## 20141202

### Online reading assignment: heat transfers

Physics 205A, fall semester 2014
Cuesta College, San Luis Obispo, CA

Students have a 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.
"Conduction is heat transferred through an object, convection is heat circulating through the air, and radiation is heat transported through light."

"It makes sense that heat per time flows from high to low and is inversely proportional to thermal resistance, and proportional to the difference in temperature. The lava lamp made sense to me about the globules moving upwards because the light bulb is at the bottom, and then they come back down because they are wax and they cool down and increase their density which brings them back down."

"I don't understand anything from the reading."

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 idea of 'radiation.' When I think of this, I think of heat being radiated directly off a surface, such a heat from a spacecraft going into space. But it seems that radiation in this context only refers to heat transfer in the form of light."

"I found emissivity confusing and why e = 0 or why e = 1 and what the constant in the equation for Stefan's law is when we are told what emissivity equals depending on whether it is a blackbody or silverbody."

"Possibly all of the material. I was unable to get to the reading, so I am not completely sure."

"I believe I have understood everything!"

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 [80%]
insulation conductivity κ: minimize [52%]
Total surface area A exposed to the outdoors: minimize [59%]

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 [70%]
thermal resistance R of the walls: maximize [67%]

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. ******************************************** [44] silver. ** [2] (There is a tie.) ** [2] (Unsure/guessing/lost/help!) ****** [6]

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. ************************** [26] silver. ************ [12] (There is a tie.) ***** [5] (Unsure/guessing/lost/help!) *********** [11]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Stefan's law looks like one giant cluster of confusion to me." (I will try to unconfuse it for you.)

"Where is there an additional negative sign in Stefan's law?" (In order to make this equation consistent with heat being added to an object when it absorbs light (positive heat flow), and heat being removed from an object when it emits light (negative heat flow).)

"Is power in watts defined the same as electrical watts? (Like, my hair dryer is 1,875 watts, are they the same?)" (Yes, watts measures the output of joules of energy per time. So compare the forced convection heat output of your hair dryer to the radiative output of say, a 100 watt light bulb! Hair gotta dry.)

"Why does color affect absorption so much?" (A silverbody has surface atoms and bonds with few available electron energy level spacings to absorb or give off photons, so there is no way to readily convert light into thermal energy, but this also means that there is no way to readily convert thermal energy into light. A blackbody has surface atoms and bonds with many available electron energy level spacings that can absorb or emit photons, so it can readily convert light into thermal energy, and also readily convert thermal energy into light.)

"Damn, I need to go snowboarding again." (Just don't be one of those snowboarders that sits down in the middle of the run for a smoke break. I hate those guys.)