20141125

Physics presentation: internal energy conservation

Sigh. This equation. The ubiquitous "Q = m·c·∆T" endlessly recited in chemistry, more than likely without much understanding. And by "understanding," I mean, "physics."

Well, now that you've seen that equation, you can't unsee it, so in this presentation we'll instead emphasize the conceptual meaning of heat transfers (the "Q" on the left side of the equality), and of changes in thermal internal energy (the m·c·∆T" on the right side of the equality), and see how this equation works in the context of principles developed earlier in this course for work and mechanical energy conservation.

First, introducing internal energy systems, and especially changes in thermal internal energy.

Recall that mechanical energy systems such as translational kinetic energy and rotational kinetic energy are concerned with the macroscopic translational motion or rotational motion of an entire object. Thermal internal energy is also concerned with a motion of an object, more specifically the random microscopic motion of the individual atoms and molecules within an object. So for this just-cooked turkey coming out of the oven, it contains a large amount of thermal internal energy due to the very active microscopic motion of its atoms and molecules.

The total thermal internal energy of an object's microscopic atomic and molecular motion depends on its mass m and its temperature T (measured in kelvins), and is expressed as joules (chemists use "calories" or "Calories" for energy--be sure to ask them why they keep using those non-Système International units). A low temperature object has very little thermal internal energy, and higher temperature it has, the more thermal internal energy it has. The intensive property of different materials (due to its atomic and molecular make-up) related to its thermal internal energy is the "specific heat capacity," in units of J/(kg·K).

(Note that while your textbook explains in detail what thermal internal energy is, it does not actually define a symbol for thermal internal energy, so for our purposes we'll use "Etherm.")

Instead of finding out how much thermal internal energy Etherm an object has, often we are more concerned with its initial-to-final change ∆Etherm, which is the final amount of thermal internal energy minus the initial amount of thermal internal energy. Notice how the common factors of mass m and specific heat capacity c are pulled out, such that the amount of change in thermal internal energy is determined by the ∆T change in temperature.

Recall that mechanical energy systems such as gravitational potential energy and elastic potential energy are concerned with the storage of energy due to height of an object in a gravitational field, or due to the compression or stretching of a spring or an elastic material from its equilibrium state. Bond internal energy is concerned with the storage of energy, but stored in the spring-like bonds between the individual atoms and molecules in solids and liquids.

During vaporization (liquid turning into gas) or melting (solid turning into liquid), these bonds are broken, freeing up individual atoms or molecules. Bond internal energy increases during these processes, much like the elastic potential energy of a spring increasing as it stretches more and more, because the bonds between atoms and molecules must be stretched further apart in order to "disconnect" them from each other.

During condensation (gas turning into liquid) or freezing (liquid turning into solid) bonds begin to form between the individual atoms or molecules. Bond internal energy decreases during these processes, much like the elastic potential energy of a spring decreasing as it stretches less, because the distances between atoms and molecules must brought be closer together in order to "connect" them to each other.

(While there is a "latent heat" equation to calculate the change in the bond internal energy during phase changes (when bonds are broken or made between individual atoms and molecules), we will focus primarily on changes in thermal internal energies, so only consider situations where there are changes in temperature, but no changes in phase.)

Next, heat--the "Q" in the "Q = m·c·∆T" equation.

Recall that work is the transfer of mechanical energy on the macroscopic level, whether put in by an external agent, or taken away because of friction/drag. Heat, then, is the transfer of internal energy on a microscopic level.

If there is no energy transferred into or out of the thermal internal energy of a system (as with the contents an extremely well-insulated Thermos® bottle), then it is effectively thermally isolated from the environment, and the heat exchanged between the system and the external environment is zero.

If the thermal internal energy of a system increases, its temperature increases, and thus external heat from the environment is positive, being added into the system (as for this blowtorch on a marshmallow).

If the thermal internal energy of a system decreases, its temperature decreases, and thus external heat from the environment is negative, being removed from the system (as for these sous-vide cooked portions of duck meat being chilled in an ice-water bath).

The direction of heat flow is determined by the relative temperatures of two objects interacting with each other--the object at a higher temperature will "heat up" (transfer energy to) an object at a lower temperature. The two objects will stop transferring energy between each other when they attain the same final temperature (such that there is no longer a direction for heat to flow), thus reaching thermal equilibrium.

(Don't ever expect heat to spontaneously flow from a lower temperature object to a higher temperature object--unless you "do work" (expend mechanical energy) to make this happen, which is why it will cost you to run a refrigerator or air conditioner to remove heat from the low temperature contents, and dump it to the warmer environment outside. We'll focus on the "natural" direction of heat flow that occurs between two different temperature objects that thermally interact with each other.)

Third, accounting for the transfers between different thermal internal energy systems, and also for the contribution to or taking from these energies by an external source of heat.

This is expressed in a format in order to emphasize the parallels with the total mechanical energy conservation equation developed earlier. The transfer/balance equation here shows how the transfer of energy (as heat exchanged by external agents and/or the environment) causes corresponding changes in the thermal internal energies of all objects in our system. Any or all of these thermal energy forms on the right-hand side of the equation can increase or decrease (due to an increase or decrease in temperature), but together all of their changes must add up to the corresponding heat exchange term on the left-hand side of this equation.

When you substitute the individual ∆Etherm terms on the right-hand side of the equation with the equivalent m·c·∆T expressions, then this results in the oh-so-familiar-but-maybe-not-quite-so-meaningless-anymore "Q = m·c·∆T" equation.

Note that in the idealized case that the system is thermally isolated from both external agents and the environment, then the left-hand size of this equation would be zero. Then the individual energy terms on the right-hand side of this equation can then trade and balance amongst themselves, instead of with the outside world.

So now let's see how this transfer/balance equation can be applied to idealized situations where heat gain/loss exchanges with the outside world are negligible compared to the exchanges within a system.

Raw seafood is placed on a block of salt that has already been heated up. The energy contained in the high-temperature block of salt is then transferred to the seafood, cooking it. While it is being cooked, does the internal thermal energy of the seafood increase, decrease, or not change? Does the thermal internal energy of the salt block increase, decrease, or not change?

Assuming that the seafood and salt block system is thermally isolated from the environment (such that Qext = 0), which thermal internal energy experienced a greater amount of change: the seafood, or the block?

Frozen meat is placed in a water bath, in order to defrost it. At the very start of this defrosting process (where the frozen meat just begins to warm up from its below-freezing temperature, and the ice crystals inside have not yet reached the melting point), does the internal thermal energy of the meat increase, decrease, or not change? Does the thermal internal energy of the water increase, decrease, or not change?

Assuming that the meat and water system is thermally isolated from the environment (such that Qext = 0), which thermal internal energy experienced a greater amount of change: the meat, or the seafood?

A shot of whiskey is mixed with a pint of beer to make a boilermaker. Assuming that the whiskey and beer have approximately the same temperature before they are mixed together, does the internal thermal energy of the shot of whiskey increase, decrease, or not change? Does the thermal internal energy of the pint of beer increase, decrease, or not change?

Assuming that the whiskey and beer system is thermally isolated from the environment (such that Qext = 0), which thermal internal energy experienced a greater amount of change: the whiskey, or the beer?

(If you haven't noticed the type of vocabulary used in this presentation, we are deliberately avoiding the confusion between "hot" (in terms of high temperature, high thermal internal energy objects) and "heat" (thermal energy transferred between objects).)

20141122

Physics presentation: sound

Apparently the ultimate test of an impressive car sound system is not how loud it is, but is the subwoofer powerful enough to vibrate the air and shred a phonebook? (Video link: " MD style Paper Shredder !!!!.")

In a previous presentation, we introduced wave parameters and standing waves along strings. Here we will extend these ideas to sound waves traveling in air.

First, sound wave parameters.

The amplitude and the frequency of a sound wave is set by the source (here, a speaker cone) which will vibrate in and out, displacing the air in front of it.

The amplitude of the speaker's in-and-out vibrations is related to the "loudness" of a sound--small amplitude vibrations barely displace the air molecules in front of the speaker from their nominal positions, while large amplitude vibrations will greatly displace the air molecules from their nominal positions. (The amount that the air is displaced will decrease as a sound wave spreads outwards from a source--thus making sounds "quieter" as you move further away from a source--but for the purposes of this presentation we'll ignore the spreading out of sounds.)

The frequency of the speaker's in-and-out vibrations is related to the "pitch" of a sound--we perceive different frequency sounds in the range from 20 Hz (very low bass notes) up to 20,000 Hz (extremely high treble notes). These are nominal values, and typically the high end is lost due to age or unsafe sound exposure. (There can be sounds with frequencies lower than 20 Hz, if the source vibrates slowly enough--we humans can't hear these infrasound frequencies, but elephants can send out and receive these type of sounds over great distances to communicate with each other. Likewise if a source vibrates quickly enough, it will produce sounds with frequencies higher than 20,000 Hz--we humans can't hear these ultrasound frequencies, but bats can send out and receive these type of sounds, typically for navigating and preying on insects.)

The next sound wave parameter is the speed that it propagates through air. Here we can see the spherical wavefront of the gun blast sound emitted from both the muzzle (and earlier wavefronts from the rear of the barrel). Note that the bullet is traveling faster than the speed of sound, resulting in a cone-shaped shock wave. More on that later.

The speed of a sound wave in air depends approximately on the square root of the absolute temperature (in kelvin, not in Celsius!), and at 273 K (0° C) would have a speed of 331 m/s, and at "room temperature" (according to some) of 293 K (20° C) sound waves would have a speed of:

v = (331 m/s)·sqrt((293 K)/(273 K)) = 342.91026079 m/s,

or to three significant figures, 343 m/s. (Some may define room temperature as high as 25° C, but apparently physicists like it a little cooler than some other people.) Essentially sound travels faster through warmer air, and slower through cooler air (neglecting changes in pressure). These sound waves are fast, but bullets and jets can certainly move faster than these speeds.

Here, an F-14 Tomcat moving faster than the speed of sound creates that characteristic cone-shaped shock wave, which to an observer sounds as a sonic boom. (Video link: "F-14_Tomcat_sonic_boom.ogg.")

The speed of a sound wave is set by the temperature of the medium (air), but the frequency of the wave is set by the source. The resulting spatial repeat interval is the wavelength, which is the distance between the "crowding" wavefronts. Note that while a single air molecule (highlighted in yellow) just moves back-and-forth (at the average random speed of an ideal gas at that temperature), the wavefronts of a wave travel from left-to-right at the speed of sound.

(This is a review of our previous discussion of one-dimensional rope and string waves.) Note the hierarchy of these wave parameters. Since the wave speed is determined by properties of the material it travels through (independent of the source), and the frequency is determined by the source (independent of the medium), these are said to be independent wave parameters. In contrast, the wavelength of the wave is dependent on both the independent speed and frequency parameters. Algebraically there is nothing wrong with expressing this relation as v = λf and f = v/λ, as long as you recognize that the dependency of λ doesn't change.

The speaker on the left oscillates back-and-forth as the source of a sound wave that travels left-to-right along this section of air. The top case is where the speaker oscillates back-and-forth with a certain frequency and a small amplitude, while the bottom case is where the speaker oscillates back-and-forth with the same frequency and larger amplitude. Which wave travels with the faster speed? Which wave has the longer wavelength?

Here both speakers oscillate back-and-forth with the same amplitude, but the top case has a lower frequency and the bottom case has a higher frequency. Which wave travels with the faster speed? Which wave has the longer wavelength?

In this setup, the a speaker oscillates back-and-forth with a given frequency and amplitude, creating the waves in the first section at left. As the wave travels from left-to-right, it is then transferred to the second section that allows the wave to travel at a much slower speed (perhaps due to a cooler air temperature, or a different gas density). Along which section does the wave have a higher frequency? Along which section does the wave have a longer wavelength?

Now we'll briefly extend our previous discussion on standing waves on strings to sound standing waves in pipes.

Sound waves can be "trapped" inside pipes of various lengths, with either open and/or closed ends, and if a sound wave matches the resonant frequency of the pipe, then the air inside will move back-and-forth in a coordinated manner--a "standing wave."

We can regard a pipe with both ends closed, or a pipe with both ends open as a "symmetric" system. If they have the same length, then the air molecules will resonate with the same fundamental frequency (in this animation, approximately 1 Hz). Note that while the standing waves have the same frequency for both pipes, the closed-closed pipe has an antinode in the middle, where air molecules slosh back-and-forth, while there are nodes at either end, where air molecules are "trapped" because they cannot move into (against) the closed ends.

In contrast, the open-open pipe has antinodes at either end, as air molecules are free to slosh back-and-forth, being exposed to the atmosphere, while there is a node in the middle, where air molecules are "trapped" because of the motion at either end.

Because the distance between consecutive nodes and antinodes in these pipes is proportional to the wavelength of the standing wave, the fundamental frequency would be the same for either closed-closed or open-open pipes.

For a closed-closed or open-open symmetric pipe, the frequency at which a source of sound waves would resonate within this pipe are multiples of the fundamental frequency f1, which depends on the wave speed v of sound (343 m/s for "room temperature"), and the length of the pipe.

We can regard a pipe with one end open, while the other end is closed (or vice versa) as a "symmetric" system. If they have the same length, then the air molecules will resonate with the same fundamental frequency (in this animation, approximately 2 Hz). Note that while the standing waves have the same frequency for both pipes, the open end has an antinode, where air molecules exposed to the atmosphere slosh back-and-forth, while the closed end has a node, where air molecules are "trapped" because they cannot move into (against) the closed ends.

(Because the distance between consecutive nodes and antinodes in these pipes is proportional to the wavelength of the standing wave, the fundamental frequency would be the same for either closed-open or open-closed pipes. Note for a given length of pipe, the asymmetric systems have a longer (bigger value) node-antinode spacing than the symmetric systems, and thus an asymmetric system would have a lower (smaller value) fundamental frequency (in this animation, approximately 2 Hz) than the symmetric system.)

For a closed-open or open-closed asymmetric pipes, the fundamental frequency is lower than a comparable symmetric pipe (note the factor of 4 instead of 2 in the denominator), and resonant frequencies are all odd multiples of the fundamental frequency (rather than integer multiples). How very, very...odd. But this follows from the asymmetry of closed-open and open-closed pipes, such that even standing wave frequency multiples are not allowed.

Let's close out with some trombone playing, which can be approximated as a symmetric pipe (the bell end is open to the air, while the mouthpiece end is open to your lungs). In order to play different notes on a trombone is to buzzing your lips, not just at any arbitrary frequency, but only at multiples of the fundamental frequency in order to resonate the air along the pipe. Higher resonant frequency notes can be accomplished either by changing the length L of the trombone by shortening the slide... (Video link: "How to play the Trombone B Flat Major Scale.")

...or by keeping the position of the slide fixed (such that L is constant), and buzzing your lips to match higher resonant frequency notes. (Video link: "Trombone B Flat 7 Octaves Attempt.") This takes a fair amount of manual and lip coordination, so keep that in mind the next time you are subjected to a novice trombone player practicing the scales.

20141121

Astronomy current events question: reflections off of Titan's Kraken Mare

Astronomy 210L, fall semester 2014
Cuesta College, San Luis Obispo, CA

Students are assigned to read online articles on current astronomy events, and take a short current events quiz during the first 10 minutes of lab. (This motivates students to show up promptly to lab, as the time cut-off for the quiz is strictly enforced!)
Preston Dyches, "Cassini Sees Sunny Seas on Titan," (October 30, 2014)
http://www.nasa.gov/jpl/cassini-sees-sunny-seas-on-titan/
NASA's Cassini spacecraft recently observed __________ Kraken Mare, a hydrocarbon sea on Saturn's moon, Titan.
(A) sunlight reflected by.
(B) a tidal wave in.
(C) lightning storms over.
(D) the start of winter at.
(E) floating icebergs atop.

Correct answer: (A)

Student responses
Sections 70178, 70186
(A) : 18 students
(B) : 6 students
(C) : 7 students
(D) : 0 students
(E) : 6 students

Astronomy current events question: Nova Delphinus 2013 fireball

Astronomy 210L, fall semester 2014
Cuesta College, San Luis Obispo, CA

Students are assigned to read online articles on current astronomy events, and take a short current events quiz during the first 10 minutes of lab. (This motivates students to show up promptly to lab, as the time cut-off for the quiz is strictly enforced!)
LaTina Emerson, "Led By Georgia State, Astronomers Image The Exploding Fireball Stage Of A Nova," (October 27, 2014)
http://hubblesite.org/newscenter/archive/releases/2014/43/full/
Georgia State University’s Center for High Angular Resolution Astronomy (CHARA) observed the expanding explosion of a nova in the constellation Delphinus by:
(A) using GPS satellite signals.
(B) trapping neutrinos in underground detectors.
(C) combining light from several telescopes.
(D) recording fluctuations in light reflected by the full moon.
(E) measuring increased aurora borealis activity.

Correct answer: (C)

Student responses
Sections 70178, 70186
(A) : 7 students
(B) : 3 students
(C) : 20 students
(D) : 5 students
(E) : 2 students

Astronomy current events question: Abell 2744 "ghost light"

Astronomy 210L, fall semester 2014
Cuesta College, San Luis Obispo, CA

Students are assigned to read online articles on current astronomy events, and take a short current events quiz during the first 10 minutes of lab. (This motivates students to show up promptly to lab, as the time cut-off for the quiz is strictly enforced!)
Felicia Chou, Ray Villard, and Mireia Montes, "Hubble Sees 'Ghost Light' From Dead Galaxies," (October 30, 2014)
http://hubblesite.org/newscenter/archive/releases/2014/43/full/
NASA's Hubble Space Telescope picked up infrared "ghost light" in galaxy cluster Abell 2744 from stars:
(A) on a collision course with each other.
(B) with extreme gravitational redshifts.
(C) ejected from broken-apart galaxies.
(D) that exploded as type II supernovae.
(E) consumed by a supermassive black hole.

Correct answer: (C)

Student responses
Sections 70178, 70186
(A) : 2 students
(B) : 3 students
(C) : 23 students
(D) : 5 students
(E) : 4 students

20141120

Physics quiz question: stretching fishing lines

Physics 205A Quiz 6, fall semester 2014
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e Conceptual Question 11.4, Problems 11.1, 11.3

"Untitled"
Eileen Delhi
https://flic.kr/p/fGR8Fi

Trilene® XL Super Strong fishing line (Young's modulus 2.0×109 N/m2) and Eagle Claw® Sportfisher fishing line (Young's modulus 3.1×109 N/m2) have the same 10.0 m length [*]. The Trilene® fishing line has a cross-sectional area 1.8 times that of the Eagle Claw®. Both fishing lines are stretched with a tension force of 98 N. The __________ fishing line will stretch more.
(A) Trilene®.
(B) Eagle Claw®.
(C) (There is a tie.)
(D) (Not enough information is given.)

[*] S. Ottolini, G. Halpin, P. LaBruzzo, "Tensile Strength of Fishing Line," http://www.santarosa.edu/~yataiiya/E45/PROJECTS/Tensile%20Strength%20of%20Fishing%20Line%20Power%20Point.ppt.

Correct answer (highlight to unhide): (B)

Hooke's law for the Trilene® and Eagle Claw® fishing lines are given by:

(F/ATri) = YTri·(∆LTri/L),
(F/AEagle) = YEagle·(∆LEagle/L),

where tension F and the original, unstretched length L are the same for both fishing lines. The Trilene® fishing line has a cross-sectional area 1.8× that of the Eagle Claw® fishing line:

ATri = 1.8·AEagle.

The amount that the Trilene® fishing line will be stretched is given by:

LTri = (F·L)/(ATri·YTri),

LTri = ((98 N)·(10.0 m))/((1.8·AEagle)·(2.0×109 N/m2)),

LTri = (2.722222222×10–7 m3)/AEagle.

Similarly, the amount that the Eagle Claw® fishing line will be stretched is given by:

LEagle = (F·L)/(AEagle·YEagle),

LEagle = ((98 N)·(10.0 m))/((AEagle)·(3.1×109 N/m2)),

LEagle = (3.161290323×10–7 m3)/AEagle.

Thus this sample of Eagle Claw® fishing line will stretch more than the Trilene® fishing line sample.

Sections 70854, 70855, 73320
Exam code: quiz06eAg7
(A) : 13 students
(B) : 49 students
(C) : 2 students
(D) : 0 students

Success level: 77%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.46

Physics quiz question: increasing tension

Physics 205A Quiz 6, fall semester 2014
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e Multiple-Choice Question 11.5

"Can You Hear Me?: The Longest Tin Can Phone Ever"
Isaac Ravishankara
http://vimeo.com/61160400

200 m of mason twine[*] (linear density 9.1×10–4 kg/m) was recently used to make the "longest tin can telephone ever." Increasing the tension in the mason twine would __________ the frequency of waves sent along it.
(A) decrease.
(B) not affect.
(C) increase.
(D) (Not enough information is given.)

[*] "#18 x 425 ft. Orange Polypropylene Twisted Mason Line," http://www.homedepot.com/p/Unbranded-18-x-425-ft-Orange-Polypropylene-Twisted-Mason-Line-65375/202957511.

Correct answer (highlight to unhide): (B)

The speed v of transverse waves along the mason twine depends on the tension F and the linear mass density (mass per unit length) µ:

v = sqrt(F/µ).

Thus increasing the tension in the twine would increase the wave speed.

Also the wavelength λ is the parameter that depends on the speed v and source frequency f, which can be varied independently of each other:

λ = v/f.

While increasing the tension F of the twine would increase the speed v (which would increase the wavelength λ), this change in the property of the medium would be independent of the frequency, which is a property of the source. Thus increasing the tension in the twine would have no affect on the frequency of the waves sent along it, which depends on the source of the waves, and not the medium.

Sections 70854, 70855, 73320
Exam code: quiz06eAg7
(A) : 10 students
(B) : 35 students
(C) : 19 students
(D) : 0 students

Success level: 55%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.75

Physics quiz archive: simple harmonic motion, waves

Physics 205A Quiz 6, fall semester 2014
Cuesta College, San Luis Obispo, CA
Sections 70854, 70855, 73320, version 1
Exam code: quiz06eAg7



Sections 70854, 70855, 73320 results
0- 6 :  
7-12 :   ***** [low = 12]
13-18 :   **********
19-24 :   ********************** [mean = 23.3 +/- 5.3]
25-30 :   *************************** [high = 30]

20141119

Online reading assignment: temperature

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 temperature.


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.
"Warming causes expansion. Cooling causes contraction. The longer the length, the less temperature change needed to expand it."

"The expansion of materials is what ultimately defines what a change in temperature 'is.'"

"Matter expands and contracts in response to changes in temperature. Different materials have different expansion coefficients."

"I found it very interesting how there are gaps in railroads, sidewalks, bridges, etc. I never thought about it but now I know that it is to prevent a warped shape from thermal expansion due to heat."

"I understand that we should buy gasoline when it is cold outside and the company does not adjust for temperature."

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.
"Maybe explain '3·α.'"

"This section seems pretty straightforward. Similar to stretch and compression of object but based on temperature instead of applied force."

"I found the presentation about linear expansion confusing. I understand that change in temperature is important, but the formula was very confusing. Change in temperature being proportional to change in length over the original length doesn't click with me yet. I would definitely benefit from some talk about this in lecture."

For solids, what is the mathematical relationship between the coefficient of volume expansion β and the coefficient of linear expansion α?
"β = 3·α."

"Whoa--what?"

"The change in temperature."

"I am not sure and obviously could use some help with this in class."

To expand these two steel beams 1.0 cm from their original lengths, the longer beam will require __________ temperature increase compared to the shorter beam.
a smaller.  *************************** [27]
the same.  ******** [8]
a larger.  ************* [13]
(Unsure/guessing/lost/help!)  ****** [6]

For a thermometer, the glass volume expansion coefficient 3αglass is __________ the alcohol volume expansion coefficient βalcohol.
less than.  ****************************** [30]
equal to.  ********** [10]
greater than.  ******* [7]
(Unsure/guessing/lost/help!)  ******* [7]

For the water level in this plastic rainwater basin to lower as the temperature falls overnight, the plastic volume expansion coefficient 3αplastic must be __________ the water volume expansion coefficient βwater.
less than.  ********************** [22]
equal to.  ***** [5]
greater than.  ******************* [19]
(Unsure/guessing/lost/help!)  ******** [8]

A certain fuel company will measure out a gallon of gasoline and sell it for the same price, whether it is cool or warm. Indicate the gallon of gasoline that has a greater:
(Only correct responses shown.)
mass: the cool gallon [46%]
density: the cool gallon [74%]

Briefly explain why a gallon of gas purchased when it is cool would be better than a gallon of gas purchased when it is warm. (In either case, the fuel company dispenses the same volume of exactly one "standard" gallon.)
"When it is cold the gasoline would have a greater density and mass because the substance is more condensed than it would be when warm. When it is warm, the substance would expand, meaning you would get the same volume of gas with less density and mass."

"The mass of a cool gallon will be greater because the molecules are moving slower so they take up less area and will sit closer together."

"Well, when it is cold it will have more energy."

"...more holla for yo' dolla."

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"The difference between the densities of gasoline is so minuscule that it really doesn't matter to buy a given volume at a certain temperature." (Yes, but that small price difference multiplied by a lot of gasoline sold = PROFIT.)

"Isn't gas stored underground where it's kept at a constant temperature? And for that matter, isn't this entire dilemma an urban myth?" (Yes, and yes, if you assume that the gasoline does not get warmed up appreciably as it gets pumped through a warm above-ground dispenser. But see the above question and answer.)

"Gas in this area just went below $3 a gallon. I think I was nine years old the last time it was that cheap."

"Are we allowed to skip the final if we are happy with our grade?" (Sure, that is entirely up to you. Your "take it or leave it" grade, which is the total of your course points without taking the final will be posted the weekend before finals week.)

"Are there any known materials that do not expand/contract due to temperature changes?" (Strictly speaking, no--but there are some exotic alloys that have very small expansion coefficients, used in super-large telescope mirrors or space-based telescope mirrors that experience large temperature swings, in order to minimize changes to their size/shape.)

"In addition to solids/liquids, will we also cover expansion/contraction of gases?" (No--that sounds dangerously like chemistry (P·V = n·R·T) talk to me...)

20141117

Online reading assignment: sound

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 sound.


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.
"The frequency of a sound wave is set by the source as it vibrates the air in front of it. The speed of a sound wave in air depends approximately on the square root of the absolute temperature."

"Wavelength is dependent on two independent factors, velocity and frequency. The fundamental frequency in pipes depends on velocity and length."

"In some regions of sound waves there are compressions and rarefactions. Compressions are where molecules are tight together causing a higher pressure, and rarefactions are where molecules are spread out and have a lower pressure."

"Transverse waves on materials do not make sound waves by themselves but do cause them by exciting the air around them."

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.
"How there can be a standing sound wave. Nor do I understand the formulas that go with it."

"I'm going to wait for the lecture to figure out what I know and what I don't."

"The difference between the independent and dependent parameters."

"What I don't quite understand is what we are learning."

For sound waves, what does the T for sound wave speeds stand for, and what is its SI (Système International) unit?
"Absolute temperature, measured in K (kelvins)."

"T is the period of the wave. The inverse of T, called the frequency, is measured in Hertz."

For sound waves in air, classify each of these parameters are being "independent" (able to be changed without affecting other independent parameters), or "dependent" (will be changed when independent values are changed).
(Only correct responses shown.)
Amplitude A: independent [63%]
Wave velocity v: independent [55%]
Frequency f: independent [60%]
Wavelength λ: dependent [73%]

For sound waves in a tube, classify each of these parameters are being "independent" (able to be changed without affecting other independent parameters), or "dependent" (will be changed when independent values are changed).
(Only correct responses shown.)
Sound wave velocity v: independent [45%]
Tube length L: independent [50%]
Fundamental frequency f1: dependent [62%]
Frequency f of sound "blown" into tube: independent [50%]

Select the standing sound waves that would resonate in the tubes below.
(Only correct responses shown.)
Tube open at both ends: all multiples of f1 [57%]
Tube open at one end, and closed at the other end: only odd multiples of f1 [65%]
Tube closed at both ends: all multiples of f1 [53%]

What musical instrument(s) do you play? List none, or as many as applicable. #justasking
"None." [15 responses]

"Guitar." [10 responses]

"Piano." [10 responses]

"Singing." [3 responses]

"Flute." [2 responses]

"Saxophone."

"Violin/viola." [3 responses]

"Ukulele."

"Tuba."

"French horn."

"Banjo."

"Clarinet."

"Didgeridoo."

"Tumbak and jembek."

"Trumpet." [2 responses]

"Trombone." [2 responses]

"Drums/percussion." [3 responses]

"Bass/bass guitar." [3 responses]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Why do resonant frequencies of asymmetrical pipes only occur in odd multiples?" (Because they have mismatched ends--one open, one closed--so all possible symmetric standing waves, which are the even multiples of the fundamental frequency, are not allowed, and only odd multiples are allowed.)

"What musical instrument(s) do you play?" (Piano, ukulele, and in grade school, the most played instrument in 20th century.)