies and silverbodies better at absorbing heat?"

"So if a black object takes in more heat and can emit more heat as compared to a white object, shouldn't the objects be the same temperature if they're in the same condition? But I know that black gets warmer than white, so it's confusing."

"I did not really understand what the units of e was or how it changed depending on the material of the object."

"Nothing too confusing, just conduction and the equations related to it."

"The formulas for Fourier's and Stefan's laws are a little scary."

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 [83%]
insulation conductivity κ: minimize [66%]
Total surface area A exposed to the outdoors: minimize [66%]

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 [71%]
thermal resistance R of the walls: maximize [83%]

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.	*************************** [27]
silver.	** [2]
(There is a tie.)	** [2]
(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.	******************** [20]
silver.	****** [6]
(There is a tie.)	**** [4]
(Unsure/guessing/lost/help!)	***** [5]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"This is an interesting area of physics that I'm glad we're getting to."

"This is starting to feel very chem-like."

"There seems to be a lot of information in these sections, I hope you will condense what we need to know in lecture!"

"There's a lot going on with Stefan's law, is there an easy way to remember the details?"

"Will the Physics 205B course be a similar pace to this course?" (The pace will be similar to the second-half of this course, about one chapter a week.)

"Is the thickness L from Fourier's law in the textbook the same as the thickness d used in the presentations? Are they interchangeable?" (Yes, and yes.)

"Will we generally be given the emissivity e for an object? Also, will we be given the Stefan-Boltzmann constant value on quizzes?" (Yes, and yes.)

"Sorry, the holidays are pretty hectic."

"Sorry P-dog, currently on vacayyy."

"Have a great Thanksgiving!"

Physics quiz question: comparing rates of radiated heat

Physics 205A Quiz 7, fall semester 2018
Cuesta College, San Luis Obispo, CA

Two solid blocks with the same temperature, but different surface areas and emissivities are placed in separate, room temperature (290 K) environments. Ignore conduction and convection heat transfers to/from the environment. The __________ object radiates more heat per time to the environment.
(A) smaller, e = 0.1.
(B) larger, e = 0.9.
(C) (There is a tie.)
(D) (Not enough information given.)

Correct answer (highlight to unhide): (B)

The power radiated by an object (where the negative sign indicates that heat is continuously leaving its surface) is given by:

Power = –e·σ·A·T⁴,

where the Stefan-Boltzmann constant σ = 5.670×10^–8 watts/(m²·K⁴) and the surface temperatures T = 600 K are the same for both objects.

For the smaller, lower emissitivity (e = 0.1) object, the power radiated is:

Power_smaller = –(0.1)·σ·A_smaller·(600 K)⁴,

and for the larger, higher emissivity (e = 0.9) object, the power radiated is:

Power_larger = –(0.9)·σ·A_larger·(600 K)⁴,

and from inspection, since 0.1 < 0.9 and A_smaller < A_larger, then the larger, higher emissivity object will radiate heat at a higher power rate than the smaller, lower emissivity object.

Sections 70854, 70855
Exam code: quiz07PeA7
(A) : 16 students
(B) : 32 students
(C) : 2 students
(D) : 0 students

Success level: 64%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.37

Physics quiz archive: temperature, thermal equilibrium, heat transfers

Physics 205A Quiz 7, fall semester 2018
Cuesta College, San Luis Obispo, CA
Sections 70854, 70855, version 1
Exam code: quiz07PeA7



Sections 70854, 70855 results

0- 6 :	** [low = 6]
7-12 :	****
13-18 :	***********
19-24 :	********************** [mean = 20.8 +/- 6.0]
25-30 :	************ [high = 30]

Online reading assignment: heat transfers

Physics 205A, fall semester 2018
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.

"This reading covers transfers of heat. Convection, where heat is transferred by the movement of a fluid. Conduction, where heat is transferred through a material directly. Radiation, where heat is traveled through electromagnetic waves and a look into their applications."

"I learned that conduction is heat transferred through solid objects, convection through fluids, and radiation in the form of light. I also learned that power is measured as the amount of energy transferred per time."

"Fourier's law of conduction seems sort of alright. Power is proportional to the temperature difference ∆T and inversely proportional to thermal resistance R, which itself is proportional to thickness of the material, and inversely proportional to its resistivity and area."

"By increasing the thickness or decreasing the Area of an a wall (material), you decrease heat flow. Heat is conducted based on the modules bouncing around (molecular energy). Metals have more free electrons, which increases this bobbling around (increasing heat conduction). I get that Q is inversely proportional to thickness/length. Also, radiation doesn't require a material medium."

"How different colors (or materials) like the blackbody and the silverbody both absorb and radiate heat differently."

"I understand how to apply Fourier's and Stefan's law to real life scenarios, but not mathematically."

"I wouldn't say I understand this chapter completely. I'm hoping class will resolve this feeling."

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.

"I have never been exposed to the equations that are involved with the different heat exchanges, so of course they will begin to make more sense once I start using them more."

"In particular I don't fully understand thermal resistance."

"I think some of the equations used for each concept might be a bit confusing for me. Other than that I think learning it in class will reinforce the idea."

"Blackbody, silverbody, and radiation were confusing, but now I am fine."

"Kind of confused by radiation. I thought black things heated up fast...but I didn't know they loose heat fast too? Also I think it would be good to see a Stefan's law problem worked out, there's a lot of symbols in there."

"I didn't quite understand Stefan's law and would greatly benefit from an overview of it in lecture."

"The calculations."

"I understood all of the concepts in this section."

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 [88%]
insulation conductivity κ: minimize [69%]
Total surface area A exposed to the outdoors: minimize [79%]

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 [74%]
thermal resistance R of the walls: maximize [77%]

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.	********************************** [34]
silver.	** [2]
(There is a tie.)	* [1]
(Unsure/guessing/lost/help!)	** [2]

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.	******************** [20]
silver.	**************** [16]
(There is a tie.)	* [1]
(Unsure/guessing/lost/help!)	** [2]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"How about more examples of blackbody and silverbody applicable to global warming and the similarities and differences to the albedo effect?"

"Interesting presentation today, I enjoyed it!"

"Heat transfers are 'cool!' If I'm snowboarding in white I will reflect more light but since silverbodies are bad at emitting heat, could I then stay warmer than the dude in all black since blackbodies emit heat better?" (Yes, for a cloudy day, or at night, or in a cave, or whenever absorbing heat from the sun or other sources (like a fireplace) is not a factor.)

Physics quiz question: rate of heat radiated by the sun

Physics 205A Quiz 7, fall semester 2017
Cuesta College, San Luis Obispo, CA

Stars can be modeled as spherical blackbodies that radiate heat out to an environment assumed to have a temperature of 0 K. The sun has a surface temperature of 5,772 K and a surface area of 6.09×10¹⁸ m². The sun radiates heat per time at a rate of:
(A) 3.45×10¹¹ watts.
(B) 1.11×10¹⁵ watts.
(C) 1.99×10¹⁵ watts.
(D) 3.83×10²⁶ watts.

Correct answer (highlight to unhide): (D)

The net power radiated by the sun (where the negative sign indicates that heat is continuously leaving the surface of the sun) is given by:

Power = –e·σ·A·((T)⁴ – (T_env)⁴),

where the emissitivity e = 1 (for an ideal blackbody), the Stefan-Boltzmann constant σ = 5.670×10^–8 watts/(m²·K⁴), the surface area A = 6.09×10¹⁸ m², the surface temperature T = 5,772 K, and the temperature of the environment is assumed to be T_env = 0 K, such that the rate of heat radiated by the sun can be solved for:

Power = –(1)·(5.670×10^–8 watts/(m²·K⁴)·(6.09×10¹⁸ m²)·((5,772 K)⁴ – (0)⁴),

Power = –3.832707188×10²⁶ watts,

or to three significant figures, the rate of hate radiated per time from the sun is 3.83×10²⁶ watts.

(Response (A) is A·σ; response (B) is T⁴; response (C) is e·σ·A·T.)

Sections 70854, 70855
Exam code: quiz07Whu7
(A) : 1 student
(B) : 3 students
(C) : 18 students
(D) : 20 students

Success level: 48%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.66

Physics quiz archive: temperature, thermal equilibrium, heat transfers

Physics 205A Quiz 7, fall semester 2017
Cuesta College, San Luis Obispo, CA
Sections 70854, 70855, version 1
Exam code: quiz07Whu7


Sections 70854, 70855 results

0- 6 :
7-12 :	******* [low = 9]
13-18 :	*************
19-24 :	******************* [mean = 18.9 +/- 5.4]
25-30 :	*** [high = 27]

Online reading assignment: heat transfer applications

Physics 205A, fall semester 2017
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 transfer applications.

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.

"I think the examples provided that show how heat can be applied helped leaven my understanding a bit. I think the best example for me was the 'Coffee Joulies' example because it reminds me a lot of the examples used in the last chapter. Unfortunately I have not yet given myself enough time at look at the textbook."

"That companies try and make products that transfer heat in order to try and make money."

"There are a lot of companies attempting to create products to inhibit or change temperature of objects."

"I learned what we will be doing in lab next week. We will be choosing which claim we will investigate, either testing the cooper cooler method, the coffee joulies method, or the space blanket."

"All of these applications seem really interesting. The most interesting was probably the insulation blanket since that has real world applications that could quite literally save some lives during snowfall. Granted, you would need pants and a jacket."

"There are several different technologies that have been created to either heat something up or cool something down. These consumer products claim to use innovative techniques."

"Examples of conduction, convection, and radiation were shown. The 'Cooper Cooler' is supposed to cool beverages via convection, 'Coffee Joulies' are supposed to cool and maintain coffee temperature at 140° F, and a space blanket is supposed to keep one warm by trapping and reflecting heat back to the user."

"I understand the idea of heat transfer, the different types of heat transfer, and energy transfer in this chapter. The various examples that we went over in class last Monday were very helpful in solidifying my understanding."

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.

"I'm still a little confused on the difference between convection, condition and radiation. I think I would benefit from some in class examples."

"I found the 'Coffee Joulies' to be confusing on how they work."

"Conduction vs. convection."

"Not very confused. Convection is the most difficult of the three heat transfers, though."

"Not much, it's mostly chemistry review."

The primary heat transfer process that the Cooper Cooler™ uses to chill beverages is:

conduction.	*************** [15]
convection.	******************* [19]
radiation.	* [1]
(Unsure/guessing/lost/help!)	* [1]

How plausible do you think these claims for the Cooper Cooler™ are?
(Only modal responses shown.)

"Chills beverages on demand forty times faster than a freezer" : somewhat plausible [50%]
"Because it's spinning and not shaking your carbonated beverages, you don't have to worry about them exploding" : not very plausible [33%]

The primary heat transfer process that the Coffee Joulies™ uses to moderate and maintain coffee temperatures is:

conduction.	*********************** [23]
convection.	*********** [11]
radiation.	* [1]
(Unsure/guessing/lost/help!)	* [1]

How plausible do you think these claims for Coffee Joulies™ are?
(Only modal responses shown.)

"One 'bean' for every four ounces of coffee cools right down to 140° F in a few seconds": somewhat plausible [47%]
"Keeps coffee at 140° for two full hours": not very plausible [50%]

The primary heat loss process that a reflective space blanket is intended to minimize is:

conduction.	******* [7]
convection.	***** [5]
radiation.	********************** [22]
(Unsure/guessing/lost/help!)	** [2]

How plausible do you think these claims for a space blanket are?
(Only modal responses shown.)

"It reflects your body heat back to you" : somewhat plausible [67%]
"You still must have insulation between you and the blanket to minimize conductive heat loss" : very plausible [47%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"Is a practice final being posted or done in class?" (It's already posted under the "Final Exam" goals page.)

"Is it possible to have the correct answer of the first problem from the last midterm?" (It's already been posted under the "Midterm 2" goals page.)

"When did you say the grades would be updated before the final? Was it this coming Friday?" (This coming Saturday. However, your lab grades may not include the last few assignments, but that will be noted if that happens.)

Online reading assignment: heat transfers

Physics 205A, fall semester 2017
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.

"I understand that the difference in colors changes the amount of radiative heat that is absorbed."

"I understand the three forms of heat transfers: conduction, convection, and radiation."

"The power through a wall is proportional to the temperature difference.As per the zeroth law of thermodynamics, heat flows from high to low temperatures!"

"Convection is the process of fluids carrying heat and conduction is heat passing directly through an object. The power of heat is stronger with a greater change in temperature and minimal resistance. Radiation is energy carried by electromagnetic waves."

"The first thing I learned is that heating chocolate bunnies is extremely funny for some reason. I also learned that convection, conduction and radiation are types of heat transfers that transport heat. Convection uses circulating air, conduction transfers through an object and radiation is in the form of light."

"There are different types of heat transfers conduction, convection and radiation. In conduction, heat is transferred through an object, e.g. when your mom says don’t touch the stove because it’s hot but you really want that mac-n-cheese and you touch it anyway and burn yourself on the stove. In convection, heat transfers with the circulation of air, e.g. old fashioned radiator it takes in cool air at the bottom and produces hot air through the top. In radiation heat is transported in the form of light, e.g. soaking in the sun on a nice beach day (I miss summer)."

"Conduction is when heat is transferred through an object; convection is when heat is circulated in the air; radiation is when heat is transferred as light."

"Insulation thickness makes it more resistant to heat. The conductivity is the opposite, transferring heat faster through the material."

"I have noted and understand the differences between convection, conduction, and radiation. Blackbodies are good absorbers and therefore are good emitters. Emissivity is a dimensionless number between 0 and 1 that is the ratio of the energy an object actually radiates to what it would radiate."

"Black absorbs; white reflects."

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.

"I found the equations confusing."

"Fourier's law confuses me."

"I am a little confused about conductivity and how it affects thermal resistance. Im sure once its explained in class it will be much clearer."

"I haven't given myself quite enough time to figure out what exactly don't understand because everything seems a little confusing. I do think that maybe i'd like to better understand thermal resistance."

"All and all in made sense. The part that somewhat got confusing was the formulas. More specifically with the applications. What each of the variables mean."

"I found Stepfan's law more confusing. I would appreciate some review on how this works."

"Colors affecting radiative absorption?"

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 [82%]
insulation conductivity κ: minimize [64%]
Total surface area A exposed to the outdoors: minimize [61%]

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 [63%]
thermal resistance R of the walls: maximize [72%]

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.	*************************** [27]
silver.	**** [4]
(There is a tie.)	[0]
(Unsure/guessing/lost/help!)	** [2]

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.	****************** [18]
silver.	*********** [11]
(There is a tie.)	** [2]
(Unsure/guessing/lost/help!)	** [2]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"Everything progressively gets more interesting. I'm excited for Physics 205B!"

"As a welder I work with heat quite a bit."

"Is the zeroth law of thermodynamic is the equilibrium of temperatures?" (Yes. Surprisingly, it is not a trivial law.)

"Does heat always flow from high temperatures to low temperatures?" (Yes, all by itself. However, if you want heat to flow from low temperatures to high temperatures (in order to cool down your refrigerator, or use air conditioning to cool down your house), then you will need to spend energy in the form of work to move heat "opposite of the way it wants to go." Also heat pumps will move heat from low temperatures to high temperatures in order to extract energy from the cool environment to heat your house, but this again requires you to spend energy in the form of work to move heat "opposite of the way it wants to go.")

"You know me just a run down of the formulas would be greatttttt." (Mmmmkay.)

Physics final exam question: smaller, hotter, and more luminous star

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

Stars can be modeled as spherical blackbodies (emissivity 1.0) that radiate heat out to an environment assumed to have a temperature of 0 K. Discuss why a smaller star must have a higher temperature than a larger star in order to radiate more heat per time than the larger star. Explain your reasoning using the properties of temperature and radiative heat transfer.

Solution and grading rubric:

• p:
  Correct. Discusses:
  1. that the size of the smaller star (if the only differing factor) would result in a lower rate of heat radiation per time;
  2. that the smaller size must then have a hotter temperature to result in a higher rate of heat radiation of per time.
• r:
  As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes. At least compares both surface area and temperature values. (May also have set the rate of heat radiated per time equal for the two stars.)
• t:
  Nearly correct, but argument has conceptual errors, or is incomplete. Typically only compares just surface area, or just temperature.
• v:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Some garbled attempt at applying Stefan's law of radiation.
• x:
  Implementation of ideas, but credit given for effort rather than merit. Approach other than that of applying Stefan's law of radiation.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.

Grading distribution:
Sections 70854, 70855, 73320
Exam code: finali0w4
p: 37 students
r: 8 students
t: 1 student
v: 1 student
x: 2 students
y: 0 students
z: 0 students

A sample "p" response (from student 0424):
Another sample "p" response (from student 6436):

Physics quiz question: radiating concrete columns cooling

Physics 205A Quiz 7, fall semester 2016
Cuesta College, San Luis Obispo, CA

"PKGarage"
Kelly Nighan
flic.kr/p/diq5GV

Two concrete columns (emissivity 0.85[*]) are at a temperature of 293 K, and are exposed all around their sides to an environment at a temperature of 273 K. The two columns have the same cross-sectional area, but the shorter column is one-half the length of the taller column. The __________ concrete column cools off faster due to the net amount of radiative heat per time transferred to/from the environment.
(A) shorter.
(B) taller.
(C) (There is a tie.)
(D) (Not enough information is given.)

[*] engineeringtoolbox.com/emissivity-coefficients-d_447.html.

Correct answer (highlight to unhide): (B)

The net power (rate of heat per time) transferred by radiation is given by:

Power = –e·σ·A·((T_obj)⁴ – (T_env)⁴),

Since both the short and tall concrete columns have the same emissivity e and temperature T_obj), and are both surrounded by the same environment temperature T_env), the net rate of heat simultaneously radiated to and absorbed from the environment them differs only because of their surface areas:

Power_short = –(0.85)·σ·A_short·((293 K)⁴ – (273 K)⁴),

Power_tall = –(0.85)·σ·A_tall·((293 K)⁴ – (273 K)⁴).

Since the taller column has more surface area (A_tall > A_short), then the taller column will have a greater net rate of heat radiated per time, thus cooling off faster than the shorter column (Power_tall > Power_short).

(Note the negative values for power here, corresponding to a net amount of heat being removed (radiated) from the object, while a positive value corresponds to a net amount of heat being put into (absorbed) by the object (in order to be consistent with the ±Q convention for removing heat from (–) or putting heat into (+) a thermodynamic system).)

Sections 70854, 70855, 73320
Exam code: quiz07p4sT
(A) : 14 students
(B) : 23 students
(C) : 15 students
(D) : 0 students

Success level: 44%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.72

Physics quiz archive: temperature, thermal equilibrium, heat transfer

Physics 205A Quiz 7, fall semester 2016
Cuesta College, San Luis Obispo, CA
Sections 70854, 70855, 73320, version 1
Exam code: quiz07p4sT



Sections 70854, 70855, 73320 results

0- 6 :	* [low = 6]
7-12 :	*************
13-18 :	***************
19-24 :	*************** [mean = 18.5 +/- 6.6]
25-30 :	******** [high = 30]

Online reading assignment: heat transfer applications

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 transfer applications.

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.

"How the Coffee Joulies work--their melting point is 140 and they turn into liquid and absorb the heat from the coffee and release it back when it begins getting cooler."

"We will be testing out some stuff related to the transfer of heat between different objects. I know that heat goes from hot to cold, so I am curious to see how the different things mentioned perform when tested."

"A Cooper Cooler chills wine in six minutes by spraying the bottle with ice water transferring heat to the ice water from the bottle. Coffee Joulies keep coffee at a comfortable 140° because the material inside the joulies has a melting temp of 140°. The solid material absorbs the heat of the coffee and releases a little bit at a time as the environment removes heat from the coffee."

"I've heard about space blankets before and how they are so light weight and not even expensive. They are a heat-reflective thin plastic sheet."

"That you lose a lot of body height from emitting it so an emergency space blanket should be silver."

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.

"There are no equations so I am confused if this will mostly be conceptual rather than a numerical section?"

"I am confused about carbonation when using a Cooper Cooler. Why does pressure build in a soda can when you shake it and not when you spin it?"

"I'm a little confused how spinning a bottle is any different than shaking it. It seems like the drink would still explode because it was still shaken up."

"How do I know it's conduction, convection or radiation for the questions below? I wasn't sure on how to know."

"I don't really understand how something can cool off faster by being spun and sprayed. It doesn't seem like it should work that well."

"It seems super interesting this method of adding Coffee Joulies to cool down your hot coffee while keeping it at a constant temperature. I don't fully understand how the coffee joulies work though and the chemical processes behind it."

The primary heat transfer process that the Cooper Cooler™ uses to chill beverages is:

conduction.	***************** [17]
convection.	************** [14]
radiation.	[0]
(Unsure/guessing/lost/help!)	**** [4]

How plausible do you think these claims for the Cooper Cooler™ are?
(Only modal responses shown.)

"The Cooper Cooler™ can chill a soda in one minute": somewhat plausible [43%]
"Because it's spinning and not shaking your carbonated beverages, you don't have to worry about them exploding": not very plausible [43%]

The primary heat transfer process that the Coffee Joulies™ uses to moderate and maintain coffee temperatures is:

conduction.	********************** [22]
convection.	********* [9]
radiation.	** [2]
(Unsure/guessing/lost/help!)	** [2]

How plausible do you think these claims for Coffee Joulies™ are?
(Only modal responses shown.)

"One 'bean' for every four ounces of coffee cools right down to 140° in a few seconds": somewhat plausible [49%]
"Keeps coffee at 140° for two full hours": not very plausible [37%]

The primary heat loss process that any blanket (regardless of type) should prevent in typical "emergency survival conditions" (on Earth) is:

conduction.	**** [4]
convection.	******* [7]
radiation.	******************** [20]
(Unsure/guessing/lost/help!)	**** [4]

How plausible do you think heavy-duty garbage bag material will be just as effective as a space blanket for typical "emergency survival conditions" on Earth?

Implausible.	***** [6]
Not very plausible.	******* [7]
Somewhat plausible.	************** [14]
Very plausible.	***** [5]
(Unsure/guessing/lost/help!)	[3]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"The negative sign that is applied to the power equation for radiation--is that strictly for when energy is taken from the object?" (Yes, since a positive sign is for when energy (heat) is given to an object.)

"It says the emergency space blankets are perfect for space because radiative heat loss is the primary means of heat loss. What is the primary means of heat loss on Earth?" (Medical research says for a patient on an operating room table, it's radiation. But if you're wearing clothes, then you've minimized radiation and conduction (by covering up exposed skin and adding layers), and then you would need to minimize convection by sheltering yourself from the wind, and wearing a hat on your head. Heat loss via evaporation is an additional factor if you are perspiring (or even just exhaling), and are surrounded by dry (and cooler) air.)

"Could you just use a few extra trash bags instead, and it would be just as effective as a space blanket?" (Let's test this in lab, before you need to resort to this in the wilderness.)

"Would silverbody blankets be ideal for space?" (NASA seems to think so. And they use gold foil instead of aluminum foil, because, reasons.)

"I might need this Cooper Cooler if it really works." (Let's test this in lab, before you ask Santa for one of these.)

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).)