Presentation: magnetism

Look at these tiny compasses. Just look at them. And look at what happens to them when a bar magnet is brought nearby. Just look at them move around. (Video link: "PH MD SC TUTE 70031A V0541 Nuclear Magnetic Resonance NMR Model.")

In this presentation we discuss the attraction and repulsion of bar magnets in terms of magnetic fields, in parallel with a previous presentation discussing the attraction and repulsion of electric charges in terms of electric fields.

First, a "direct" model of magnetic forces.

By convention we label the two magnets that exert forces on each other as "source" and "test" poles, where the source magnet (with a north N and a south S pole) is said to be exerting a force on the test magnet (with a north n and a south s pole).

Throughout this discussion, don't worry about the magnitude of the forces these magnets exert on each other, as will focus only the direction of these magnetic forces. This force is attractive if the ends of the source magnet and test magnet face each other with opposite poles, and repulsive if the ends of the source magnet and test magnet face each other with like poles.

Note the convention where the source bar magnet (held stationary) has square ends, while the test bar magnet, which would be free to turn about a fixed center, is drawn like a compass with pointy ends.

Second, a more sophisticated "indirect" or two-step model of magnetic forces.

Instead of a source magnet directly exerting a force on a test magnet, in this two-step model, the source magnet is said to create a magnetic B field everywhere around it, and it is this magnetic field that exerts a force on a test magnet.

In order to visualize the magnetic field created by a source magnet, let's imagine filling space with many test magnets, all of them small enough that the only significant force exerted on them is due to the source magnet, and not due to the test magnets on each other. The directions of all of these test magnets shows us how the "influence" of the source magnet at each and every location in space--it is this influence throughout space that is the magnetic field of the source magnet.

Instead of drawing tiny test magnets at each and every location in space to represent the magnetic field of a source magnet, we place a series of test magnets end-to-end, and trace this magnetic field line. We can then replace the line of test magnets with magnetic field lines everywhere, where the direction of these lines denotes the direction of the north ends of the test magnets.

Let's focus on the first step of this two-step model. The source magnet, with a north pole N and a south pole S creates a magnetic field everywhere around it. (Again, don't worry about the magnitude of this magnetic field). The direction of all magnetic fields make closed loops, each coming out of the N pole (which is the "source" of magnetic field lines), and going into the S pole (which is the "sink" of magnetic field lines).

Notice how these magnetic field lines form closed loops, coming out of the N pole of the bar magnet, and coming into the S pole of the bar magnet.

Now what? If there is another magnet anywhere in the presence of this magnetic field, this magnetic field will exert a force on the test bar magnet's north pole n and south pole s. (Again, don't worry about the magnitude of this magnetic force). The direction of the magnetic force on the n pole is along the direction of magnetic field lines, and the force on the s pole is directed against the direction of magnetic field lines. As a result, the test magnet will often twist and move around corresponding to the forces exerted on its n and s poles.

Here, from before, we show the magnetic field lines filling in all space surrounding a source magnet with N and S poles. This magnetic field will cause a magnetic force to be exerted along a field line on the test magnet's n pole, and a magnetic force to be exerted against the field line on the test magnet's s pole. If we hold the middle of these test magnets stationary, but allow them to twist around, they will all align themselves accordingly where n poles "obey" and s poles "disobey" the field lines. (In any case, as a check the direction of the force on any test magnet poles should be attractive or repulsive depending on whether it is the opposite or same end as the source magnet N and S poles.)

Note that all of these magnets have both a north pole and a south pole. While there is speculation on the existence of magnetic monopoles, we will only consider magnetic dipoles, such as these bar magnets.

Presentation: denser and denser-er (revised)

Before we begin, a recap on the stellar remnants from two previous presentations on the lives and deaths of medium-mass stars and massive stars.

A white dwarf is the remnant of a medium-mass star, but to scale, it is much larger than the two possible remnants of a massive star--a neutron star, and a black hole, which actually has zero size (but is surrounded by an event horizon, which we'll discuss later).

However, while the white dwarf is the largest of these stellar remnants, it is not the most massive--a neutron star is more massive, and a black hole is more massive still.

All of these stellar remnants are incredibly dense, but which stellar remnant is densest? Least dense?

Since we have already discussed the behavior of dense white dwarfs (isolated or in close binary star systems) in a previous presentation, we will look at the other denser and "denser-er" stellar remnants--neutron stars and black holes. ("Dense and 'densibility?'")

First, dense neutron stars.

Imagine being in graduate school in astronomy, if you were Jocelyn Bell in the 1960s. Suppose she was given instructions to show up early on the first day of class with all the other students, in a muddy field behind campus, wearing old clothes. She was given gloves, wire cutters, and a sledgehammer, and was directed to a massive pile of wooden stakes and spools of cable. "Welcome to astronomy graduate school--your research project will be radio astronomy--and you will be building the school's first radio telescope."

After weeks of pounding in wooden stakes and stringing up cables to form a radio telescope mesh, Jocelyn Bell got down to actual radio astronomy, listening to whatever radio signals were detected and noting anything unusual or strange. This is the actual trace of an interesting repeating radio signal she noticed (along with an audio recording of a similar signal). (Video link: "2-10-denseanddenser-discovery.")

These mysterious signals--named "pulsars"--repeat at very precise intervals. The best model we have for explaining these pulsing radio signals is this "lighthouse model," which sends out light beams in certain directions, and the rotation--whether slow or fast--determines the interval between signals. (Video link: "Beacon, San Luis Obispo County Regional Airport, CA.")

Recall that the core of a massive main-sequence star will begin to collapse and implode at the end of its supergiant phase, crushing itself into a neutron star, concentrating its magnetic fields. These strong magnetic fields, with north and south poles, capture stray charged particles, forcing them to emit radio waves in certain directions. No one initially expected that the radio pulse evidence would lead there, but neutron stars turn out to be the answer to the mystery of pulsars. (Video link: "The Oblique Rotator Model for a Pulsar.")

Second, "denser-er" black holes.

This is the main outlet for water in Lake Berryessa water to pass through the Monticello Dam, and is a fair analog for a black hole. If you were swimming far from this dangerous portal, you wouldn't notice its presence, but if you were unfortunate to find yourself near it, you would definitely "feel" its presence. Provided you could swim fast enough, you will can make it back to safety, but there is an imaginary boundary around it at which you could not escape, no matter how fast you can swim--this boundary, or point-of-no-return, can be said to be its "event horizon." (Video link: "The Black Hole—The Glory Hole.")

Now instead of funnels and water, consider stars and black holes and space-time. All objects distort and curve space-time around themselves, and if you're far away from the distorted space-time around massive objects, where space-time is flat (in this crude two-dimensional model), you won't "see" this flatness, but you'll "feel" its flatness because you would not experience any gravitational forces. If you were near a massive object's distorted space-time, again you wouldn't be able to "see" this puckering, but you would "feel" it because your motion would tend to slide down this curved space-time towards the object--which is how gravitational forces work.

Note the space-time depressions around stars, but also the funnel-shaped distortion caused by something that doesn't seem to be there at all--this is the effect of a black hole on space-time. Remember that black holes can't be seen, but its effects on space-time--its gravitational field--can definitely be felt. And like the Lake Berryessa water outlet, you can get near it and get back out to safety provided you can move quickly enough, but there is an imaginary boundary around it at which you could not escape, no matter how fast you can move--this boundary, or point-of-no-return, for everything including light is the black hole's "event horizon." (Video link: "Black hole deforms space.")

So what would it be like to get close to a black hole, in the presence of its distorted space-time, and try to enter it?

Your textbook discusses tidal effects and "spaghettification," and time dilation effects in more detail, but let's try to model how an object would stretch out while circling closer and closer to a black hole, while apparently taking an infinite amount of time (as observed from afar) to circle and enter the event horizon--the point of no return around itself--where not even light can escape.

Here's that same funnel shape representing distorted space-time caused by a black hole. We'll throw in some marbles, taking care to throw them in a tight clump. At first, the clump of marbles will begin slowly spread out, but after circling the "black hole" the marbles will spread out from each other, forming a long line--spaghettification--due to this funnel shaped distortion of space-time!. Although we can't really show the effect of time dilation, there is a crude analog to this from the nature of this funnel-shaped distortion of space-time, as it seems like it takes longer and longer for the marbles to get down further and further into the throat of the funnel. (Video link: " Gravity Well (Reuben H. Fleet Science Center, San Diego, CA).")

So next time you see a charity donation funnel, try this for yourself--don't just roll in one coin, toss in several very closely bunched together, and watch the tidal effects stretch them out, and think about time dilation effects as they seem to take longer and longer to get further and further down the "throat" of curved space-time!

The evidence for black holes, even though we can't "see" them--is to "feel" for them, by looking at the effect of their gravity--their distorted space-time--on nearby companion stars. (Video link: "Black hole and companion star.")

In the subsequent in-class activity you will distinguish between companion stars with compact objects--whether white dwarfs, neutron stars, or black holes.

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

Astronomy 210, spring semester 2013
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, and on 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.

"I really liked the section that talked about the end of the sun (and Earth), it sounds awesome but incredibly unfortunate. Thank god I wont be there."

"I found it interesting which stars live the longest and how they all eventually die. The red dwarfs live the longest and if you are smaller then you live longer."

"I found the type II supernova bounce very interesting. I think its very interesting that all of the energy/motion takes effect in the outer layer (smaller ball)."

"I found it interesting how if close-pair binary stars are born at the same time, the one that dies off first will gather hydrogen from its neighboring star and depending on how fast it gathers the energy, will determine how it's ending will be."

"What I found interesting was the fate of the sun and the end of the earth section. It's amazing how the sun fuses hydrogen into helium and becoming more luminous, but also scary that in five billion years the sun will end the world."

"The difference between the nova and a type Ia supernova."

"Everything I've been taught about black holes from movies and TV shows were false."

"Everything was interesting! The whole death of stars subjects is my favorite so far!"

"I didn't even know black holes actually existed. I thought they are more of a made up concept."

Describe something you found confusing from the assigned textbook reading or presentation preview, and explain why this was personally confusing for you.

"After reading about black holes I have a basic understanding of what they are, but what confused me was the concept of someone falling into a black hole and what it would look like to a viewer. Time dilation is discussed but this didn't click with me. I get the person if supposedly falling slowly but I need to hear another explanation maybe."

"How an explosion happens with stars, because it doesn't seem possible."

"I think all of it is confusing in my opinion."

A Hummer H2 and a SmartCar ForTwo will travel the same distance with a full tank of gas. Briefly explain how this is possible.

"A Hummer has a huge gas tank, but gets almost no miles to the gallon, while a SmartCar has a tiny gas tank and can get a large amount of miles per gallon."

"The size of the tank and the miles per gallon of the vehicles are inversely proportional."

Briefly explain the difference between a nova and a type Ia supernova.

"Nova is a star exploding, a type la supernova is a big star exploding."

"A nova is an explosion on a white dwarf and can occur many times, whereas a type Ia destroys the star completely leaving behind nothing. Both occur due to nuclear reactions happening on the star."

"A white dwarf stealing hydrogen from a companion star slowly is a nova, and a type Ia supernova is when it steals hydrogen quickly."

"A nova is an old star flaring up or a violent explosion involving a white dwarf, whereas a type la supernova occurs when a white dwarf in a binary system receives enough mass to exceed a certain limit and then collapses."

The first rule of astronomy class is...

"Don't talk about astronomy class." (5 responses.)

"Don't talk about/never trust astrology." (2 responses.)

"To pass astronomy class."

"Cheeseburgers for all."

"Respect/call you P-dog!" (3 responses.)

"Probably do the reading."

"Go to class." (2 responses.)

"You are part of the universe and not just an observer."

"Don't be tardy for the party?"

"Don't fall into a black hole?"

"I have no idea..." (2 responses.)

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

"What is the ring around the moon on some nights, it also happens sometimes with the sun?" (Ice crystals in the sky can cause a halo around the moon or sun.)

"Can we have a short class because it's almost spring break?" (I'm not sure Wednesday night qualifies as "almost" spring break.)

"Explain black holes the best you can. This topic is so cool. Thanks P-dog." (Imma try. Imma try real hard.)

"You should show us the Astronomy Picture of the Day at the beginning of the class period instead of a cartoon. (You should make the Astronomy Picture of the Day website your homepage in your browser. Just saying.)

Online reading assignment: advanced electricity

Physics 205B, spring semester 2013
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 advanced electricity concepts.

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 video on how to make a lighter with a battery and a gum wrapper is pretty cool! Definitely gonna try that. I'm surprised there is that much heat created."

"I think it's interesting that if the current is split across two parellel resistors, the sum of current running through those resistors is more than the current running through the original wire. Sorry that wasn't the best explanation."

I found it interesting to learn why electricians keep one hand behind their back so they do not complete the circuit and get a charge running through their body."

"How series and parallel circuits work."

"Reading about energy in circuits was interesting to me. Energy has always been interesting, and I am excited that we are learning a little about it."
"I found out that as more appliances are plugged into the same household circuit, the resistance becomes dangerously low which can cause overloaded outlets and lead to fires. This was personally interesting because I didn't know this could happen in the household."

"I never knew that plugging in multiple sockets to an outlet decreases the resistance. Interesting."

"Runaway current is an interesting topic because some people do stupid things like trying to plug in too many things into an outlet."

"I thought it was interesting how they make electric fences. By touching the fence you complete the circuit through yourself and the ground and get a nasty shock."

Describe something you found confusing from the assigned textbook reading or presentation preview, and explain why this was personally confusing for you.

"I am a little confused about the I²·R equation and how to use it."

"Still confused on a lot of the equations, trying to remember what all the symbols stand for."

"Confusing how to combine series and parallel circuits."

"Ammeter and voltmeters, I don't quite follow."

"I'm confused about the difference between power and energy. Just because they are so similar."

"I am confused on what exactly causes the fire with the gum wrapper and battery."

"Although energy is interesting, I find it confusing as well. Also reading about the effects of an electrical current on the body made me realize how dangerous electricity really is."

"The equations. They are always confusing to me. So many freaking letters; I want to punch the computer. I feel like I have a better grasp on concepts when I read the blog.

"Power dissipation."

Plugging in and using additional electrical appliances in the same household circuit would increase the total __________ in the wiring inside the house.

current. : 17 responses
voltage. : 2 responses
(Both of the above choices.) : 1 response
(None of the above choices.) : 0 responses
(Unsure/guessing/lost/help!) : 4 responses

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

"The power dissipation equation seemed simple enough, but is the final answer work = joules per second?" (Yes, and joules per second is also watts.)

"I am having a hard time separating power from energy. The textbook says that power is the rate at which energy conversion takes place?" (Yes. Energy is energy. The rate at which energy is used per time is power. Think of money as money. The rate at which money is spent per time is...scary.)

"I was confused about the difference between the ampere and the watt, is the ampere so much wattage passed per time?" (Watt? Anyways, charge is charge. How much charge flows per time is current, measured in amperes. And energy is energy; how much energy is used per time is power, measured in watts. Units can get pretty crazy in electromagnetism, and we're not done yet.)

"One time I had a car battery in the back of my truck and I was driving and saw a flash and thought it was just a reflection but then I went around this turn and I saw a spark in the back of my truck. I pulled over and found that a grounding rod had rolled on top of the battery and was connecting the terminals, the end of the grounding rod burned through my truck liner....crap!" (Yes, shorting out a car battery (as was done in the first season of Breaking Bad) is basically much more dangerous version of using a gum wrapper to short out a AA battery.)

Astronomy current events question: nearby star system WISE J104915.57-531906 brown dwarfs

Astronomy 210L, spring semester 2013
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!)

Barbara K. Kennedy, "'The Closest Star System Found in a Century," March 11, 2013
http://science.psu.edu/news-and-events/2013-news/Luhman3-2013

The brown dwarfs in the newly-discovered nearby WISE J104915.57-531906 system are:
(A) carbon-rich stars.
(B) warm black holes.
(C) white dwarf remnants.
(D) too small to start fusion.
(E) absorb more light than they emit.

Correct answer: (D)

Student responses
Sections 30678, 30679, 30680
(A) : 6 students
(B) : 0 students
(C) : 0 students
(D) : 34 students
(E) : 9 students

Astronomy current events question: Jupiter "hot spots"

Astronomy 210L, spring semester 2013
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!)

Jia-Rui Cook and Elizabeth Zubritsky, "'Hot Spots' Ride a Merry-Go-Round on Jupiter," March 14, 2013
http://www.nasa.gov/mission_pages/cassini/whycassini/cassini20130314.html

Images from NASA's Cassini spacecraft provides evidence that Jupiter's "hot spots" are created by conditions similar to Earth's:
(A) mantle circulation.
(B) atmosphere and oceans.
(C) gravitational fluctuations.
(D) shield volcanoes.
(E) northern lights.

Correct answer: (B)

Student responses
Sections 30678, 30679, 30680
(A) : 5 students
(B) : 28 students
(C) : 12 students
(D) : 2 students
(E) : 3 students

Astronomy current events question: building blocks of life on Mars?

Astronomy 210L, spring semester 2013
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!)

DC Agle and Dwayne Brown, "NASA Rover Finds Conditions Once Suited for Ancient Life on Mars," March 12, 2013
http://www.nasa.gov/mission_pages/msl/news/msl20130312.html

NASA's Mars rover Curiosity shows that ancient Mars could have supported living microbes by drilling into sedimentary rock and finding:
(A) DNA fragments.
(B) fossilized microorganisms.
(C) chemical building blocks of life.
(D) water-bearing crystals.
(E) air pockets.

Correct answer: (C)

Student responses
Sections 30678, 30679, 30680
(A) : 0 students
(B) : 6 students
(C) : 31 students
(D) : 11 students
(E) : 2 students

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

Astronomy 210, spring semester 2013
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, and on 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.

"Novae and type la supernovae. They were the only thing I kind of understood."

"I thought it was interesting that stars die and give birth to new stars. I didn't really ever think about it in that way. I just look up and see the stars the way they are. Now I know that some stars are maybe 'sons' or 'daughters' of dead stars."

"Black holes, totally radical!"

"It's interesting how main-sequence stars die after they use up all their hydrogen."

"How the entirety of a low mass star is used for fuel as opposed to a massive star which just uses its core. A low mass star circulates hydrogen throughout its whole body to the core for fusion."

"Higher mass stars can burn other elements like carbon, oxygen, and neon when their supply of hydrogen and helium runs out. I always assumed all stars only used hydrogen and helium as fuel, but after reading this section I found out that if a star has enough mass it could also burn up carbon, oxygen-neon, and silicon besides hydrogen and helium."

Describe something you found confusing from the assigned textbook reading or presentation preview, and explain why this was personally confusing for you.

"I am lost and really confused by the presentation."

"This reading was pretty straightforward. Nothing was too confusing."

"What was confusing to was the capacity for the medium-mass stars and the low-mass stars. Why doesn't the medium-mass star have more capacity, as it's bigger?"

"Black holes, the effect they have on space."

"It was so much information...BRAIN OVERLOAD"

"I'm still trying to understand the star cluster H-R diagram. I need more work figuring out where massive stars, medium-mass stars, and low-mass stars are along the main sequence line. The analogy of star clusters being like a house party helps me understand the order of star formation."

A Hummer H2 and a SmartCar ForTwo will travel the same distance with a full tank of gas. Briefly explain how this is possible.

"What confused me from the presentation is how a Hummer and a SmartCar could travel the same distance with a full tank of gas. Since I thought the Hummer would be able to go farther than the SmartCar since it was bigger."

"The comparison between the Hummer and SmartCar was a bit confusing because you would think a SmartCar would go longer miles because of the higher miles per gallon."

"A Hummer has a larger tank of gas but gets less miles per gallon and a SmartCar ForTwo has a smaller tank of gas but gets more miles per gallon so they equal out distance-wise."

"A Hummer has a huge gas tank with horrible gas mileage, and a SmartCar has a tiny gas tank with excellent gas mileage."

Briefly explain the difference between a nova and a type Ia supernova.

"If a nova happens then the star will eventually recharge with hydrogen but if a type la supernova happens then everything is gone."

"A nova is a white dwarf flaring up, a type Ia supernova is the destruction of the white dwarf."

"A nova is caused by a slow biuldup of hydrogen on a white-dwarf and can happen multiple times. A type Ia supernova occurs when hydrogen build up quickly on a white dwarf, causing a massive explosion destroying the white dwarf and the neighboring star."

"I'm completely lost."

The first rule of astronomy class is...

"Don't talk about astronomy class." (2 responses.)

"Learn astronomy."

"Have fun." (2 responses.)

"Show up early."

"To be awesome! :P." (2 responses.)

"P-dog is the one and only supreme ruler of Astronomy 210. You do not question P-dog. You dare not look P-dog in the eye. You will die a slow and painful death."

"No cuts, no buts, no coconuts?"

"To ask questions!"

"What is the evidence?"

"Get to class on time and be prepared."

"Always be a stargeek?"

"I have no idea..." (5 responses.)

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

"What causes different elements to have different spectra?"(Different energy levels, which electrons will make different jumps up or down to absorb/emit different energy photons.)

"I don't have a 'study buddy' for this class, so when I miss class or an assignment, and don't know the correct answers for flashcard questions, where can I turn for help?" (See me after class. Come to office hours. Make an appointment for an office visit. E-mail me. Whatever it takes.)

"Can we see a picture of Mrs. P-dog? Like what does she look like?" (Didn't I show you this picture of Mrs. P-dog and me from 15 years ago already?)

Online reading assignment: circuit analysis

Physics 205B, spring semester 2013
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 circuit elements and circuit analysis.

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 loved the electrochemistry unit in my chemistry class, so I'm interested to learn about electricity from a physics point of view!"

"I've learned this before in high school, I liked it then because I was taking an electronics class. Not too sure how I like it now."

"I really liked seeing the group of kids messing with an electric fence, passing the shock. It's fun to do that."

"If we follow a complete loop in an electrical circuit, such that we wind up were we started, the electrical potential rises and the electrical potential drops must all add up to zero. This is interesting because it all goes in a loop."

"The video of the electric current going through plywood is really fascinating."
"Kirchhoff was a pretty cool guy. He analyzed circuits and wasn't afraid of anything."

"Connecting resistors in a different manner increases or decreases the current. I'd like to see this in person."

"I think that it is interesting that we are going over material that everyone can easily relate to."

"Nothing really."

"It's all interesting and I am looking forward to learning about how circuits work."

"The video of the metal melter was really cool and i want to try it!"

"How your car battery supplies less voltage due to the internal resistance of the battery when supplying amps to your starter."

"I find the loop rule interesting. It seems like such a simple concept but makes so much sense."

Describe something you found confusing from the assigned textbook reading or presentation preview, and explain why this was personally confusing for you.

"Formalas."

"I am a little confused on what the point of resistors are, I get that the more resistance the lower the current flow, but why?"

"I found the concept of resistance a little confusing. I just need some clarification on how it can be measured or used."

"Electricity in general has always been confusing to me. I think it's confusing how electrons flow in the opposite direction of what we call 'current.'"

"Nothing was too confusing."

If you go through a resistor along the direction of current, the amount of current __________ and the voltage ___________.

decreases, decreases. : 2 responses
decreases; remains the same. : 4 responses
decreases; increases. : 2 responses
remains the same; decreases. : 2 responses
remains the same, remains the same. : 1 response
remains the same; increases. : 1 response
increases; decreases. : 0 responses
increases; remains the same. : 1 response
increases; increases. : 0 responses
(Unsure/guessing/lost/help!) : 7 responses

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

"I finally figured out everything for this past midterm and now I have to start all over!" (It's the circle of life. Or at least, of introductory physics.)

Online reading assignment question: helpful/unhelpful Midterm 1 astronomy study tips

Physics 205B Reading assignment 14, spring semester 2013
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.

Describe something notable that either helped or did not help with studying for this midterm. Selected comments may be discussed in class. (Graded for completion.)

The following are all of the student responses to this question, verbatim and unedited.

"the explanation of the problems helps, better when we do the whole problem start to finish in terms of things to think about or look for when setting up to do a problem"

"Working with a study group!"

"going over the blogs helps because they explain concepts. P-dogs tests are very conceptual, so they help a lot"

"Using the answer key for the textbook helps me better understand how to go about a problem."

"having worked out solutions to related problems helps me study and understand the material better."

"Going over the quizzes helps, the flashcard questions help when you have the correct answers."

"practice midterm and the study guide"

"four exams this week"

"Going over the previous quizzes and seeing previous years tests are quite helpful."

"Studying practice problems help. I like working out problems, but the conceptual material is sometimes hard to figure out how to study for. I can sort of gain insite from doing work out problems to understand the concepts."

"Online websites, reviewing your quizzes, and reading the text books."

"Everything is pretty well explained regarding what is going to be covered, i just procrastinated a lot! :("

"The worked out equations from past midterms have been really helpful. I find them to be useful when you are stuck on similar problems."

"The practice time to do problems as groups is always helpful, and the time in lab to use the formulas is helpful."

"cramming doesnt help in the test."

"study groups"

"It helps the most to go over what we work on in class with the problems and the steps to go about them, otherwise the material can be overwhelming of where to focus."

"Being in class is a huge help. As long as I'm in class, I learn something."

"Doing practice problems is helpful. Study groups tend to be unproductive, so they are not helpful!"

Online reading assignment question: confusing Midterm 1 physics topics

Physics 205B Reading assignment 14, spring semester 2013
Cuesta College, San Luis Obispo, CA

130324Phys205B-confusing-c
http://www.flickr.com/photos/waiferx/8587698704/
Originally uploaded by Waifer X

Wordle.net tag cloud for confusing topics covered in Midterm 1, generated by responses from Physics 205B students at Cuesta College, San Luis Obispo, CA (www.wordle.net/show/wrdl/6522935/Untitled).

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.

List at least three words describing confusing subjects covered in class (up through this midterm). (Graded for completion.)

[Responses have been edited to consolidate related common subjects.]

Student responses
Section 30882

lenses, raytracings, virtualimages
electricfields, electricforces, conceptualquestions
electricfields, electricforces
flustering, deceivinglycomplex
microscopes, charges
correctivelenses, nearpoint
electricfields, interference, diopters
polarization, raytracings, TIR
nothing
interference, polarization, electricfields, electricforces
polarization, raytracings, lenses
electricpotential, capacitance, electricfields
interference, polarization, electricpotentialenergy
telescopes, electricfields, electricforces, interference, diffraction
polarization, refraction, electricpotential
somanyformulas
interference, diffraction

Describe your most confusing subject, and briefly explain why this subject interested you. (Graded for completion.)

The following are all of the student responses to this question, verbatim and unedited.

"I think that lens diagrams and explaining how they work is pretty confusing. I just need to study it harder."

"I find it confusing how to solve conceptual equations, even when I understand the equations for the problem."

"i am still having trouble with electric fields and forces problems, mainly because i have a hard time remembering what some of the terms mean in the equations."

"Polarizes, the concept just took me a while to wrap my head around. Also understanding the difference between electric field and force equations."

"calculating force of a charge"

"the corrective lens part of the class"

"Diopters. I guess this is more of a complaint than a confusing thing. I just think that the may we explain magnification is stupid. The prescription for glasses should be the magnification needed, not an inverse of it. Some people just want to feel more self-important by making things harder for everyone else."

"Polarizers were difficult to understand. Some of these concepts are just hard to follow. I sort of get them, but I need to truly understand these concepts."

"Diffraction, double slit and single slit"

"for some reason i can get to explain how this topics work or how they relate to something i can do."

"The most confusing thing we have covered this semester has been polarizers for me. I have a hard time picturing what is going on in my head."

"I had to put the most work into forces and fields to grasp the concepts."

"polirizer because i dont get them a lot."

"The most confusing subject was electrical fields and forces because they are so similar to eachother."

"Polarizers still confuse me despite all your efforts to explain, i still just feel like there are so many directions that light can come at you from, how can you say a specific direction?"

"Some of the wavelength stuff. Defraction, internal reflection. Lots of formulas and measurements that require the right reference point."

"Interference of all kind confuses me. Double slit, single slit, constructive, and destructive are all confusing to me."

Online reading assignment question: interesting Midterm 1 physics topics

Physics 205B Reading assignment 14, spring semester 2013
Cuesta College, San Luis Obispo, CA

130324Phys205B-interesting-c
http://www.flickr.com/photos/waiferx/8586572081/
Originally uploaded by Waifer X

Wordle.net tag cloud for interesting topics covered in Midterm 1, generated by responses from Physics 205B students at Cuesta College, San Luis Obispo, CA (www.wordle.net/show/wrdl/6522906/Untitled).

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.

List at least three words describing interesting subjects covered in class (up through this midterm). (Graded for completion.)

[Responses have been edited to consolidate related common subjects.]

Student responses
Section 30882

polarization, sound , wavelength
lenses, images, electromagnetism
microscopes, TIR, interference
lasers, polarization, opticalinstruments
intuitivelyconfusingconcepts
hyperopia, reflection, incidentangle
telescopes, microscopes
raytracings, refraction, telescopes
testcharges, telescopes, interference
optics, electricfields, diffraction
refraction, lenses, microscopes
microscopes, interference, diffraction
polarization, TIR, interference
electricpotentialenergy, polarization, raytracings
lenses, lenses, lenses
myopia, electricfields, reflection
onlysortofdangerous
microscopes, refraction, polarization,

Describe your most interesting subject, and briefly explain why this subject interested you. (Graded for completion.)

The following are all of the student responses to this question, verbatim and unedited.

"I think the polarization of light is pretty interesting. I always wondered how polarized sunglasses worked."

"I liked studying images because it was interesting to learn how they work."

"I thought it was fascinating that, with corrective lenses, I'm looking at a virtual image"

"I find it interesting how the lenses in a microscope work."

"i thought that corrective optics was a cool subject because it was cool distinguishing the difference between real and virtual images,"

"Corrective optics, it just really blew my mind that everything I see through my contacts from 0.33m and on is a virtual image."

"how polarization works and how two perpendicular polarizations will not allow light to pass through"

"electrons and how the act"

"Refraction, I like the concept of total internal reflection. Floating under the surface of a pool, it can be almost a perfect mirror if the surface is calm enough."

"Test charges. I am not sure why it is interesting to me."

"Corrective optics because I never knew what went into correcting your eye sight"

"the refraction and reflection material was interesting since i could some how related to chemistry also the microscopes and lenses helped me visualize how a microscope works."

"The most interesting thing we have covered this semester to me has been the microscope. I have recently started using microscopes in biology and it is interesting to see how they actually function."

"I love snell's law because I see it in action all the time. I see glass in a whole new light."

"ray tracings because i love to draw ray tracings."

"It was interesting how the lens created different images, for cameras, eyes, glasses, telescopes and microscopes."

"I enjoyed learning about reflection because it really made sense to me and I could relate to it because I have witnessed it in my life on a regular basis."

"I like that we are just now getting into electricity. It seems most applicable and even a little intuitive."

"I really like refraction. Because I am a marine biology major, I spend a lot of time looking into water, so now I understand the physics behind the distortion of things in the water!"

Presentation: advanced electricity

Gum wrapper (foil-covered paper), cut into a strip, with a narrow center section: check. AA battery: check. Completing an electrical circuit to start a fire: priceless. (Video link: "How to Make A Prison Lighter From a Bubble Gum Wrapper. MUST SEE! :)")

In this presentation we introduce a few additional concepts as we further implement Kirchhoff's circuit rules in analyzing electrical circuits.

The videos shown in this presentation, as with many videos shown in this class, should never be attempted at home. Even if many of these videos use items commonly found at home. Even if these items are found, well, within the very walls of your house itself!

First, using the digital multimeters used in laboratory to measure current, or to measure voltage.

You have already used these digital multimeters to measure the resistance of Christmas light bulbs, when isolated from an electrical circuit, when the proper plugs and correct dial settings are used. These digital multimeters can also measure current or electric potential rises/drops of items wired in an active electrical circuit, provided the proper plugs and correct dial settings are used. And since measurements are being made on an item while the circuit is "live," some care must be taken in not only using the proper plugs and correct dial settings, but also in properly connecting the digital multimeter to the item of interest.

When measuring the current passing through a light bulb (or any other circuit element), the current that passes through the light bulb must also pass through the digital multimeter. This means that the wiring in the circuit must be "broken" open to connect the digital multimeter to measure current (making it an ammeter).

Does it matter whether the ammeter is connected to measure current before it passes through the light bulb, or to measure current after it passes through the light bulb?

Why must an ammeter have a resistance that is ideally zero (or at the very least, a very low resistance value)?

When measuring the amount of electrical potential used by a light bulb (or any other circuit element), the digital multimeter must be connected to both before and the current flows through the light bulb. This means that the wiring in the circuit is not modified in order to connect the digital multimeter to measure electric potential (making it an voltmeter), as it "feels" the amount of electric potential before and after the light bulb, and reports the difference (whether a drop or rise).

Why must a voltmeter have a resistance that is ideally infinity (or at the very least, a very high resistance value)?

Second, the rate of energy per time (power) used by a circuit element.

"Joule heating" is the historical term for the power (or rate of energy used per time) continuously used by a circuit element of resistance R due to the amount of current I flowing through it, as in these radiating coils.

(For the purposes of this class, we will consider the resistance R of materials to be constant with respect to temperature, although the resistivity of many materials will typically be strongly dependent on the temperature.)

Recall from a previous presentation that a change in electric potential ∆V represents the "potential" potential energy used by a charge. For each charge q that uses a certain amount ∆V of electric potential, the amount of electric potential energy used is ∆EPE = q·∆V. For a continuous flow of charges per time as in a current I, the rate of electric potential energy used per time (that is, power) is ∆EPE/∆t = (q·∆V)/∆t = (q/∆t)·∆V = I·∆V.

(If instead the ∆V represents a rise in electric potential (as in a battery) instead of a drop, then I·∆V represents the continuous rate of electric potential energy per time provided by the battery.)

Substituting in Ohm's law (I = ∆V/R), the two parameters I and ∆V in the power equation can each be substituted out, yielding two other equivalent equations for power, most notoriously, the "Twinkle, Twinkle, Little Star" form:

Twinkle, twinkle, little star
Power equals I-squared R.

Believe me, you won't get this out of your head. Ever.

So endless hours of amusement await you when solving power dissipation (or source) problems, so use caution when you use these equations, and more importantly, use only the equation you really need.

All household electrical outlets provide 120 volts of electric potential and operate independently of each other, and thus are wired in parallel to each other to the same 120 volt electromotive source. Recall that from a previous presentation that as more resistors are added in parallel to a circuit, the equivalent resistance decreases. This means that as more and more appliances are plugged into the same household circuit and turned on, then the equivalent resistance may become dangerously low.

This may result in a very large runaway current flowing through the wires leading to the overloaded outlets. Even though the wires ideally have a low resistance r, a large enough current I will cause the wire to heat up appreciably (as the rate of joule heating is I²·r), resulting in the insulation around the wires, and the joists and walls in contact with it to catch on fire.

In order to prevent runaway currents from happening in a household circuit (well, should common sense fail to prevent overuse of the outlets), a circuit breaker is designed to "trip" and interrupt the current should it become dangerously high. If this should happen, protocol is to first unplug any and all appliances in that part of your house before resetting the breaker to re-complete the circuit. (Video link: "A 6 Amp AC breaker trips on DC at 240 Volts 30 Amps.")

Remember the gum wrapper and AA battery fire starter at the start of this presentation? That is an analog for an older-type fuse that would have a slightly greater resistance than the wires, such that the fuse would experience higher joule heating and sacrificially melt first before the wires would themselves heat up, breaking the circuit. Unlike modern circuit breakers, a fuse is no longer functional after melting, and must be completely replaced. A highly dangerous, but unfortunately common workaround for when a replacement fuse was not readily available was to push a penny (or other small piece of conducting metal) into the fuse socket in order to re-complete the circuit--hopefully after unplugging any and all appliances in that part of the house!

Physics midterm question: unpolarized light, different polarizer sets

Physics 205B Midterm 1, spring semester 2013
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 22.45

Unpolarized light is incident on two sets of ideal polarizers. Discuss why a greater fraction of the incident light passes through polarizer set 1 than through polarizer set 2. Explain your reasoning using the properties of polarizers and polarization.


Solution and grading rubric:

• p:
  Correct. One-half of the unpolarized light will pass through the first polarizer in either set, but the first polarizer in set 1 will let vertically polarized light pass through, while the first polarizer in set 2 will let horizontally polarized light pass through. From Malus' law, the fraction of polarized light that passes through a polarizer is given by cos²θ, where θ (the angle between the polarization of light and the transmission axis of the polarizer) would be 15° for set 1, and 75° for set 2. Since (1/2)·cos²(15°) > (1/2)·cos²(75°), more light will pass through the set 1 than through set 2.
• r:
  As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes.
• t:
  Nearly correct, but argument has conceptual errors, or is incomplete.
• v:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner.
• x:
  Implementation/application of ideas, but credit given for effort rather than merit.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.
  

Grading distribution:
Section 30882
Exam code: midterm01p0C4
p: 26 students
r: 3 students
t: 4 students
v: 0 students
x: 0 students
y: 0 students
z: 0 student

A sample "p" response (from student 1337):
Another sample "p" response (from student 3698):

Physics midterm question: objects at diverging lens focal points

Physics 205B Midterm 1, spring semester 2013
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 23.63

Two diverging lenses available from a commercial optics supplier have focal length f = –6 cm and f = –9 cm, respectively[*]. An object placed 6 cm in front of the 
f = –6 cm lens creates an image. This same object placed 9 cm in front of the f = –9 cm lens also creates an image. Discuss why these images will have the same size. Explain your reasoning by using the properties of lenses, thin lens equations and/or ray tracings.

[*] edmundoptics.com/optics/optical-lenses/double-concave-dcv-spherical-singlet-lenses/uncoated-double-concave-dcv-lenses/1756.

Solution and grading rubric:

• p:
  Correct. Uses ray diagram and/or thin lens equation(s) to explain why the virtual images produced by objects placed at the focal points of diverging lenses must have the same size with respect to the original object.
• r:
  As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes.
• t:
  Nearly correct, but argument has conceptual errors, or is incomplete.
• v:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner.
• x:
  Implementation/application of ideas, but credit given for effort rather than merit.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.
  

Grading distribution:
Section 30882
Exam code: midterm01p0C4
p: 18 students
r: 2 students
t: 5 students
v: 11 students
x: 8 students
y: 0 students
z: 0 student

A sample "p" response (from student 1515):

Physics midterm problem: microscope construction

Physics 205B Midterm 1, spring semester 2013
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 24.42

The converging lenses available from a commercial optics supplier have focal lengths f = +0.30 cm and f = +0.45 cm, respectively[*]. A Physics 205B student would like to use these two lenses to construct a microscope with a "tube length" L (the distance from focal point to focal point) of 5.0 cm, where the f = +0.30 cm lens is used as the objective. Solve for (a) the angular magnification of this microscope, and (b) the distance from the object to the objective lens. (The near point of the Physics 205B student is 25.0 cm.) Show your work and explain your reasoning.

[*] edmundoptics.com/optics/optical-lenses/double-convex-dcx-spherical-singlet-lenses/uncoated-double-convex-dcx-lenses/1748.

Solution and grading rubric:

• p:
  Correct. Determines (a) angular magnification to be –930×, and (b) the object must be placed 0.32 cm in front of the objective lens.
• r:
  Nearly correct, but includes minor math errors. Determines angular magnification, but does not explicitly solve for the distance for the object in front of the objective lens, but instead understands that it must be held very near outside the focal point (0.30 cm) of the objective lens.
• t:
  Nearly correct, but approach has conceptual errors, and/or major/compounded math errors. At least has magnification, and some attempt in finding the distance for the object in front of the objective lens
• :
  Implementation of right ideas, but in an inconsistent, incomplete, or unorganized manner.
• x:
  Implementation of ideas, but credit given for effort rather than merit.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.

Grading distribution:
Sections 30882
Exam code: midterm01p0C4
p: 19 students
r: 2 students
t: 4 students
v: 6 students
x: 2 students
y: 0 students
z: 0 students

A sample "p" response (from student 1224):