## 20161130

### 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).)