20170424

Online reading assignment: radioactive decay rates

Physics 205B, spring semester 2017
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.
"Radioactive decay is exponential."

"Honestly, not much. This whole concept is very foreign to me."

"Half-life is the time for one-half of a statistically large sample to decay. Half-life is proportional to the time constant and the decay constant."

"I totally understand how half lives work because I have learned it before in other classes like chemistry and geology. Those classes really helped with this lecture!"

"I understand half-life and radioactive decay as I have had it in math."

"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."

"After a molten sample solidifies, it will start anew with having radioactive atoms with no daughter atoms. Melting a sample 'resets' its solidification age."

"Radiation is hella bad for you."

"Haven't got to it yet."

Describe what you found confusing from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically identify the concept(s) that you do not understand.
"I would like more practice in obtaining the half-life given the time constant. Also, I want to know how equations look for radioactive dating or if you just count up the ratio of daughter atoms to radioactive atoms."

"What solidification age is and how was supposed to know which had an older solidification age."

"The concepts seem pretty easy to understand. Doing some problems to see how the different constants work would be great!"

"Very confused as to what is going on and am having trouble understanding activity (half-life decay) and half life equations."

"It all is pretty confusing, but it should clear up after lecture."

"Not sure yet."

"I'm good."

State the SI units for activity (radioactive decays per time).
"Becquerel."

"Bq."

"I believe that it is Bq = 1 decay/second."

"A becquerel, Bq, is the unit for one disintegration per second."

For a radioactive decay process, the time constant τ ("average lifetime") is __________ half-life T1/2.
less than.  *** [3]
equal to.  ************ [12]
greater than.  ******** [8]
(Unsure/guessing/lost/help!)  * [1]

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). [21%]
Sample with older solidification age: adioactive sample with daughter atoms. [75%]

Describe what changes in a sample when it melted and then solidified that resets its solidification age as determined by radioactive dating.
"Gaseous daughter atoms are released. This tells us how long ago the sample started with radioactive atoms with no daughter atoms."

"After a molten sample solidifies, it will start over with having radioactive atoms with no daughter atoms. Melting a sample restarts its solidification age."

"Not sure what is meant by 'solidification age.'"

"Uhh...what?"

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Not sure what the relationship between half-life and the time constant is." (As long as you realize they're not the same thing, but can be converted from one to the other (by a factor of ln(2)).)

"Is exponential decay just an overview and half-life is more specific?" (They're related to each other mathematically, as are the exponential decay and half-life decay equations. Both either will give you the same results computationally.)"

"I have always associated half-life with chemistry. It is interesting to explore this concept from a physics standpoint."

"Do you prefer Rockstar or Monster energy drinks?" (RockStar sugar-free tastes much better than Monster sugar-free.)

20170421

Astronomy current events question: early galaxy dark matter

Astronomy 210L, spring semester 2017
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!)
Reinhard Genzel, Natascha M. Förster Schreiber, Hannelore Hämmerle, "Event Horizon Telescope Ready to Image Black Hole" (March 16, 2017)
mpg.de/11170451/early-galaxies-dark-matter
Very distant, early galaxies may have less dark matter than current galaxies, based on mapping the distribution of their stars':
(A) orbital velocities.
(B) brightness fluctuations.
(C) matter-antimatter ratios.
(D) neutrino emissions.
(E) fast radio bursts.

Correct answer: (A)

Student responses
Sections 30679, 30680
(A) : 10 students
(B) : 9 students
(C) : 8 students
(D) : 6 students
(E) : 0 students

Astronomy current events question: Event Horizon Telescope

Astronomy 210L, spring semester 2017
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!)
Jonathan Amos, "Event Horizon Telescope Ready to Image Black Hole" (February 16, 2017)
bbc.com/news/science-environment-38937141
The Event Horizon Telescope is a "virtual telescope" that will combine data from many different __________ to image the gas and dust surrounding the supermassive black hole at the center of the Milky Way galaxy.
(A) radio telescopes.
(B) quantum supercomputers.
(C) underground neutrino detectors.
(D) gamma ray bursts.
(E) space telescopes.

Correct answer: (A)

Student responses
Sections 30679, 30680
(A) : 20 students
(B) : 2 students
(C) : 1 student
(D) : 1 student
(E) : 9 students

Astronomy current events question: LL Pegasi spiral gas pattern

Astronomy 210L, spring semester 2017
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!)
Stuart Wolpert, "Using a Powerful Telescope, Scientists View Spiral Pattern of Gaseous Emissions around LL Pegasi and Its Companion Star" (March 16, 2017)
newsroom.ucla.edu/releases/astronomers-observe-a-dying-red-giant-stars-final-act
A 3-D map of the spiral pattern of gases surrounding the red giant and companion star in the LL Pegasi system was made from __________ observations by the Atacama Large Millimeter/submillimeter Array in Chile.
(A) radio wavelength.
(B) magnetic field.
(C) gravitational wave.
(D) neutrino emission.
(E) gamma ray.

Correct answer: (A)

Student responses
Sections 30679, 30680
(A) : 13 students
(B) : 0 students
(C) : 8 students
(D) : 4 students
(E) : 5 students

Online reading assignment: flux laws & devices

Physics 205B, spring semester 2017
Cuesta College, San Luis Obispo, CA

Students have a bi-weekly online reading assignment (hosted by SurveyMonkey.com), where they answer questions based on reading their textbook, material covered in previous lectures, opinion questions, and/or asking (anonymous) questions or making (anonymous) comments. Full credit is given for completing the online reading assignment before next week's lecture, regardless if whether their answers are correct/incorrect. Selected results/questions/comments are addressed by the instructor at the start of the following lecture.

The following questions were asked on reading textbook chapters and previewing 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.
"The concept of magnetic flux. For any imaginary or actual area A (such as that enclosed by a wire loop) in the presence of a (uniform magnitude and direction) magnetic field B, the magnetic flux ΦB is the product of the magnetic field magnitude B and the area A."

"The concept of Faraday's law and the relationship between electromagnetic force going through a wire loop and the magnetic flux changing while going through the loop. The magnetic flux must always be changing, and if it is constant, then the electromagnetic force is zero."

"According to Faraday's law, in order for emf to be induced, the flux has to change."

"Change in magnetic flux is necessary to induce current. Even if a magnetic field is present, it will not induce current if it remains constant."

"What I was able to understand from tonight's reading is that the magnetic flux ΦB is the product of the magnetic field B and the area is A. The perpendicular sign '⊥' means the maximum value for the magnetic flux ΦB."

"I think I get the basic concepts relating to how a changing magnetic flux creates an induced current, and how that is applied in the coil and transformer we saw. I also get the voltage can vary in a transformer, because the number of 'windings' in the core corresponds to number of turns, i.e. N."

"How to convert grams to newtons."

"Haven't gotten to it yet."

"Nothing really..."

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.
"Faraday's Law. I didn't understand the concept of it."

"I'm not quite sure what's going on with Lenz's law. I don't see what's going on with the coils inducing a loop & creating a magnetic field."

"How to properly use all the symbols in each formula, because it seems like they are all over the place."

"How a coil resists change. I understand the comparison to throwing a brick, but a brick has mass that requires force to displace."

"The rotating coil generator was unclear. I think some clarification of Lenz's law would be very helpful. I can't seem to visualize what is going on in the explanations on the blog. You LOST me on the transformer part for sure. So, the more you could explain about that, the better."

"The physics of changing a transformer into a metal melter could be described a bit better in detail."

"I am confused on mostly every part of this. Very confused on magnetic flux."

"Nothing that I can think of."

"A lot of things man, a lot."

State/describe the symbol used for magnetic flux, and give its SI units.
B, the weber."

"An O with a capital I running through it, followed by a small B; Teslas times meters squared (T·m2), or webers."

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 [91%]
Constant non-zero magnetic flux: no induced current in loop. [57%]
Magnetic flux increasing in strength: induced current in loop. [83%]
Magnetic flux decreasing in strength: induced current in loop. [83%]

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). [39%]
Power: no change. [30%]

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). [39%]
Power: no change. [40%]

Explain why a transformer that has the same number of primary coils and number of secondary coils would not be useful.
"It wouldn't be useful because it wouldn't change the voltage or the current at all."

"The difference in number of coils is what allows the step-down or step-up effect to occur."

"There would be a one-to-one relationship, which means that there would be no change in the induced current or voltage."

"Because they would nullify each other."

"I have no idea."

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Dude I still don't understand the whole hand thing."

"Help me."

"Where does the flux capacitor show up in all of this?"

"I gotta say, the example relating to the induction forge made it really click how powerful magnetic fields are. I guess it sounds silly, since its one of the most fundamental and powerful forces in the universe, but for some reason that short clip made it click."

20170420

Astronomy quiz question: stellar evolution states in star cluster

Astronomy 210 Quiz 6, spring semester 2017
Cuesta College, San Luis Obispo, CA

At right is an H-R diagram of a star cluster. The stars in the dashed box are __________ stars that have __________ their main-sequence life.
(A) massive; not yet begun.
(B) massive; already ended.
(C) low-mass; not yet begun.
(D) low-mass; already ended.

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. The luminous stars at the top of the H-R diagram are massive stars that evolve faster than medium-mass and low mass stars, so a star cluster containing medium-mass stars that are in their main-sequence stage (and low mass stars that have not reached the main-sequence) will have massive stars that have already left the main-sequence.

Section 30674
Exam code: quiz06n4cI
(A) : 3 students
(B) : 16 students
(C) : 0 students
(D) : 1 student

Success level: 82% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.40

Astronomy quiz question: stellar evolution states in star cluster

Astronomy 210 Quiz 6, spring semester 2017
Cuesta College, San Luis Obispo, CA

At right is an H-R diagram of a star cluster. The stars in the dashed box are __________ stars that have __________ their main-sequence life.
(A) massive; not yet begun.
(B) massive; already ended.
(C) low-mass; not yet begun.
(D) low-mass; already ended.

Correct answer (highlight to unhide): (C)

All stars in a star cluster are born at the same time, but undergo stellar evolution at different rates depending on their masses. The dim stars at the bottom of the H-R diagram are low-mass stars that slower faster than medium-mass and massive stars, so a star cluster containing medium-mass stars that are in their main-sequence stage (and massive stars that are leaving the main-sequence) will have low-mass stars that have not already reached the main-sequence.

Section 30676
Exam code: quiz06sJ4c
(A) : 2 students
(B) : 3 students
(C) : 32 students
(D) : 2 students

Success level: 83% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.18

Astronomy quiz archive: stellar evolution

Astronomy 210 Quiz 6, spring semester 2017
Cuesta College, San Luis Obispo, CA

Section 30674, version 1
Exam code: quiz06n4cI


Section 30674
0- 8.0 :  
8.5-16.0 :   **** [low = 13.5]
16.5-24.0 :   ****
24.5-32.0 :   ******** [mean = 25.8 +/- 7.6]
32.5-40.0 :   **** [high = 40.0]


Section 30676, version 1
Exam code: quiz06sJ4c


Section 30676
0- 8.0 :  
8.5-16.0 :   ** [low = 15.0]
16.5-24.0 :   ************
24.5-32.0 :   **************** [mean = 26.1 +/- 5.9]
32.5-40.0 :   ********* [high = 36.5]

20170419

Online reading assignment: the Milky Way (SLO campus)

Astronomy 210, spring semester 2017
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.
"The Milky Way is a beautiful galaxy, to understand how stars grow and die, and in the consuming of other smaller galaxies create more stars, is very fascinating."

"I liked the comparison of the lights at eye level in the mirrored room with the flat disk shape of the Milky Way."

"How so many of the stars in our galaxy aren't visible to us."

"William and Caroline Herschel mapping our galaxy in 1785--it gives us an idea of the progression of the astronomical science."

"I really liked how you said that even though we do not know what dark matter is made of it, it doesn't mean that it doesn't exist. and then compared it to the Zodiac killer. Very cool!"

"I liked the part on dark matter because there was something oddly comforting about it. It's like being surrounded by an invisible blanket; we're pretty sure it's there all around us but we can't prove that it's true."

"I found self-sustaining star formation cool; how the death of stars can lead to the birth of new stars and how this process repeats over and over again."

"I found it very interesting that the arms and spurs of galaxies are maintained by either fusing with a dwarf spiral galaxy or by the explosions of type II supernovae massive stars which adds to the growth and repair of a galaxies spiral arms. I thought this was interesting as I never thought of it as something that would be necessary for a galaxy to need to do."

Describe something you found confusing from the assigned textbook reading or presentation preview, and explain why this was personally confusing for you.
"I didn't understand how the Milky Way's flat disk shape works out to be seen as just a strip of stars."

"How Shapley could estimate the distances of globular clusters. Don't understand how he calculated true luminosities of Cepheid variable stars."

"I am still a little confused on how you can map the Milky Way and how we know what it looks like, when we can't see much of it from Earth."

"The positioning of the global clusters and how we receive the information that tells us the location of the Milky Way center."

"What the rotation curve is and what it means."

"Dark matter is confusing, 'cause liiiiiike... what is it?"

"Dark matter, because how exactly can you find the dark matter?"

"The dark matter was kind of confusing to me. I don't understand how there's just a weird thing on the outside that causes gravity to spread. Its also confusing about how we have learned so much about the galaxy but some of it is just assumptions because we cannot see much due to the gas and dust."

"Why are spurs a separate thing? They look like arms but they have a different category. Is it due to size, composition, etc? I'm interested to know why they have a name besides arms."

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.  ***** [5]
(Never been able to see it.)  * [1]
(Unsure/guessing/lost/help!)  * [1]

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%.  ****** [6]
5%.  ** [2]
10%.  ********* [9]
50%.  *** [3]
100%.  ** [2]
(Unsure/guessing/lost/help!)  ********** [10]

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?
"Do I have my mobile phone? If so, I'd whip out that selfie cam. Even if it was dead I could use the reflection of the screen."

"Find a puddle of water and use that as your mirror. This works very well actually, trust me I know."

"Funny, I am going to Coachella this weekend, and I was laughing because I have never packed a mirror before while camping. But this weekend, YES, no bad hair at Coachella."

"Shave your head."

"Ask a squirrel or lizard what they think."

"Put on a ball cap and call it a day! No need to look in a mirror while you're camping."

"Run my fingers through my hair to see if my hair is different from normal."

Look at PimpStar Rims (*.html) for cars, or MonkeyLectric Rims (*.html) for bikes. Briefly explain how they work.
"Images are sent to the wheels using WiFi, and the wheel has a microprocessor and wireless adapter to interpret the signal and display it using the LED lights."

"Persistence of vision illusion from rapidly blinking lights coordinated to create patterns when swept across our field of vision."

"By spinning fast the display an image what would be un seen while standing still."

"Lights blink rapidly in a coordinated pattern."

"They use a strip of different colored lights in the wheel. So then, when the wheel is spinning, it looks like the entire wheel is one massive blob of color."

"The human brain can only process visual stimuli so quickly, so a sequence of lights blurs into a word."

"Okay, I have a bit of a hard time with this. But its something to do with the flashing patterns."

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"The subject matter in this class just gets more interesting and more mind boggling every time."

"It's interesting and cool to see how all the different astronomers work together, even if they don't live at the same time. They appreciate each other's work and continue it when they die. If astronomers didn't work together like this, we wouldn't know half of what we know now about space and stars and galaxies." (Kind of like self-sustaining star formation?)

"The Milky Way is beautiful and fascinating. It's one of my favorite things to look up at while I'm stargazing." (Me, too.)

"The puddle thing works really well too if you want to do your makeup. You know, just in case Mrs. P-dog wants to do her makeup while in the wilderness. #protip" (Or Mrs. P-dog could find a mysterious time portal in the middle of the desert.)

"Every time you post a link on these assignments, I cannot help but to click on it, and then immediately hit the back button, but it's too late; I have to redo the assignment." (Ctrl-click or mouse-right-click on the link to open a new tab/window, without affecting the reading assignment window. #protip)

"Do you consider staying in an RV camping?" (No, but what about if you built your own "RV" yourself?)

"P-dog, is this camping question like the house party? Why are there reocurring events we aren't actually having?" (I think you need to get out more.)

20170418

Online reading assignment: the Milky Way (NC campus)

Astronomy 210, spring semester 2017
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.
"That stars change in various ways throughout time and many variable stars pulsate like beating hearts."

"Dark matter--it's a cool way to explain how objects in the Milky Way behave, even though we have little idea what dark matter is made of."

"That our sun has traveled around our galaxy plenty of times. crazy to think that the sun revolves around the galaxy."

"The shape of the Milky Way galaxy, as I never really thought of being anything other than just a stream of stars."

"Dark matter is quite interesting. It's especially fascinating since we don't really know what it's made of."

"It was interesting to learn about spiral arms in comparison of a starfish of a Milky Way and how they are not fixed structures but they keep 'growing back.'"

"How there are spiral arms and also spurs in the Milky Way. I thought it was just made up of multiple arms, not spurs or much else really."

"The Milky Way--I wish I could see it in real life instead of just in pictures. Although it also made me hungry. Curse the people that thought it would be a good idea to name chocolate bars after astronomy references. Now all of us astronomy students can't catch a break. "

Describe something you found confusing from the assigned textbook reading or presentation preview, and explain why this was personally confusing for you.
"How stars stop pulsating when they evolve out of the instability strip."

"The way the galaxy's spiral arm stars light up then get dark, and how dark matter works."

"There was nothing confusing about these presentations. Everything was pretty straightforward."

"I honestly didn't get the whole concept of globular cluster. I get that our galaxy has them but I don't know how to spot one."

"It will always confuse me that there is no end to space and how there are more and more galaxies outside of the Milky Way."

"I found the gravity and orbits part of the slides in your blog to be a little confusing. I don't understand the difference between 'centered' and 'spread-out' gravity."

"Why is this always the hardest part to fill out?"

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.  ***** [5]
Not very much.  ****** [6]
Barely seen it.  *** [3]
(Never been able to see it.)  *** [3]
(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%.  ***** [5]
5%.  ** [2]
10%.  ********** [10]
50%.  [0]
100%.  [0]
(Unsure/guessing/lost/help!)  [0]

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?
"I wouldn't care what condition my hair is in."

"I never have bad hair days. Just planned crazy hair days."

"You could ask your friend if your hair looks like a galaxy with spiral arms or an unremarkable blob-shaped galaxy."

"Look in the body of water around you or a pot or pan you have for cooking."

"Assuming my vehicle is there and for the sake of the question there are no mirrors on the car, I'd use the reflection off a window. Even if I had my phone but it was dead I could use the screen as a mirror to check."

"See if any of the other campers run away screaming, "Monster!" or "Cool! That bear has an afro!""

"Feel my hair, see if it's tangled, or poofy from humidity."

"I would look at my own shadow. I always have a bad hair day so wouldn't be surprised."

Look at PimpStar Rims (*.html) for cars, or MonkeyLectric Rims (*.html) for bikes. Briefly explain how they work.
"It flashes lights at intervals that we perceive at images as it moves, due to the speed we process optical stimulae."

"They use a computer to program LED bars to light up and create certain images as they spin."

"Parts of the rims light up at certain points, making it look as if there is one stationary object when it is just a moving circle of blinking lights."

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"When are we ever going to use the telescopes at night?" (Hopefully this week, weather permitting, and allowing time for the ground to dry out from the rain. I'll give you a heads-up on this on Thursday.)

"Do you have bad hair days?" (I accept the fact that bad hair days are better than having no hair at all. Still, that's what baseball caps are for.)

"Do you have MonkeyLectric Rims for your bike?" (Yes.)

"Is this Milky Way stuff going to be on the next midterm?" (No, the second midterm covers only up to stellar evolution. The midterm study guide has already been posted online.)

"Are we going to have a movie day in class and watch Zodiac?" (Nah, instead we'll use the "free day" next week to review for the midterm, and for an extra-credit group worksheet.)

"If it is speculated that there is a supermassive black hole at the center of most galaxies, how could gravity be spread out so evenly throughout the galaxy?" (Actually this is how we know that the "evenly spread out gravity" in galaxies cannot come from the central supermassive black hole, but instead from "evenly spread out" dark matter.)

"If the vast amount of matter in the Milky Way is 'dark matter,' is there dark matter in our solar system and near Earth?" (Astronomers are already looking for evidence of that right now.)

"Please go over and explain these presentations and reading from the textbook a little more extensively this week."

20170417

Online reading assignment: generators

Physics 205B, spring semester 2017
Cuesta College, San Luis Obispo, CA

Students have a bi-weekly online reading assignment (hosted by SurveyMonkey.com), where they answer questions based on reading their textbook, material covered in previous lectures, opinion questions, and/or asking (anonymous) questions or making (anonymous) comments. Full credit is given for completing the online reading assignment before next week's lecture, regardless if whether their answers are correct/incorrect. Selected results/questions/comments are addressed by the instructor at the start of the following lecture.

The following questions were asked on reading textbook chapters and previewing 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.
"That there is more than one way to turn motion through a magnetic into electric energy."

"A magnetic field can be used to generate current. Single-pass generators generate current as a metal object moves through a magnetic field in one direction. The object must then be move back to the starting point, which generates current with opposite polarity."

"Why rail generators are 'single-use.' You cannot have the rod run along the rails infinitely, as the rails themselves cannot go on infinitely. Because they go on finitely, they must be 'reset' before able to be used again."

"I understand that there can be two different generators. One can only be used once before having to be reset. The other is continuous."

"Generators and motors are basically the same thing because they work pretty much the same way. Continuous generators do not have to be reset."

"I understand that single-pass generators can only be used once before resetting and a continuous generator does not explicitly need to be reset in order to provide motional emf and current. I also know that in a single-pass generator, as long as the rod is made to move through the magnetic field, the bottom end of the rod becomes negatively charged, while the top end of the rod becomes positively charged."

"Nothing."

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 a rotating rod in a magnetic field generates current."

"Continuous generators, specifically the rotating coil."

"More of the right-hand rule stuff and unsure about motional emf and what that means."

"How would bringing a wire loop back out of a magnetic field create an emf with opposite polarity than if it was brought in?"

"I do not understand exactly how the motion through an magnetic field creates electricity that can be stored. Where would the + and – wires go? Do they go to a battery or a capacitor? Does it matter?"

"The formula makes sense when looking at it, but in class discussion and examples would help with a better understanding of how it all works and how to use the RHR correctly! I also do not have a firm grasp on continuous generators and how those would work."

"A lot of things."

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 ↑.  ********* [9]
down ↓.  ****** [6]
left ←.  * [1]
right →.  **** [4]
into the page ⊗.  * [1]
out of the page ⊙.  [0]
(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.  *** [3]
out away from the center of rotation  ************ [12]
into the page ⊗.  * [1]
out of the page ⊙.  ***** [5]
(No direction, as this quantity is zero.)  [0]
(Unsure/guessing/lost/help!)  ** [2]

Explain what a generator is supposed to "generate."
"Generators create electricity."

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

"Voltage."

"It generates current and motional emf."

"A generator is supposed to generate electrical energy by converting mechanical energy into that type of energy."

Explain the meaning of "motional" in the term "motional emf."
"A motional emf is created by when moving an object through a magnetic field."

"Energy created by movement."

"Electromotive force that is generated by moving a conductive object through a magnetic field."

"Velocity."

"I'm unsure of what it means."

"Unsure/lost/guessing/help!"

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"An alternator in a car is a small generator. Alternators have a solenoid that spins mechanically and rubs against brush magnet (coils) creating electricity to supply power back to a car's battery. The primary reason for a failing alternator is worn-out brush magnets. These can be replaced with some mechanical skill for a couple bucks compared to a $100+ alternator." (Just be aware that replacing brush magnets (wire coils that produce magnetic fields from a current) with permanent magnets will actually convert your alternator to a magneto, which behaves differently than an alternator. An alternator needs a small amount of current put into it so the brush magnet coils inside will create a magnetic field, and the rotating solenoid inside moves through these magnetic fields to produce a greater amount of output current. A magneto does not need a small amount of current put into it to create a magnetic field for its rotating solenoid, but the output current characteristics may have specifications different enough that you shouldn't put it back in your car. However, you could instead connect it to a wind or water turbine to produce electricity, as you wouldn't need to worry about the input current to make it work.)

"Would using the current generated in a single-pass generator affect it?" (No, but you would just need to continuously put the same amount energy in to move the generator component through the magnetic field that is converted to electrical energy.)

"How would you reset a single pass generator?" (You would need to pick up the coil or rod, and move it back to its original position. Then start moving it again.)

"When would you use a single-pass generator?" (Practically speaking, only to explain the basics of generating electricity. Later we'll go over more useful and complicated generators that produce alternating current (AC) electricity.)

"Could you go over the examples for the generators that are on the blog?"

"Why are the motional emf values so inconsistent for a rotating coil and why do they change direction?" (The orientation of a rotating coil continuously changes with respect to the stationary magnets field, so the emf values will correspondingly vary as well.)

"I'm still having trouble wrapping my brain around how the magnetic field, and the subsequent force, acts on a metal coil, like in the last example."

"My hand hurts after so many contortions for the RHR1 and RHR2." (Prepare yourself for RHR3 later on.)

20170415

Physics quiz question: voltmeter reading in open circuit

Physics 205B Quiz 5, spring semester 2017
Cuesta College, San Luis Obispo, CA

An ideal 1.5 V emf source is connected to a switch, a light bulb, a resistor, and an ideal voltmeter. While the switch is open, the voltmeter reading is:
(A) zero.
(B) finite, non-zero value below 1.5 V.
(C) exactly 1.5 V.
(D) some finite value above 1.5 V.
(E) ∞.

Correct answer: (C)

From applying Kirchhoff's loop rule to the bottom loop of the circuit, which includes the 0.90 Ω resistor, 1.5 V emf source, and the voltmeter, the voltmeter will "read" the rise in electric potential from the 1.5 V emf source, and the drop in electric potential of the 0.90 Ω resistor:

Vvoltmeter = – I·(0.90 Ω) + (1.5 V).

However, no current will pass through the upper part of the circuit, which includes the 0.90 Ω resistor, the 4.0 Ω light bulb, and the open switch (which has an infinite resistance). This means that there is no drop in electric potential due across the 0.90 Ω resistor, and thus the voltmeter reading will be:

Vvoltmeter = – (0 A)·(0.90 Ω) + (1.5 V) = 1.5 V.

Sections 30882, 30883
Exam code: quiz05vLeY
(A) : 15 students
(B) : 5 students
(C) : 6 students
(D) : 1 student
(E) : 0 students

Success level: 23%
Discrimination index (Aubrecht & Aubrecht, 1983): –0.04

Physics quiz question: resistive circuit element with most current

Physics 205B Quiz 5, spring semester 2017
Cuesta College, San Luis Obispo, CA

An ideal 4.5 V emf source is connected to two light bulbs, a resistor, and an ideal ammeter. The __________ has the most current flowing through it.
(A) 2.0 Ω light bulb.
(B) 3.0 Ω light bulb.
(C) 5.0 Ω resistor.
(D) (There is a two-way tie.)
(E) (There is a three-way tie.)
(F) (Not enough information is given.)

Correct answer: (E)

From Kirchhoff's junction rule, the current passing through the 4.5 V emf source is equal to the sum of the current that passes through both light bulbs, and the current that separately passes through the 5.0 Ω resistor:

Iemf = Ilight bulbs + Iresistor.

Kirchhoff's loop rule is then applied to the (clockwise) path that passes through the 4.5 V emf source, the light bulbs, and back to just before the 4.5 V emf source:

0 = +(4.5 V) – Ilight bulbs·(3.0 Ω) – Ilight bulbs·(2.0 Ω),

which can then be solved for the amount of the current that passes through the light bulbs:

Ilight bulbs·(3.0 Ω) – Ilight bulbs·(2.0 Ω) = +(4.5 V),

Ilight bulbs = +(4.5 V)/(3.0 Ω + 2.0 Ω),

Ilight bulbs = 0.90 A.

Similarly, Kirchhoff's loop rule can also be applied separately to the (clockwise) path that passes through the 4.5 V emf source, the 5.0 Ω resistor, and back to just before the 4.5 V emf source:

0 = +(4.5 V) – Iresistor·(5.0 Ω),

which can then be solved for the amount of the current that passes through the resistor:

Iresistor·(5.0 Ω) = = +(4.5 V),

Iresistor = +(4.5 V)/(5.0 Ω),

Iresistor = 0.90 A.

Thus the amount of current that passes through the two light bulbs is equal to the amount of current that separately passes through the resistor, making this a three-way tie.

Conceptually this can also be seen by looking at the equivalent resistance of the two light bulb that are connected in series:

Rlight bulbs = 3.0 Ω + 2.0 Ω = 5.0 Ω.

Since the current that passes through the 4.5 emf source splits up to either go through the light bulbs (which together have a total resistance of 5.0 Ω) or instead go through the 5.0 Ω resistor, then it can be seen that equal amounts of current will pass separately either through the light bulb or through the resistor.

Sections 30882, 30883
Exam code: quiz05vLeY
(A) : 3 students
(B) : 3 students
(C) : 5 students
(D) : 5 students
(E) : 10 students
(F) : 0 students

Success level: 38%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.75

Physics quiz question: USB-C communication connection wire power dissipation

Physics 205B Quiz 5, spring semester 2017
Cuesta College, San Luis Obispo, CA

The communication connection wire within a Type-C USB cable has a resistance of 5.6 kΩ, and while attached to a smartphone, 80 µA of current passes through this wire. While this current passes through the communication connection wire, the electrical power dissipated within it is:
(A) 1.4⨉10–8 W.
(B) 3.6⨉10–5 W.
(C) 0.45 W.
(D) 2.5⨉103 W.

[*] androidauthority.com/usb-type-c-and-3-1-explained-656552/.

Correct answer (highlight to unhide): (B)

Given the resistance R and the current I that passes through the communication connection wire, the voltage drop across it is given by Ohm's law:

V = I·R,

V = (80⨉10–6 A)·(5.6⨉103 Ω) = 0.448 V.

The power dissipated in the communication connection wire is then:

P = I·∆V,

P = (80⨉10–6 A)·(0.448 V) = 0.00003584 W,

or to two significant digits, 3.6⨉10–5 W.

Alternatively, the power can be directly calculated by making a substitution for ∆V using Ohm's law:

P = I·∆V = I·(I·R),

P = I2·R,

P = (80⨉10–6 A)2·((5.6⨉103 Ω) = 3.6⨉10–5 W.

(Response (A) is I/R; response (C) is ∆V = I·R; and response (D) is ∆V·R= I·R2.)

Sections 30882, 30883
Exam code: quiz05vLeY
(A) : 6 students
(B) : 17 students
(C) : 0 students
(D) : 3 students

Success level: 65%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.86

Physics quiz archive: circuits (2)

Physics 205B Quiz 5, spring semester 2017
Cuesta College, San Luis Obispo, CA
Sections 30882, 30883, version 1
Exam code: quiz05vLeY



Sections 30882, 30883 results
0- 6 :   *** [low = 3]
7-12 :   ****
13-18 :   ******* [mean = 17.7 +/- 7.0]
19-24 :   *********
25-30 :   *** [high = 27]

20170414

Astronomy current events question: galaxy F01004-2237 supermassive black hole

Astronomy 210L, spring semester 2017
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!)
Amy Pullan, "Stars Regularly Ripped Apart by Black Holes in Colliding Galaxies" (February 27, 2017)
sheffield.ac.uk/news/nr/stars-ripped-apart-black-holes-1.684679
Observations of the F01004-2237 galaxy's supermassive black hole's ___________ as it ripped apart a nearby star suggest that such events may be very common.
(A) Doppler shifts.
(B) gravitational wobbles.
(C) magnetic field.
(D) brightness fluctuations.
(E) absorbed light wavelengths.

Correct answer: (D)

Student responses
Sections 30679, 30680
(A) : 3 students
(B) : 5 students
(C) : 3 students
(D) : 15 students
(E) : 6 students

Astronomy current events question: hydrogen-volcano greenhouse effect?

Astronomy 210L, spring semester 2017
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!)
Rebecca Valli, "Volcanic Hydrogen Spurs Chances of Finding Exoplanet Life" (February 27, 2017)
mediarelations.cornell.edu/2017/02/27/volcanic-hydrogen-spurs-chances-of-finding-exoplanet-life/
Scientists propose that planets with hydrogen-spewing volcanoes could create conditions that harbor life, by producing:
(A) hydrocarbon nutrients.
(B) protective ozone.
(C) flammable lava.
(D) a greenhouse effect.
(E) ingredients for water oceans.

Correct answer: (D)

Student responses
Sections 30679, 30680
(A) : 7 students
(B) : 2 students
(C) : 1 student
(D) : 19 students
(E) : 3 students

Astronomy current events question: SDSS 1557's metal-rich asteroids

Astronomy 210L, spring semester 2017
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!)
Bex Caygill, "First Evidence of Rocky Planet Formation in Tatooine System" (February 27, 2017)
ucl.ac.uk/news/news-articles/0217/270217-First-evidence-of-rocky-planet-formation-in-Tatooine-system
The SDSS 1557 system's white dwarf and brown dwarf may be orbited by shattered metal-rich asteroids, based on measurements of:
(A) Doppler shifts.
(B) gravitational wobbles.
(C) magnetic field strengths.
(D) brightness fluctuations.
(E) absorbed light wavelengths.

Correct answer: (E)

Student responses
Sections 30679, 30680
(A) : 2 students
(B) : 3 students
(C) : 8 students
(D) : 6 students
(E) : 13 students

Online reading assignment: magnetic fields of current-carrying wires and loops

Physics 205B, spring semester 2017
Cuesta College, San Luis Obispo, CA

Students have a bi-weekly online reading assignment (hosted by SurveyMonkey.com), where they answer questions based on reading their textbook, material covered in previous lectures, opinion questions, and/or asking (anonymous) questions or making (anonymous) comments. Full credit is given for completing the online reading assignment before next week's lecture, regardless if whether their answers are correct/incorrect. Selected results/questions/comments are addressed by the instructor at the start of the following lecture.

The following questions were asked on reading textbook chapters and previewing presentations on magnetic fields of current-carrying wires and loops.


Selected/edited responses are given below.

Describe what you understand from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically demonstrate your level of understanding.
"This section covered two magnetic fields exert forces on different test objects. Also, how magnetic fields are created by current-carrying wires."

"RHR2 is used to determine the direction of the B field created by a straight section of current-carrying wire. RHR3 is used to determine the direction of the B field created by a circular loop of current-carrying wire."

"The conceptual model for how magnetic fields and current function in the universe is best described as any wire loop in which current can run through that can be modeled as having a north and south pole. In this model we can use the vectors or direction of magnetic current and describe the nature of magnetic fields and how they impact moving charges and vice versa."

"Tonight reading assignment talked about the RHR2 and RHR3.The difference between RHR2 and RHR3 is that they the fingers represent different magnitudes. RHR3 is also used for circular loop of a current carrying a wire; when one curls their fingers in the direction of the current along the wire and the thumb points int the direction of the magnetic B field."

"Straight wires create a magnetic field that is circular around itself. Inside a loop, the magnetic field is perpendicular to the direction of the current."

"A current-carrying wire also produces a magnetic field of its own. As current travels through a wire it presents a circular pattern directing the magnetic field. A specific machine using these magnetic waves for a real-world application is with that of magnetic resonance imaging. A long solenoid is used to produce magnetic fields which exert forces on hydrogen atoms so prevalent in the body to to create an image of the physical location of internal structures."

"This section is on wires and loops. A source object creates a magnetic field everywhere around it. All wires have a magnetic loop around it even if they are closed or not."

"I understand how a magnetic field will exert a magnetic force on a test object such a straight section of current-carrying wire and also how to use the RHR 1. I get that the thumb is used either for the velocity direction of a positive charge, or for the direction of 'conventional current' along a straight section of wire."

"I know the right hand rules; that's really it."

Describe what you found confusing from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically identify the concept(s) that you do not understand.
"I was a little confused on how to know whether there would be magnetic forces on each other."

"I fill preety confident knowing I can comprehend an equation which works through these concepts and applies the different value to the formulas we will work with. I still have a little bit of trouble grasping the conceptual idea of magnetic fields and how for example north and south poles function in the real world."

"What I found a little confusing was the right-hand rules 2 and 3. I think that doing some examples actually using those hand gestures in class (like we did for right hand rule 1) would help very much!"

"Still somewhat confused on the hand rules. It seems that depending on which way you orient your hand you could have the same direction with your thumb and multiple possible directions for your B and F values."

"It is difficult to understand via online how to accurately use the RHR 2/3 so in class examples and explanation of this technique would be highly beneficial. I also don't really understand how the magnetic field works in terms of a straight wire and loops."

"I would like some more clarification on current-carrying wire examples."

"Can we go over more concepts in class? I don't even know what I'm confused on."

"I need to read the textbook."

"I just skimmed it."

"I think I'm okay this time around."

"All good!"

State/describe the symbol used for the "permeability of free space," and give its SI units.
0, where 'µ' is the symbol for micro and '0' is a subscript zero; Tesla·meters/Amperes."

"Permeability (symbolized by µ0) is measured in henries per meter, or newtons per squared-amperes."

"Didn't get to it yet."

"Can't find this answer."

State whether it is possible or not possible for the following pairs of objects to exert magnetic forces on each other.
(Only correct responses shown.)
The ends of two bar magnets: possible [79%]
The end of a bar magnet, and a stationary charge: not possible [42%]
The end of a bar magnet, and a moving charge: possible [58%]
Current flowing through a wire, and a stationary charge: not possible [46%]
Current flowing through a wire, and a moving charge: possible [71%]
Current flowing through a wire, and another wire with current in it: possible [58%]

For the magnetic field created by current in a long straight wire, indicate which right-hand finger(s) point along which directions.
(Only correct responses shown.)
Current I in long straight wire: thumb [88%]
Magnetic field B: curled fingers [88%]

For the magnetic field created by a current in a circular loop of wire, indicate which right-hand finger(s) point along which directions.
(Only correct responses shown.)
Current I in circular loop of wire wire: curled fingers [75%]
Magnetic field B: thumb [75%]

Explain the similarities/differences between a circular current loop, and a solenoid.
"There is no particular difference between the two, a solenoid is a large coil containing a large number of close turns and may be regarded as a current circular loop."

"A solenoid has a lot more turns than just a circular loop. Otherwise they're almost the same."

"A solenoid is essentially stacked current loops, so the magnetic effects stack as well, i.e. it gets more concentrated in the center."

"They are both a circular loop of wire with a current through it; a solenoid can act more like a magnet."

"I have no idea"

"I do not know what a solenoid is..."

"Not sure--I am kind of confused with this one!"

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"I didn't fully understand how the movement of a charge affects the capacity to exert magnetic forces. Does a charge only create a magnetic field if it is moving?" (Yes, and a magnetic field exerts a force only on a moving (test) charge.)

"Does the circular right hand rule work the same way on your left hand for a negative charge? (Only if you consider electron flow instead of (positive) current flow.)

"Are there any examples we do with the left hand? I hope not, because the right-hand rules are already confusing me!"

"I think a bit more practice with different hand rules examples would be helpful."

"Why do I feel mentally handicapped when reading this book?"

"Throwing up physics gang signs are fun!"