## 20151202

### Online reading assignment: heat transfers

Physics 205A, fall semester 2015
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 clearly understand the heat transfers of convection, conduction and radiation. I also understand the difference between blackbody, which is good at absorbing heat and emitting heat. Where as silver body is good at reflecting light but poor at absorbing heat."

"I understand that heat conduction is where heat is transferred through an object. Also, that convection is the transport of heat via circulating air. And that radiation is where heat is transported in the form of light."

"Conduction is heat transferred through an object. Power is the measurement of this transferred heat per unit time. Expressed either in joules/second or watts. Resistance is the measurement of how difficult this transfer of heat is. It is proportional to the materials thickness. Convection is the transfer of heat through masses of moving air. Radiation is the transfer of hear through light. Objects that are good at absorbing radiation heat must also be good at emitting or radiating heat."

"Not much."

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.
"Stefan's law. I understand what the law is, I'm just unclear about using it in a problem."

"No specific confusion yet, though I'm expecting to run into more after class discussion and examples."

"The only concepts I don't understand are the equations themselves. Everything else is a matter of practice. The concepts can be a little confusing, but I understand the basics."

"The language in describing the concepts is frustrating."

"I'm confused about the whole online presentation. There is a lot of equations with a whole bunch of variables that it is making it impossible to understand."

"What the constants represent in these equations and why they're important. Why does power use κ and convection use σ?"

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 [89%]
insulation conductivity κ: minimize [67%]
Total surface area A exposed to the outdoors: minimize [77%]

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 [75%]
thermal resistance R of the walls: maximize [84%]

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. ********************************************** [46] silver. **** [4] (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. *********************************** [35] silver. ***************** [17] (There is a tie.) *** [3] (Unsure/guessing/lost/help!) ** [2]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"I am confused why black releases heat faster but absorbs heat fast as well. How could this happen?" (Radiation/absorption is a two-way street. It would be very odd for a material to absorb heat, but not be able to radiate heat. That material would basically start absorbing all the heat in the universe yet not be able to re-release it, thus becoming infinitely hot, while the rest of the universe reaches absolute zero. So it's a good thing that a blackbody is an efficient radiator as well as being an efficient absorber.)

"Why would they paint the entire SR-71 Blackbird black if it causes the entire object to absorb heat quicker? Wouldn't it be more effective to only paint the area around the engines black to radiate heat and the rest silver to resist an increase in temperature?" (That would be true if the engines were the only hot part of the SR-71 Blackbird and that heating up from absorbing sunlight were important factors, but due to how fast it flies, the amount of air continuously ramming into it heats the whole aircraft up such that radiating heat from all of its black-painted surfaces is the only way to make sure the entire thing doesn't fall apart from melting.)

"Why do snowboarders and skiers wear tinted goggles?" (Bigger question: why do snowboarders take smoke breaks right in the middle of a run? #whatsupwiththat)

"Are we going to have to test our calligraphy skills and write that fancy script "P" every time we use Fourier's law or Stefan's law?" (Yes, unless you promise not to confuse "power P" with "pressure P," momentum "p," or "density ρ.")

"The melting chocolate bunnies was somewhat disturbing."

"Starting to wreck this class! #yolo"