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.)