## 20191127

### 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!"