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!"

Physics midterm question: comparing net force for afloat vs. submerged sinking block

Physics 205A Midterm 2, fall semester 2019
Cuesta College, San Luis Obispo, CA

A solid object is (a) partially submerged in water as it sinks with increasing speed, then while (b) completely underwater it still sinks with increasing speed. Discuss why the magnitude of the net force on the object is greater for case (a) than for case (b). Ignore friction and drag. Explain your reasoning using the properties of Newton's laws, Archimedes' principle (buoyant forces), and free-body diagrams.

Solution and grading rubric:

• p:
  Correct. Recognizes that:
  1. each block (a) or (b) has two vertical forces acting on it:

     Weight force of Earth on block (downwards, magnitude w = m⋅g, same for both (a) and (b)),
     Buoyant force of water on block (upwards, magnitude F_B = ρ_water⋅g⋅V_sub, less for (a)); and

  2. block (a) has a downwards weight force, and an upwards buoyant force much less than the magnitude of the weight force; and
  3. block (b) has the same downwards weight force as (a), also with an upwards buoyant force less than the magnitude of the weight force, but with a magnitude greater than the magnitude of the buoyant force in (a) (as more volume is submerged); and
  4. from Newton's second law, the downwards net force for (a) has a greater magnitude than the downwards net force for (b), as demonstrated by either explicit comparison of vector lengths and/or comparing terms in ΣF = +F_B − w equations for each case.
  May either draw a free-body diagram, and/or discuss these forces and Newton's laws in words.
• r:
  As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes. Typically does not explicitly demonstrate Newton's second law via vector addition (different up vectors drawn much less than, or a little less than the same down vector for each case; and/or comparing same/different quantities in ΣF = +F_B − w equations for each case).
• t:
  Nearly correct, but argument has conceptual errors, or is incomplete. At least recognizes that the object has a greater buoyant force once it is fully submerged.
• v:
  imited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Some constructive attempt at relating the buoyant force to the density of the fluid and volume displaced (Archimedes' principle) and/or Newton's first law.
• x:
  Implementation/application of ideas, but credit given for effort rather than merit. Appeals to some other properties of fluids and densities other than Archimedes' principle and Newton's laws.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.
  

Grading distribution:
Sections 70854, 70855
Exam code: midterm02sQm5
p: 13 students
r: 12 students
t: 8 students
v: 15 students
x: 4 students
y: 0 students
z: 0 students

A sample "p" response (from student 3372):

Physics quiz question: partially submerged block resting on bottom

Physics 205A Quiz 5, fall semester 2019
Cuesta College, San Luis Obispo, CA

A solid block has 75% of its volume below water, while resting on the bottom of a water tank. The force with the largest magnitude is the:
(A) weight force of Earth on the block.
(B) buoyant force of water on the block.
(C) normal force of tank bottom on the block.
(D) (There is a tie.)
(E) (Not enough information is given.)

Correct answer (highlight to unhide): (A)

The block has three vertical forces acting on it:

Weight force of Earth on block (downwards, magnitude w = m·g).
Buoyant force of water on block (upwards, magnitude F_B = ρ_fluid·g·(Volume submerged)).
Normal force of tank bottom on block (upwards, magnitude N).

Because the block is stationary in the vertical direction, from Newton's first law all of the up and down forces must sum to zero. This means that the two upwards forces (buoyant force and normal force) are together equal to the one downwards force (weight), such that the weight force has the largest magnitude of these three forces.

Sections 70854, 70855
Exam code: quiz05Gu1L
(A) : 30 students
(B) : 2 students
(C) : 5 students
(D) : 14 students
(E) : 0 students

Success level: 58%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.67

Physics quiz archive: rotations, torque, pressure, buoyancy, fluid flow

Physics 205A Quiz 5, fall semester 2019
Cuesta College, San Luis Obispo, CA
Sections 70854, 70855, version 1
Exam code: quiz05Gu1L



Sections 70854, 70855 results

0- 6 :	** [low = 6]
7-12 :	*********
13-18 :	***********
19-24 :	***************** [mean = 19.8 +/- 6.3]
25-30 :	************* [high = 27]

Online reading assignment: static fluids

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 static fluids.


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.

"Mass density is mass of a substance divided by its volume."

"Pressure as force density is force divided by area. Pressure as energy density is energy divided by volume."

"The concept of pressure being force over surface area. As well as energy density conservation. If the surrounding pressure of an object increases then the ρ·g·∆y of the object will decrease and vice versa. As for the buoyant force, it's all dependent on the object's volume and the density of the fluid it is submerged in."

"That pressure and gravitational potential energy have an inverse relationship. So, for example, as a submarine goes further underwater in the y direction, its pressure increases while its gravitational potential energy decreases. The opposite is true for a balloon flying into the sky."

"In the example of a swimmer fully submerged underwater, I understand the application of Newton's first law in that all the forces acting on the swimmer balance out. This is given by the two forces of a downward weight force and upwards buoyant force balancing out."

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.

"After going through the presentation preview, I was confused about the fluid density at first but then took another glance and realized that it is simply the kilograms divided by meters cubed because it is a 3D object it must be cubed."

"Something I didn't understand from the reading is pressure and depth in a static fluid. I don't understand the formula. I need an example of how to use it and what the variables mean."

"I was a little confused about the concept of buoyancy. I could definitely use some review of that equation."

"Archimedes' principle is a little confusing. When we draw our diagrams do we treat it as we would a normal force? Also, I feel like the book did a bad job at explaining some of this stuff. None of it seems too difficult by any means."

"The units and some equations that you use when looking at the problems. Hopefully will go over in class to clarify."

What is the numerical value for atmospheric pressure (P_atm, at sea level), in units of Pa?

"101,325 Pa."

"1.013 × 10⁵ Pa, which is also 1 atm."

To three significant digits, what is the numerical value for the density of water, in units of kg/m³?

"1,000 kg/m³."

To two significant digits, what is the numerical value for the density of air (at 20° C), in units of kg/m³?

"1.2 kg/m³."

For the air pressure surrounding the balloon as it rises from ground level to the upper atmosphere, indicate the changes in each of the energy density forms of the atmosphere.
(Only correct responses shown.)

ρ_air·g·∆y: increases [61%]
∆P: decreases [56%]

For the water pressure that surrounded these cups as they were taken deep underwater, indicate the changes in each of the energy density forms of the water.
(Only correct responses shown.)

ρ_water·g·∆y: decreases [44%]
∆P: increases [66%]

For the submerged diver floating underwater, Newton's __________ law applies, and the (downwards) weight force and (upwards) buoyant force on the diver are __________.

first; balanced.	******************************** [32]
second; unbalanced.	****** [6]
(Unsure/lost/guessing/help!)	*** [3]

Using ρ·g·V, the density of the __________ should be included in the calculation of the magnitude of the buoyant force on the diver.

diver.	*********** [11]
water.	*************************** [27]
(Unsure/lost/guessing/help!)	*** [3]

For the red ship (barely) afloat, Newton's __________ law applies, and its (downwards) weight force, the (downwards) oil platform's weight force, and the (upwards) buoyant force on the red ship are __________.

first; balanced.	**************************** [28]
second; unbalanced.	********** [10]
(Unsure/lost/guessing/help!)	*** [3]

Using ρ·g·V, the density of __________ should be included in the calculation of the magnitude of the buoyant force on the red ship.

seawater.	*********************** [23]
air.	** [2]
red ship.	************ [12]
(Unsure/lost/guessing/help!)	**** [4]

Using ρ·g·V, the volume of the red ship's __________ should be included in the calculation of the magnitude of the buoyant force on the red ship.

underwater portion.	*********************** [23]
above water portion.	** [2]
total volume, both underwater and above water.	************* [13]
(Unsure/lost/guessing/help!)	[3]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"Please go over these!"

"I would love if we could spend a generous amount of time calculating different pressures."

"Yikes! These were challenging for me. Hopefully I will feel better about this material after lecture."

"I do not understand the concept behind the red ship's buoyancy and I am having a hard time understanding the reasoning behind the formulas."

"Are we given the equations on the tests?" (Yes--you can see which equations were given on past quizzes and exams, so you wouldn't need to memorize those.)

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.)

Physics midterm question: floating ebony-balsa wood cubes

Physics 205A Midterm 2, fall semester 2018
Cuesta College, San Luis Obispo, CA

A wooden cube is made by gluing ebony (denser) and balsa (less dense) pieces together. Both pieces have the same volume. The total density of the cube is less than that of water. The cube is carefully placed into water such that it floats "top-heavy" (ebony on top of balsa). The cube is then turned over such that it floats "bottom-heavy" (balsa on top of ebony). Discuss which orientation will float higher (or if there is tie), and why. (Ignore any water that may soak into the wood pieces, and the thin layer of glue between the two wood pieces.) Explain your reasoning using the properties of densities, volumes, forces, Newton's laws, Archimedes' principle (buoyant forces), and free-body diagrams.

Solution and grading rubric:

• p:
  Correct. Recognizes that:
  1. each block ("bottom-heavy" or "top-heavy") has two vertical forces acting on it:

     Weight force of Earth on block (downwards, magnitude w = m⋅g),
     Buoyant force of water on block (upwards, magnitude F_B = ρ_water⋅g⋅V_sub);

     and
  2. because each block ("bottom-heavy" or "top-heavy") is stationary in the vertical direction, then its downwards weight force must have the same magnitude as its upwards buoyant force, due to Newton's first law; and
  3. since the mass of each block ("bottom-heavy" or "top-heavy") does not matter which type of wood is stacked above the other, the magnitude of the weight is the same, making the magnitudes of the buoyant forces the same; such that
  4. the amount submerged volume underwater for both blocks must be the same.
  
  Thus the buoyant forces on each block ("bottom-heavy" or "top-heavy") are equal, and thus the amount of volume submerged for either block must be the same.
• r:
  As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes. May somehow claim that the cube will float differently when "bottom-heavy" or "top-heavy," or does not explicitly conclude that the cube will float at the same water level whether "bottom-heavy" or "top-heavy."
• t:
  Nearly correct, but argument has conceptual errors, or is incomplete. At least recognizes that the weight force on the block is unchanged whether "bottom-heavy" or "top-heavy," but somehow has different buoyant forces acting (thus Newton's first law would not apply to at least one of the blocks); or has different weights and different buoyant forces acting on the blocks, but for each block these forces are balanced via Newton's first law.
• v:
  imited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Some constructive attempt at relating the buoyant force to the density of the fluid and volume displaced (Archimedes' principle) and/or Newton's first law.
• x:
  Implementation/application of ideas, but credit given for effort rather than merit. Appeals to some other properties of fluids and densities other than Archimedes' principle and Newton's laws.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.
  

Grading distribution:
Sections 70854, 70855
p: 21 students
r: 17 students
t: 13 students
v: 6 students
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 0921):

Physics quiz archive: rotations, torque, pressure, buoyancy, fluid flow

Physics 205A Quiz 5, fall semester 2018
Cuesta College, San Luis Obispo, CA
Sections 70854, 70855, version 1
Exam code: quiz05Ro74



Sections 70854, 70855 results

0- 6 :
7-12 :	******** [low = 9]
13-18 :	**************
19-24 :	************************* [mean = 19.6 +/- 5.2]
25-30 :	***** [high = 30]

Online reading assignment: static fluids

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 static fluids.


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.

"Pressure as a force density is pressure equal force divided by area. I also understood that pressure as a energy is pressure equals energy divided by volume."

"For energy density conservation, the change in pressure and the gravitational potential energy density balance each other and equal 0. If an object is floating while submerged in water, Newton's first law applies and the weight force of the object and the upward buoyant force are equal."

"Pressure is a force per unit density with units of Pa (pascals), and we can think of it as energy per unit of volume. And since its energy/unit volume we can compare it to PE_grav per unit volume. We learned a new force (buoyant force which we can calculate by the equation (F_B = ρ·g·V). For an object that is fully submerged (and floating underwater), Newton's first law applies because the downwards weight force and upwards buoyant force cancel out."

"Yay Newton's laws again! They really must be legit if they even work underwater! The new fancy 'p' (ρ) is fluid density."

"The volume of an object when calculating buoyancy needs to be the portion of the object that is submerged underwater."

"I am sorry about this sir, but unfortunately I did not have enough time to preview the online presentation. I will have to choose 'Honestly, I just didn't get to it (yet).'"

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 didn't understand ρ·g·Δy."

"I was confused by the equation for pressure and why ΔP and the rest of the equation need to have opposite signs."

"The expanding of the weather ballon and shrinkage of the cups."

"That buoyant force only depends on the density of the liquid and not the density of the submerged or floating object."

"I found buoyancy kind of confusing in the way that I don't quite know what the whole concept of it is. It involves the density of the fluid and the volume of the object but I don't understand how those interact."

"The buoyant force is confusing for me."

"I don't really understand much about buoyancy. I am guessing it increases the deeper an object is under a liquid?"

"How would use these different equation on a test problem."

"I found most of this information confusing."

"I don't really have any questions."

What is the numerical value for atmospheric pressure (P_atm, at sea level), in units of Pa?

"101,325 Pa."

"1.013×10⁵ Pa."

"14.70 pounds per square inch?"

"0?"

"Giga?"

To three significant digits, what is the numerical value for the density of water, in units of kg/m³?

"1.00×10³ kg/m³."

"1,000.00 kg/m³?"

"1,000 kg/m³? 1.00×10³ kg/m³? or 0.001×10⁶ kg/m³? Not really sure how to go about getting three significant figures."

"0.333?"

To two significant digits, what is the numerical value for the density of air (at 20° C), in units of kg/m³?

"1.3 kg/m³."

"1.29 kg/m³."

"1.2754 kg/m³."

"5?"

For the air pressure surrounding the balloon as it rises from ground level to the upper atmosphere, indicate the changes in each of the energy density forms of the atmosphere.
(Only correct responses shown.)

ρ_air·g·∆y: increases [73%]
∆P: decreases [73%]

For the water pressure that surrounded these cups as they were taken deep underwater, indicate the changes in each of the energy density forms of the water.
(Only correct responses shown.)

ρ_water·g·∆y: decreases [56%]
∆P: increases [73%]

For the submerged diver floating underwater, Newton's __________ law applies, and the (downwards) weight force and (upwards) buoyant force on the diver are __________.

first; balanced.	********************************** [34]
second; unbalanced.	****** [6]
(Unsure/lost/guessing/help!)	**** [4]

Using ρ·g·V, the density of the __________ should be included in the calculation of the magnitude of the buoyant force on the diver.

diver.	******** [8]
water.	******************************* [31]
(Unsure/lost/guessing/help!)	***** [5]

For the red ship (barely) afloat, Newton's __________ law applies, and its (downwards) weight force, the (downwards) oil platform's weight force, and the (upwards) buoyant force on the red ship are __________.

first; balanced.	************************ [24]
second; unbalanced.	******************* [14]
(Unsure/lost/guessing/help!)	****** [6]

Using ρ·g·V, the density of __________ should be included in the calculation of the magnitude of the buoyant force on the red ship.

seawater.	********************************* [33]
air.	* [1]
red ship.	***** [5]
(Unsure/lost/guessing/help!)	***** [5]

Using ρ·g·V, the volume of the red ship's __________ should be included in the calculation of the magnitude of the buoyant force on the red ship.

underwater portion.	*********************** [23]
above water portion.	****** [6]
total volume, both underwater and above water.	********** [10]
(Unsure/lost/guessing/help!)	***** [5]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"In the energy density conservation equation, pressure and gravitational potential energy will always correspond to each other, right? So does that mean if one decreases the other would have to increase because it cancels out?" (Yes.)

"I don't understand the energy density conservation. I understand that one change has to be negative and one change has to positive to cancel each other out, but how do you know when one is positive or negative?" (PE_grav depends on y, such that if you go higher or lower, then Δy will be positive (for increasing height) or negative (for decreasing height).)

"I felt like I understood that as elevation increases so does gravitational potential energy density, but I was wondering if that is also true in the submarine example. I would think so because pressure increases the deeper you go underwater but does PE_grav increase as well?" (Since the submarine descends to a lower level underwater, then PE_grav decreases, such that ΔPE_grav will be negative (making ΔP positive, and so pressure increases the deeper the submarine goes underwater.)

"So if something is floating would that make it applicable to Newton's first law or is that something completely different?" (If it is floating and stationary (not sinking or rising), then Newton's first law must apply.)

"I'm confused as to which density (that of the diver or that of the water) should be used for the diver completely underwater." (The density of water, which is the fluid surrounding the diver. The buoyancy force on an object is exerted from the stuff the object is (partially/fully) submerged in.)

"For an object that was completely submerged and floating underwater that Newton's first law applies because the downwards weight force and upwards buoyancy force balance out. So for an object that is 'partially' submerged, is that considered a Newton's second law case? Since one force clearly has to be greater than the other, or else the object would be 'fully' submerged." (No, since the partially submerged object is still floating (and is stationary, so its motion is constant, then Newton's first law still applies.)

"Why is it that some people float on water and others don't?" (The surrounding fluid is not able to exert enough buoyancy force on some people to support them, even when they fully submerged.)

"Can you do more questions in class?" (We will have time for that today.)

"I kind of just took my best guess at the questions above." (Don't worry, you still get full credit for trying and completing this assignment.)

"Sorry dude but I couldn't do the reading assignment because this part of the semester is too difficult to manage."

Online reading assignment: heat transfers

Physics 205A, fall semester 2017
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 understand that the difference in colors changes the amount of radiative heat that is absorbed."

"I understand the three forms of heat transfers: conduction, convection, and radiation."

"The power through a wall is proportional to the temperature difference.As per the zeroth law of thermodynamics, heat flows from high to low temperatures!"

"Convection is the process of fluids carrying heat and conduction is heat passing directly through an object. The power of heat is stronger with a greater change in temperature and minimal resistance. Radiation is energy carried by electromagnetic waves."

"The first thing I learned is that heating chocolate bunnies is extremely funny for some reason. I also learned that convection, conduction and radiation are types of heat transfers that transport heat. Convection uses circulating air, conduction transfers through an object and radiation is in the form of light."

"There are different types of heat transfers conduction, convection and radiation. In conduction, heat is transferred through an object, e.g. when your mom says don’t touch the stove because it’s hot but you really want that mac-n-cheese and you touch it anyway and burn yourself on the stove. In convection, heat transfers with the circulation of air, e.g. old fashioned radiator it takes in cool air at the bottom and produces hot air through the top. In radiation heat is transported in the form of light, e.g. soaking in the sun on a nice beach day (I miss summer)."

"Conduction is when heat is transferred through an object; convection is when heat is circulated in the air; radiation is when heat is transferred as light."

"Insulation thickness makes it more resistant to heat. The conductivity is the opposite, transferring heat faster through the material."

"I have noted and understand the differences between convection, conduction, and radiation. Blackbodies are good absorbers and therefore are good emitters. Emissivity is a dimensionless number between 0 and 1 that is the ratio of the energy an object actually radiates to what it would radiate."

"Black absorbs; white reflects."

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 found the equations confusing."

"Fourier's law confuses me."

"I am a little confused about conductivity and how it affects thermal resistance. Im sure once its explained in class it will be much clearer."

"I haven't given myself quite enough time to figure out what exactly don't understand because everything seems a little confusing. I do think that maybe i'd like to better understand thermal resistance."

"All and all in made sense. The part that somewhat got confusing was the formulas. More specifically with the applications. What each of the variables mean."

"I found Stepfan's law more confusing. I would appreciate some review on how this works."

"Colors affecting radiative absorption?"

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 [82%]
insulation conductivity κ: minimize [64%]
Total surface area A exposed to the outdoors: minimize [61%]

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 [63%]
thermal resistance R of the walls: maximize [72%]

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.	**** [4]
(There is a tie.)	[0]
(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.	****************** [18]
silver.	*********** [11]
(There is a tie.)	** [2]
(Unsure/guessing/lost/help!)	** [2]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"Everything progressively gets more interesting. I'm excited for Physics 205B!"

"As a welder I work with heat quite a bit."

"Is the zeroth law of thermodynamic is the equilibrium of temperatures?" (Yes. Surprisingly, it is not a trivial law.)

"Does heat always flow from high temperatures to low temperatures?" (Yes, all by itself. However, if you want heat to flow from low temperatures to high temperatures (in order to cool down your refrigerator, or use air conditioning to cool down your house), then you will need to spend energy in the form of work to move heat "opposite of the way it wants to go." Also heat pumps will move heat from low temperatures to high temperatures in order to extract energy from the cool environment to heat your house, but this again requires you to spend energy in the form of work to move heat "opposite of the way it wants to go.")

"You know me just a run down of the formulas would be greatttttt." (Mmmmkay.)