20180423

Physics quiz question: half-life of Cs-137

Physics 205B Quiz 7, spring semester 2014
Cuesta College, San Luis Obispo, CA

A medical radiotherapy source was stolen in Goiânia, Brazil, releasing radioactive contamination resulting in four deaths and 249 survivors before most of its contents was recovered. Over 25 years, the activity of the unrecovered portion of the source still in the environment has decreased from an estimated 7.0×1012 decays/second to 3.9×1012 decays/second. The half-life of this material is:
(A) 10 years.
(B) 14 years.
(C) 30 years.
(D) 36 years.

[*] wki.pe/Goi%C3%A2nia_accident.

Correct answer (highlight to unhide): (C)

The activity of a sample is given by:

R = R0·(1/2)(t/T1/2),

where T1/2 is the half-life. Solving for the T1/2 using the time t = 25 years that it took for the activity to drop down from R0 = 7.0×1012 decays/second down to R = 3.9×1012 decays/second, then:

(R/R0) = (1/2)(t/T1/2);

ln(R/R0) = (t/T1/2)·ln(1/2);

T1/2 = t·ln(1/2)/ln(R/R0);

T1/2 = (25 years)·ln(1/2)/(ln((3.9×1012 decays/second)/(7.0×1012 decays/second)) = 29.6250371579 years,

or to two significant figures, the half-life of this sample is 30 years.

(Response (A) is t·ln(R/R0)·ln(1/2); response (B) is t·(R/R0); response (D) is –t/ln(1/2).)

Student responses
Sections 30882, 30883
Exam code: quiz07bC4n
(No responses recorded.)

Online reading assignment: radioactive decay rates

Physics 205B, spring semester 2018
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 presentations on radioactive decay rates.


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.
"Exponential and half life decay are both covered well in general chemistry so it's somewhat of a review. But the M&Ms® example is still helpful."

"I understood the introduction to radioactive decay and half-lives. I feel like most of this is from previous biology and chemistry classes."

"Thankfully, this presentation was a bit of a refresher from my other physical science classes. Radioactive dating helps us determine how long the radioactive substance has been around, assuming that there are no daughter atoms to begin with."

"I understand that older substances have more daughter atoms in them. Different elements have different half-lives."

"When there are more daughter atoms the sample is older. The daughter are released when the sample that is being melted, which will then determine the age of the material/sample."

"Radioactive decay can be used to determine the age of an unknown material. This is done using half-lives to measure the time these radioactive materials decay in the unknown material."

"Gaseous daughter atoms can be compared to the amount of radioactive atoms left in a sample to determine how long ago the sample started. After a molten sample solidifies, it will start anew with having radioactive atoms with no daughter atoms."

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 actually feel okay with this stuff thanks to chemistry and other classes I've taken that have dealt with radioactive decay. Going through the slides one more time will help me understand things more."

"There wasn't anything too confusing from the presentation preview. I have learned about radioactive decay in other classes before."

"I don't understand the equation for half-life decay, or radioactive dating."

"I would like to go over problems to get more familiar with the equations."

"I'm confused with the section on radioactive dating, or carbon dating. How exactly is the age of something determined?"

"I don't understand the process to calculate the half life of a material or why melting an object affects its half-life."

"How is the clock reset when matter is solidified after melting if gaseous daughter particles escape?"

"I am struggling with the melting and solidification ages."

"At the moment I am not sure what I find confusing. Most of the lecture made sense to me as far as I am concerned."

State the SI units for activity (radioactive decays per time).
"Decays/s."

"s–1."

"Bq, or becquerel."

"Curies?"

"Lambda?"

"Not sure."

For a radioactive decay process, the time constant τ ("average lifetime") is __________ half-life T1/2.
less than.  ** [2]
equal to.  ***** [6]
greater than.  ******************* [19]
(Unsure/guessing/lost/help!)  ** [2]

Two samples are each comprised of 800 unstable atoms that will undergo radioactive decay. The remainder of one sample is 200 inert, stable atoms not involved in a radioactive decay process. The remainder of the other sample is 200 daughter atoms of the radioactive decay process.
(Only correct responses shown.)
Sample with more activity (decays/time): (there is a tie). [24%]
Sample with older solidification age: radioactive sample with daughter atoms. [45%]

Describe what changes in a sample when it melted and then solidified that resets its solidification age as determined by radioactive dating.
"Daughter atoms are released and are compared to the radioactive atoms that are left in the sample. Melting it resets its solidification age."

"Since daughter atoms (which are used to date the substance) are gaseous, when the substance is melted they are released. Therefore, they are no longer in the substance effectively resetting the age of the substance."

"There are technically no radioactive atoms or daughter atoms in the sample anymore, so it can reset its solidification age."

"Gaseous daughter atoms are released. This tells us how long ago the sample started with radioactive atoms with no daughter atoms."

"When a substance is liquified the once solidified daughter atoms are released into the air. When the substance is allowed to cool back into a solid the daughter atoms to not solidify back into the original state. This makes its a less reliable in radioactive dating."

"Not sure."

"Hmm..."

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"I really still think I need help with Lenz's law and Faraday's law."

"The semester after spring break is harder than I thought. But I think I will be able to catch up for the final."

"I need to play Half-Life again."

"I'm excited to eat M&Ms®!!!" (Wait for it--we'll get to do this in the last lab.)

20180420

Astronomy current events question: Fast-Evolving Luminous Transient (FELT) explosions

Astronomy 210L, spring semester 2018
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!)
Ray Villard, Armin Rest , "Kepler Solves Mystery of Fast and Furious Explosions" (March 26, 2018)
http://hubblesite.org/news_release/news/2018-18
NASA's Kepler space telescope's observations of Fast-Evolving Luminous Transient (FELT) explosions may be explained by stars that expel __________ before becoming supernovae.
(A) antimatter.
(B) gas and dust.
(C) dark matter.
(D) neutrinos.
(E) x-ray pulses.

Correct answer: (B)

Student responses
Sections 30679, 30680
(A) : 3 students
(B) : 20 students
(C) : 4 students
(D) : 6 students
(E) : 6 students

Astronomy current events question: volcanic interactions with Mar's oceans?

Astronomy 210L, spring semester 2018
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!)
Robert Sanders, "Mars' Oceans Formed Early, Possibly Aided by Massive Volcanic Eruptions" (March 20, 2018)
eps.berkeley.edu/news/mars’-oceans-formed-early-possibly-aided-massive-volcanic-eruptions
Researchers propose that Mars' oceans formed much earlier and shallower than previously thought, due to:
(A) volcanic activity.
(B) comet bombardment.
(C) global cooling.
(D) perchlorate rains.
(E) global sandstorms.

Correct answer: (A)

Student responses
Sections 30679, 30680
(A) : 29 students
(B) : 6 students
(C) : 2 students
(D) : 1 student
(E) : 1 student

Astronomy current events question: life in Venus' dark clouds?

Astronomy 210L, spring semester 2018
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!)
Terry Devitt, "Is there life adrift in the clouds of Venus?" (March 30, 2018)
news.wisc.edu/is-there-life-adrift-in-the-clouds-of-venus/
Researchers propose that dark patches in Venus' atmosphere could be caused by __________ in its clouds.
(A) hydrocarbon hurricanes.
(B) black lightning.
(C) floating microbial life.
(D) sulfuric acid rain.
(E) volcanic outgassing.

Correct answer: (C)

Student responses
Sections 30679, 30680
(A) : 0 students
(B) : 0 students
(C) : 26 students
(D) : 6 students
(E) : 7 students

20180419

Astronomy quiz question: star cluster with medium-mass protostars

Astronomy 210 Quiz 6, spring semester 2018
Cuesta College, San Luis Obispo, CA

A star cluster with medium-mass protostars would also have __________ at the same time.
(A) massive main sequence stars.
(B) red dwarfs.
(C) white dwarfs.
(D) giants.

Correct answer (highlight to unhide): (A)

All stars in a star cluster are born at the same time, but undergo stellar evolution at different rates depending on their masses. Medium-mass stars start out as protostars, then reach their main sequence stage, and subsequently become giants, planetary nebulae, and then white dwarfs. So a star cluster with medium-mass protostars cannot have giants nor white dwarfs at the same time, as these are stages that follow the protostar stage of a medium-mass star.

A red dwarf is a low-mass star that has evolved from its protostar stage to its main sequence stage; this takes much longer for a medium-mass star to evolve from its protostar stage to its main sequence lifetime. So a star cluster with medium-mass main sequence stars cannot have red dwarfs at the same time.

A massive star will evolve from its protostar stage to its main sequence stage at a rate much rapidly than medium-mass stars. Thus it is plausible that a star cluster with medium-mass protostars can have massive main sequence stars at the same time.

Section 30676
Exam code: quiz06s0ho
(A) : 17 students
(B) : 5 students
(C) : 10 students
(D) : 10 students

Success level: 44% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.55

Astronomy quiz question: star cluster with medium-mass main sequence stars

Astronomy 210 Quiz 6, spring semester 2018
Cuesta College, San Luis Obispo, CA

A star cluster with medium-mass main sequence stars would also have __________ at the same time.
(A) red dwarfs.
(B) supergiants.
(C) white dwarfs.
(D) giants.

Correct answer (highlight to unhide): (B)

All stars in a star cluster are born at the same time, but undergo stellar evolution at different rates depending on their masses. Medium-mass stars start out as protostars, then reach their main sequence stage, and subsequently become giants, planetary nebulae, and then white dwarfs. So a star cluster with medium-mass main sequence stars cannot have giants nor white dwarfs at the same time, as these are stages that follow the main sequence lifetime of a medium-mass star.

A red dwarf is a low-mass star that has evolved from its protostar stage to its main sequence stage; this takes much longer for a medium-mass star to evolve from its protostar stage to its main sequence lifetime. So a star cluster with medium-mass main sequence stars cannot have red dwarfs at the same time.

A supergiant is a massive star that has evolved from its protostar stage to its main sequence stage, and subsequently used up all of its core hydrogen exit its main sequence stage. Since massive stars evolve much more rapidly than medium-mass stars, then it is plausible that a star cluster with medium-mass main sequence stars can have supergiants at the same time.

Section 30674
Exam code: quiz06nG4s
(A) : 8 students
(B) : 5 students
(C) : 4 students
(D) : 6 students

Success level: 28% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.43

Astronomy quiz archive: stellar evolution

Astronomy 210 Quiz 6, spring semester 2018
Cuesta College, San Luis Obispo, CA

Section 30674, version 1
Exam code: quiz06nG4s


Section 30674
0- 8.0 :  
8.5-16.0 :   **** [low = 11.0]
16.5-24.0 :   ***
24.5-32.0 :   ******** [mean = 27.3 +/- 8.9]
32.5-40.0 :   ******** [high = 40.0]


Section 30676, version 1
Exam code: quiz06s0ho


Section 30676
0- 8.0 :   **** [low = 7.5]
8.5-16.0 :   ******
16.5-24.0 :   *********** [mean = 23.8 +/- 9.4]
24.5-32.0 :   ***********
32.5-40.0 :   ********** [high = 40.0]

20180418

Online reading assignment: the Milky Way (SLO campus)

Astronomy 210, spring semester 2018
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 presentations on the Milky Way's shape, size and composition and spiral arm structure and formation.


Selected/edited responses are given below.

Describe something you found interesting from the assigned textbook reading or presentation preview, and explain why this was personally interesting for you.
"It's pretty crazy to think we don't know what our galaxy looks like for sure."

"That we are able to observe that our galaxy has a disk shape even though we cannot get a real image of what it is like from an outside point of view."

"The fact that the entire galaxy is not made up of stars. and that the stars we do see is only apart of the milky way galaxy."

"The spiral arms--I think galaxies are so unique and colorful, learning more about the shapes interests me."

"How we are able to perceive our position in the Milk Way by observing star clusters above and below us."

"Dark matter--that we're able to observe galaxies and deduce their gravitational patterns without really understanding where it comes from or being able to quantify those characteristics."

"The description of dark matter was both interesting and confusing, because why does dark matter effect nothing on the outside of the halo? Why does it occur in a perfect sphere if there are multiple sources?"

"The analogy where we looked at dark matter in the terms of a criminal at large. It really makes sense to have its existence put in terms of this situation. I really liked the phrase 'though the case may never be definitively solved, doesn't mean that this person (dark matter) doesn't exist.' Why is this interesting to me personally? I think any time information that is confusing to me can be put into some type of cool analogy I am set."

"Dark matter, despite the fact that we do not know much about it."

"Dark matter is really interesting. I think just for the simple fact that we don't fully understand it, I am intrigued by it."

"Dark matter, because I didn't know it made up the majority of the Milky Way."

"Density waves interest me but also very much confuse me!? I really like the fact it helps make our beautiful galaxy even more so. I also think it's badass that our galaxy gets its spiral arms from swallowing others!"

"Density waves and how they are caused! Gravitational interactions from the collision of thin disk galaxies (with no spiral arms) pulling in dwarf galaxies causing 'ripples' that form density waves."

"I have no doubt that there were many things that would have been interesting."

"All of it."

"All I know is that I want PimpStar Rims on my car. It would be so cool.

Describe something you found confusing from the assigned textbook reading or presentation preview, and explain why this was personally confusing for you.
"I'm confused about all the information that we can find in the H-R diagram. I can't keep track of it."

"The section on globular clusters and cepheid variables is super-confusing. I don't really understand how those things work at all, honestly."

"Dark matter, because it's cool and strange."

"Dark Matter. How the heck does that stuff work?"

"Dark matter...all I know is that there is not much to learn since not much is known of it in the first place."

"How is the spinning of the Milky Way related to the way light-up rims spin on car tires?"

"How Milky Way spiral arms can grow back such as the legs of sea stars."

"Density waves! What kind of time frame are we talking about with these stars living and dying? How solid is the evidence we have and how can we improve from what we have already? What's the next evolution is solidfying or discovering more solid information about of true shape and size?"

"Nothing."

"I know everything so I'm not confused at all."

"Everything is slightly confusing for me until P-dog breaks it down for us in class."

In your experience, how much of the "Milky Way" (the band of faint stars across the celestial sphere) have you been able to see in the night sky?
As much as can be seen with the naked eye.  *************** [15]
Not very much.  ********** [10]
Barely seen it.  ****** [6]
(Never been able to see it.)  **** [4]
(Unsure/guessing/lost/help!)  *** [3]

Using the most powerful light-gathering optical telescopes in the darkest skies, __________ of the stars in our entire galaxy can be observed from Earth.
1%.  ********** [10]
5%.  **** [4]
10%.  **************** [16]
50%.  * [1]
100%.  [0]
(Unsure/guessing/lost/help!)  ******* [7]

If you did not have access to a mirror while camping, what could you do to find out whether or not you're having a bad hair day?
"Find still water."

"I would look at my reflection from a metal spoon."

"A camera! Take a picture and see how it looks!"

"Know that I'm gonna have one regardless."

"Only leave the tent at night so nobody can tell."

"Assume it's bad and put a hat on."

"I'd leave it, lol it's just camping. I don't think the birds, bears, and deer are going to care too much over a few fly away hairs."

"Ask a friend/fellow camper to describe to you the state of your hair to you."

"You could feel with your hands to see if any hair is going in any crazy directions."

"Use my shadow."

Look at PimpStar Rims (*.html) for cars, or MonkeyLectric Rims (*.html) for bikes. Briefly explain how they work.
"They hook up lights to your rims that light up with your car."

"The images are created using a strobe modulation technique. LED lights are timed to light up at certain points in a full rotation so that, to the naked eye, a still image is created with multiple flashing images."

"As the wheel is spinning the individual lights are programmed to blink at a certain time and gives the viewer an image. If the wheels were not spinning and the lights still blinked the same you wouldn't see an image."

"The lights that flash are timed and synced up to the speed that the wheel goes in order to light up at the correct time making really cool patterns and light combos on the face of the wheel."

"It has to do with lag and certain timing of when lights need to flash or not flash."

"Persistence of vision: flickering lights plus motion/movement."

"Voodoo."

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
I liked the spinning light-up rims analogy."

"What do the light-up wheels have to do with us learning about the Milky Way?"

"Do you have PimpStar Rims on your car?" (I have them on my bicycle.)

"Do you think we will ever be able to really understand dark matter? (Probably, hopefully it's a graduate student that makes that discovery, he/she will get full credit and a Nobel Prize.)

"Can you get fired for __________ inside a classroom?" (The first of astronomy class is you do not talk about astronomy class. #siristhisatest)

"I have two exams before tomorrow's quiz, so other than quiz prep, I haven't had much time to focus on the new material. I'll get to read tomorrow!"

Do we (the students) have to always ask a question or make a comment for every weekly survey in order to get credit?" (No, this last section is optional. However, if you leave a lot of the sections above blank, then you can make up for that by leaving something substantial here.)

Is our final cumulative? (Yes, but selectively cumulative, not "everything cumulative." You'll be given a definitive list of things on a study guide, and if something's not on that list, then it's not on the final exam.)

"Hello?" (Yo.)

"No comment." (You just did.)

"I have been watching a lot of slime videos and all of the galaxy pictures look like slime I could make."

Online reading assignment: flux laws & devices

Physics 205B, spring 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 presentations on flux laws and devices.


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 am beginning to understand generators better now, and better understand how moving a magnet creates energy than when I did the last reading assignment."

"The magnetic flux ΦB is the product of the magnetic field magnitude B and the area A. The maximum magnetic flux occurs if the magnetic field is perpendicular to the surface."

"Magnetic flux is an area multiplied by a magnetic field. Faraday's law says that an induced emf occurs in a wire loop when the magnetic flux through it changes."

"Magnetic flux ΦB is the product of the magnetic field magnitude B and the area A. Units of T·m2 or webers."

"Induced current opposes ΦB change."

"Induced emf, which is produced by changing magnetic flux."

"The slide-rail generator and Faraday's law. The faster the rod moves, the more area there is, thus more emf is produced. In addition, if magnetic flux is constant, an emf can not be produced."

"Faraday's law states that an induced emf in a wire loop occurs while the magnetic flux through it changes. If flux is constant there is no emf. Induced current always opposes the magnetic flux. Differing primary and secondary coil turns allow emf to be stepped up or down."

"How transformers work to step up or down the voltage from the primary loop to the secondary loop."

"I'm not sure I really understand any of this lesson."

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 slide-rail generator--how the force generated on charges through the rod makes the ends charged."

"I'd like clarification on what magnetic flux is, I'm still confused by it after reading the examples."

"How magnetic flux works with the induced current."

"How and when to apply Lenz's law."

"I am confused in the situational uses of these equations. More in-class assistance and instruction would be helpful."

"I found the section on transformers pretty confusing. Also the step-down vs. step-up stuff didn't make a lot of sense to me."

"A little bit of everything is confusing. I just need to make the connections between the different piece...Lenz's law is not yet understood."

"I definitely need a lot of explaining on this stuff I cannot grasp the concepts from just reading the lectures online."

"What is Lenz's law? I don't know what is used for and what context it is useful. Really some explanation for me here would go a long way."

"How to incorporate RHR3 to Lenz's law."

"I'm pretty confused about most of this lesson."

State/describe the symbol used for magnetic flux, and give its SI units.
"Magnetic flux ΦB is the product of the magnetic field magnitude B and the area A. Units of T·m2 or webers."

The symbol looks like a circle with a vertical line through it and it is the product of a magnetic field and an area."

For each situation involving magnetic flux and a wire loop, determine whether or not there would be an induced current in the loop.
(Only correct responses shown.)
Constant zero magnetic flux: no induced current in loop [87%]
Constant non-zero magnetic flux: no induced current in loop. [39%]
Magnetic flux increasing in strength: induced current in loop. [83%]
Magnetic flux decreasing in strength: induced current in loop. [52%]

For an ideal transformer that "steps-down" voltage from its primary coils at 120 V to its secondary coils at 2.1 V, determine what happens to the current and to the power from its primary coils to its secondary coils.
(Only correct responses shown.)
Current: stepped-up (increases). [30%]
Power: no change. [39%]

For an ideal transformer that "steps-up" voltage from its primary coils at 1.5 V to its secondary coils at 220 V, determine what happens to the current and to the power from its primary coils to its secondary coils.
(Only correct responses shown.)
Current: stepped-down (decreases). [30%]
Power: no change. [35%]

Explain why a transformer that has the same number of primary coils and number of secondary coils would not be useful.
"There would be no change in emf as the ratio of N2 to N1 would be 1."

"The transformer would not be able to regulate voltage to step it up or down. The primary coil and secondary coil turns cannot be the same amount."

"The whole point is that they have a different number of turns in order for voltages to be stepped up or down."

"Because nothing is being transformed."

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Following this pace is getting hard."

"Please go over these examples I am very lost on this subject!"

"Help..."

20180417

Online reading assignment: the Milky Way (NC campus)

Astronomy 210, spring semester 2018
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 presentations on the Milky Way's shape, size and composition and spiral arm structure and formation.


Selected/edited responses are given below.

Describe something you found interesting from the assigned textbook reading or presentation preview, and explain why this was personally interesting for you.
"I enjoyed learning about the Milky Way. I thought it was interesting to see how large it is compared to everything else, which blows my mind."

"The Milky Way, considering I can see it at my house, so learning about it will be interesting."

"That even though we are in the Milky Way, we can still tell what size and shape it is by using different methods."

"The fog model because it explains why we can only see part of our Milky Way and not the whole thing, and it is one of the easier models to understand."

"That all the stars in our galaxy are actually in their own orbit, they're not static in their position in space. As Earth moves around the sun, the sun is moving in its own orbit around the Milky Way."

"Dark matter."

"The formation theroies, because they started to give me a sort of eye-opening experience as to crazy is was that the Milky Way Galaxy had been created and how everything had fallen into place to create the galaxy, let alone create life on Earth."

"Okay, this is going to sound dumb, but since we have been studying spiral and elliptical galaxies, it just hit me hard that we are in a spiral one. We are in between the Sagittarius and Perseus Arms! Like I knew the Milky Way was spiral, but I now I'm looking at it and it's crazy that life exists in these things."

Describe something you found confusing from the assigned textbook reading or presentation preview, and explain why this was personally confusing for you.
"Without being able to see the entire Milky Way, how are we able to ascertain its shape and look?"

"Globular clusters--it's weird how they can determine how far we are from the center of the Milky Way."

"Dark matter is confusing, because what is it?"

"The density waves were a little confusing for me, and also the radio wave/far-IR maps of the Milky Way."

"I found the spiral arms and spurs to be confusing, what are they made of? Just living and dying stars?"

"How stars form inside of the spiral arms. It says that they form inside gas, but where does the gas come from? Does it come from stars that have died?"

"That spiral arms are just an illusion? Density waves? Why does it have to be like this?"

In your experience, how much of the "Milky Way" (the band of faint stars across the celestial sphere) have you been able to see in the night sky?
As much as can be seen with the naked eye.  *** [3]
Not very much.  ******* [7]
Barely seen it.  ** [2]
(Never been able to see it.)  ***** [5]
(Unsure/guessing/lost/help!)  [0]

Using the most powerful light-gathering optical telescopes in the darkest skies, __________ of the stars in our entire galaxy can be observed from Earth.
1%.  **** [4]
5%.  [0]
10%.  ******* [7]
50%.  ** [2]
100%.  [0]
(Unsure/guessing/lost/help!)  **** [4]

If you did not have access to a mirror while camping, what could you do to find out whether or not you're having a bad hair day?
"Use my phone camera."

"Use your phone, you don't even have to turn it on--if you get it in the right light you can see your reflection."

"Use the back of a metal spoon or any shiny metal object if you camp that way."

"Fill a pot or bucket with water, set it on the ground and let the water get still, then look!"

"Reflection off a car window."

"Look at the reflection in a river or body of water."

"The facial expressions of the people I am with."

"Ask a fellow camper. Or don't bother, since we are camping, odds are that other campers are having just as much of a bad hair day as me."

"Just touch your hair."

"Look at your shadow to see an outline of your hair to see if its frizzy or not."

Look at PimpStar Rims (*.html) for cars, or MonkeyLectric Rims (*.html) for bikes. Briefly explain how they work.
"The wheels have built in LEDs that flash or strobe at a certain speed, matching the speed of the wheel, to create the illusion of a still image."

"They are basically led lights hooked up to your rims and use wifi to display whatever image you choose to show while your wheels are spinning."

"A strip of lights cause the illusion of the entire wheel to light up when moving--you know, technology."

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

"Can we, as a class, go camping for our final and look at the stars all night with telescopes?" (I feel that I can't trust you enough yet to tell me if I were having a bad hair day while camping.)

"Would you ever get PimpStar Rims?" (I already have something like that on my bicycle tires.)

"Will there be any extra credit opportunities for this semester?" (At least one more, next week.)

"Do you like Star Wars?" (#happybeeps)

20180416

Physics quiz archive: circuits (2)

Physics 205B Quiz 5, spring semester 2018
Cuesta College, San Luis Obispo, CA
Sections 30882, 30883, version 1
Exam code: quiz05z0m6



Sections 30882, 30883 results
0- 6 :  
7-12 :   ***** [low = 9]
13-18 :   *************** [mean = 18.4 +/- 4.4]
19-24 :   ******************* [high = 24]
25-30 :  

Online reading assignment: generators

Physics 205B, spring 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 presentations on generators.


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.
"A single-pass generator cannot indefinitely continue to slide the rod along the rails to generate a constant motional emf and current."

"More common generators have a coil that rotates between the north pole and south pole of an external magnet. This also generates a motional emf that can be measured with a voltmeter, or made to generate current, but the values of the motional emf (and current) will fluctuate over each cycle of rotation, or even change direction."

"A Faraday disk is an example of a continuous generator, while motional emf are examples of single-pass generators."

"Motional emf is generated when a metal rod is moved through a magnetic field. The way to calculate motional emf is via multiplying the length of the rod by the speed of the rod by the magnetic field through which it is going through."

"Motional emf arises because a magnetic force acts on the charges in a conductor that is moving through a magnetic field."

"Single-pass generators can only be used once before having to be reset. Hoop-drag and rail generators are two types of single-pass generators."

"There are different kinds of generators. Some need to be reset (single-pass) before they can be used again, while some generators are continuous and can keep providing current and emf."

"There are many different types of generators that work in different ways. We are still using the right-hand rules for these generators."

"Single-pass generators can only be used once before having to reset them. In a uniform magnetic B field, electrons in a metal rod experience a downwards pointing force, and as long as the rod is made to move through the field, its bottom will be negative and the top positive. Therefore, moving this rod in a B field makes it a battery. Continuous generators do not need to be reset to provide emf and current. An example of this is a Faraday disk, where the north and south pole lie between the external magnet. All you do is crank it to generate emf. Rotating coils are also commonly used."

"How to use the right-hand rule when applying it to generators and all kinds of magnetic fields."

"Learned more on how to use my hands to solve problems."

"I understand that I don't know how generators work."

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 would like to learn more on the motional emf."

"How to determine the direction of magnetic force for a rod that is being rotated from one end."

"Still not really intuitively understanding how generators work and what the 'resetting' is about. I get the idea of having to 'move the snowboard back up the rail,' but I don't really understand what is meant by resetting in a generator, especially for the rotating rod (Faraday disk)."

"More explanation on how to use the right-hand rules for the generators would be helpful."

"It is hard for me to use the right-hand rule to determine where the forces are going with a rod moving through magnetic fields for single-pass generator diagrams."

"The motional emf animation, and how the variables relate as well as the units."

A metal rod moves to the right along a magnetic field that points into the page. The direction of the magnetic force on (fictitious) positive charges in the rod is:
up ↑.  *************** [15]
down ↓.  ******** [8]
left ←.  *** [3]
right →.  * [1]
into the page ⊗.  [0]
out of the page ⊙.  *** [3]
(No direction, as this quantity is zero.)  [0]
(Unsure/guessing/lost/help!)  ** [2]

A metal rod pivoted at one end rotates counterclockwise in a magnetic field that points out of the page. The direction of the magnetic force on (fictitious) positive charges in the rod is:
in towards the center of rotation.  ***** [5]
out away from the center of rotation  ******************* [19]
into the page ⊗.  *** [3]
out of the page ⊙.  ** [2]
(No direction, as this quantity is zero.)  [0]
(Unsure/guessing/lost/help!)  *** [3]

Explain what a generator is supposed to "generate."
"Generators generate a motional emf."

"Emf and current."

"Generates a current and motional emf, eventually creating power."

"Electric potential energy."

"Electricity?"

Explain the meaning of "motional" in the term "motional emf."
"That the emf is created through motion."

"Energy is created via movement of a metal rod through a magnetic field."

"It means something is constantly moving to produce the emf."

"The motion of the bar is what creates the emf, thus 'motional emf.'"

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"This is all becoming pretty foreign to me, any worksheets and practice in class will definitely benefit me!"

"what are the benefits of making a single-pass generator over a continuous generator?" (Single-pass generators are not very practical (as you must stop producing electricity in order to "reset" the system), but are much easier to understand conceptually. More practical, continuous generators (that don't have to, or automatically "reset" by returning to their starting point) are a bit more difficult to understand.)

"Aren't you still technically putting in some energy for the system to reset a continuous generator, just as in the single-pass generator?" (You actually put energy into all generators (they just convert your kinetic energy into electric potential energy), but you wouldn't have an abrupt break trying to reset a continuous generator (and can generate electricity uninterrupted), compared to a single-pass generator like a slide-rail generator, where you would have to pick up the rod at the end of a (finite) set of rails, pick it up to put it back to the beginning (or you would need to stop the rod, and start sliding it backwards along the rails.)

"I'm excited that we are learning useful stuff for the zombie apocalypse." (I'm excited, too.)

20180415

Physics quiz question: speed of moving rod

Physics 205B Quiz 6, spring semester 2011
Cuesta College, San Luis Obispo, CA

A conducting rod 0.50 m in length moves at a constant speed through a uniform 0.15 T magnetic field. The left end 
of the rod is 0.030 V higher in potential than the right end. The speed of the rod is:
(A) 0.0023 m/s.
(B) 0.40 m/s.
(C) 2.5 m/s.
(D) 10 m/s.

Correct answer (highlight to unhide): (B)

The magnitude of the potential difference between the ends of the rod as it moves through this magnetic field (the "motional emf" ε) is:

ε = v·B·L,

where the speed of the rod would then be:

ε/(B·L) = v = (0.030 V)/((0.15 T)·(0.50)) = 0.40 m/s.

(Response (A) is ε·B·L; response (C) is B·L/ε; response (D) is B/(L·ε).)

Section 30882
Exam code: quiz06eMf0
(A) : 0 students
(B) : 8 students
(C) : 0 students
(D) : 0 students

Success level: 100%
Discrimination index (Aubrecht & Aubrecht, 1983): 0

Physics quiz question: higher potential end of moving rod

Physics 205B Quiz 6, spring semester 2013
Cuesta College, San Luis Obispo, CA

A conducting rod moves at a constant speed through a uniform magnetic field. The __________ of this rod is at a higher potential.
(A) top.
(B) bottom.
(C) middle.
(D) (None of the above choices, as the entire rod has the same potential.)

Correct answer (highlight to unhide): (D)

Since the angle between the magnetic field and direction of the velocity of the (fictitious) positive (and negative) charges in the rod is 180°, then there is no force exerted on the charges in the rod (as the magnitude of the force depends on the sine of this angle), so no charges are displaced to either end of the rod, and thus there is no potential difference between either end.

Section 30882
Exam code: quiz06h4cK
(A) : 11 students
(B) : 5 students
(C) : 2 students
(D) : 14 students

Success level: 44%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.75

20180413

Astronomy current events question: Stratospheric Aerosol and Gas Experiment

Astronomy 210L, spring semester 2018
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!)
News release, "Earth's Atmosphere: New Results from the International Space Station" (March 20, 2018)
esa.int/Our_Activities/Human_Spaceflight/Research/Earth_s_atmosphere_new_results_from_the_International_Space_Station
The Stratospheric Aerosol and Gas Experiment (SAGE) instrument on the International Space Station gathers data on the components of Earth's upper atmosphere by:
(A) releasing probes.
(B) scooping up molecules.
(C) observing the sun and the moon.
(D) firing lasers.
(E) listening for static.

Correct answer: (C)

Student responses
Sections 30679, 30680
(A) : 6 students
(B) : 7 students
(C) : 17 students
(D) : 2 students
(E) : 5 students

Astronomy current events question: end of Kepler Space Telescope mission nears

Astronomy 210L, spring semester 2018
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!)
Alison Hawkes, "NASA’s Kepler Spacecraft Nearing the End as Fuel Runs Low" (March 14, 2018)
nasa.gov/feature/ames/nasa-s-kepler-spacecraft-nearing-the-end-as-fuel-runs-low
NASA's Kepler Space Telescope is expected to end its mission to search for extrasolar planets when it:
(A) is captured by Jupiter's gravity.
(B) goes too far away to send back signals.
(C) runs out of fuel.
(D) is displayed in a museum.
(E) crashes into the sun.

Correct answer: (C)

Student responses
Sections 30679, 30680
(A) : 5 students
(B) : 1 students
(C) : 31 student
(D) : 0 students
(E) : 0 students

Astronomy current events question: artificial intelligence survey of moon craters

Astronomy 210L, spring semester 2018
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!)
Don Campbell, "New Technique Uses AI to Locate and Count Craters on the Moon" (March 12, 2018)
utsc.utoronto.ca/news-events/breaking-research/new-technique-uses-ai-locate-and-count-craters-moon
Researchers used __________ to identify 6,000 previously undiscovered craters on the moon.
(A) online volunteers.
(B) seeing-eye dogs.
(C) artificial intelligence.
(D) radar.
(E) moonquake data.

Correct answer: (C)

Student responses
Sections 30679, 30680
(A) : 2 students
(B) : 0 students
(C) : 25 students
(D) : 5 students
(E) : 5 students

20180411

Online reading assignment: medium-mass stars, massive stars, neutron stars and black holes (SLO campus)

Astronomy 210, spring semester 2018
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 presentations on the evolution of medium-mass stars, massive stars, neutron stars and black holes.

Selected/edited responses are given below.

Describe something you found interesting from the assigned textbook reading or presentation preview, and explain why this was personally interesting for you.
"You always use cool things to compare it to like using a car mpg/tank/range comparison to a main-sequence lifetime."

"Hummer/SmartCar anecdote. It really messed with my perspective."

"The Hummer vs. SmartCar analogy, because the car scenario makes sense and seems like it would transfer over to the stars, but it just doesn't. It gives me a new appreciation for the low-mass stars."

"We can't compare cars to stars. Stars are completely different and store energy different."

"That many smaller stars will outlive the sun."

"To discover that the bigger stars die faster than the smaller ones. This makes sense as the bigger stars have faster fusion rates which means that the run out of hydrogen faster. The smaller ones fuse at a slower rate and therefore take longer to die and this makes sense to me."

"I think it's amazing that not a single red dwarf star has died of old age anywhere in the universe! Thats how old they can be."

"That the larger the star the shorter the lifetime. I always would have thought the opposite for some reason."

"The death of stars--I think it's cool that the mass of the stars affects the way in which a star may die."

"To find out how stars die and about the white dwarfs and how they die alone. RIP."

"Nova explosions--it's hard to believe that something like that takes place in this universe!"

"The subject of star death. I have often heard people use the term 'supernova' before, but never really thought anything about it other than the fact that it was some form of explosion. So while I have always known what it really refers to on a very, very basic level. But really being able to see all the visuals and learn about the different types of star deaths was just really interesting to me personally. However, I am having a hard time pairing the right explosion with the stars they go with."

"The video link of the stacked ball drop. I had never seen nor heard of this. It was then easier to image the core implosion-explosion giving me a visual to see."

"Pulsars, because they're described as mysterious signals...cool."

"I found the mass difference of stars and black hole fascinating. The concept of black hole is so popular, yet so unknown."

"Reading about what black holes are and how they work."

"That black holes are not visible at all but you can still tell it's there."

"To learn about what would happen if you jumped into a black hole. I just thought this was cool because when I was a kid me and my friends would constantly talk about jumping in black holes like it was some joke and it was nice to learn, or at least get the idea of what would actually happen if you did that."

"'Spaghettification.' I've been anxious to learn a little more about black holes, so knowing that it would seem to be an infinite amount of time to an observer before something falling in circles and enters the event horizon is super interesting and scary to me."

"I found black holes quite interesting, specifically the way gravity works in black holes. The way space-time works and how the object going into the black hole looks to observers is very strange."

"Nothing really."

Describe something you found confusing from the assigned textbook reading or presentation preview, and explain why this was personally confusing for you.
" How at the end of a medium-mass stars lifetime that it becomes a giant before it dies off. Wouldn't it just get smaller as it dies?"

"How a white dwarf can steal hydrogen from a neighboring star?"

"Novae were confusing. I don't really get how a white dwarf is able to siphon off the energy from another star."

"That stars actually die--they have a life, they live their life, and then they die?"

"Matching the end-of-life stage with the corresponding main-sequence star mass."

"Navigating the deaths/explosions of specific stars and could use in-class clarification."

"Having a hard time sorting through the information to compartmentalize each actual stage and pairing it with the star it goes with. "

"The process by which heavy elements are created."

"How a supergiant can implode-explode? I understand the concept of exploding but the idea of a star imploding first confuses me."

"When type II supernovae make heavier elements and ultimately explode, how do these heavier elements get all the way to us?"

"Density confuses me so much, so if you can explain it in class that would help a lot."

"How do they know exactly what happens in a black hole if everything is ultimately destroyed?"

"What black holes really do."

"Nothing was confusing to me, it was all pretty straightforward"

A Hummer H2 and a SmartCar ForTwo can travel the same distance with a full tank of gas. Briefly explain how this is possible.
"The Hummer can hold more gas, but uses it more quickly, so it evens out."

"A Hummer H2 and a SmartCar ForTwo can travel the same distance on a full tank of gas because of the differences in their fuel capacity and efficiency. The Hummer is an extremely inefficient car, but to compensate for that fact it is made with an extremely large gas tank capable of carrying a lot of fuel. The SmartCar is a much smaller car, and while it's many times more efficient than the Hummer, it has a proportionally appropriate much smaller gas tank. Because of the differences in how much fuel these cars use and how much they can carry, despite their major size difference, they both end up having the same range."

"The Hummer H2 contains a larger tank for the gas therefore is able to hold more gas, and SmartCar ForTwo has great mileage but the gas tank is significantly smaller therefore cannot hold much gas. so the two cars travel the same distance."

"The Hummer would need to make more pit stops to refuel?"

"They both have the same size gas tank?"

Match the end-of-life stage with the corresponding main-sequence star.
(Only correct responses shown.)
Black hole: massive main sequence star [76%]
Neutron star: massive main sequence star [43%]
White dwarf: medium-mass main-sequence star [41%]
(No stellar remnant observed yet: low-mass main-sequence star [38%]

Match the type of explosion (if possible) with the corresponding main-sequence star.
(Only correct responses shown.)
Type II supernova: massive main sequence star: [73%]
Type Ia supernova: medium-mass main-sequence star [65%]
Nova: medium-mass main-sequence star [43%]
Low-mass main-sequence star: (no explosion possible) [54%]

If you were to leap into a black hole, your friends would typically watch you falling in for __________ before you entered the event horizon.
seconds.  ***** [5]
hours.  ** [2]
days.  [0]
a year.  [0]
many years.  *** [3]
forever.  ************************* [25]
(Unsure/guessing/lost/help!)  ** [2]

The first rule of astronomy class is...
"Bring snacks?"

"It's boring but important?"

"Shoot for the stars?"

"Always pay attention?"

"Show up?"

"Do the reading assignments?"

"Ask questions?"

"Look up?"

"Having fun learning about space and our universe?"

"To be informed of and to abide by all student policies, and to call you P-dog?"

"Do your work every week?"

"Try your hardest?"

"What P-dog says, goes?"

"I don't remember. Don't die?"

"Do not talk about astronomy class."

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Why isn't the carbon-oxygen core of a white dwarf not hot enough to convert the carbon and oxygen into heavier elements? (Even though the temperature is really hot, it doesn't have enough pressure/gravity to squeeze carbon together to fuse, unless more mass is added to it by another star.)

"Is it possible for another star to move close enough to our sun to cause mass transfer and accretion disks?" (It's too late for that, it should have been a star that was born nearby at the same time as our sun.)

"Why don't pulsars collapse in on themselves?" (They don't have quite enough mass and gravity to crush themselves into a zero-size black hole, but they did have enough mass and gravity to crush all their atoms and compact them into neutrons.)

"I know that in the in-class work we will have to fill in a stellar evolution chart, the topic is still feeling way over my head. I'm sure you will go into more detail about it before we do the assignment, but I just thought I'd put it out there."

"What were to happen to an object that we observe going into a black hole? Would we ever see it come out the other side or would the black hole appear to change?" (We won't ever be able to see what happens to the object after it enters the event horizon surrounding the black hole (as no light from inside the event horizon can escape). However, we would then wind up with a black hole that has slightly more mass and gravity.)

"What is our next quiz going to be on?" (This stuff--the birth, lives, and deaths of stars.)

"Is the final exam going to be cumulative? (Yes, but selectively cumulative, not "everything cumulative." You'll be given a definitive list of things on a study guide, and if something's not on that list, then it's not on the final exam.)

"Would you jump into a black hole to potentially live...F-O-R-E-V-E-R...rrrr?" (I don't think I'd enjoy the spaghettification process. Technically, you'd definitely die, it's just that no one would live long enough to be able to watch the process complete itself.)

"How was your spring break?" (Mrs. P-dog and I had an awesome time in the mountains. How about yours?)

"'sup?" (Chilling. You?)

"I plan on studying more."