Monday, October 26, 2009
Lab Today
Today, we'll do a lab on resonant LCR circuits, which should help out with our ongoing project.
Friday, October 23, 2009
Slides from today's lecture
You can find the slides Prof. Harrell used today (and many more he did not) here if you are interested.
Monday, we'll finish off magnetic induction ... so finish reading the appropriate chapter in Griffiths over the weekend if you would.
Monday, we'll finish off magnetic induction ... so finish reading the appropriate chapter in Griffiths over the weekend if you would.
Thursday, October 22, 2009
Tomorrow's recitation
Tomorrow, owing to an unavoidable funding-related meeting, I will not be able to make the recitation. Instead, Prof. Harrell will give a lecture about magnetism in real materials - where does permanent magnetism originate from, and why do magnets stick to your fridge? It will be mostly qualitative, but highly practical and interesting stuff.
And, yes, it might show up on the next exam or final, if you need another reason to show up ;-)
And, yes, it might show up on the next exam or final, if you need another reason to show up ;-)
Wednesday, October 21, 2009
HW 7.1 hint
Check this. The first problem is an awkward one, but valuable for precisely that reason. The first time you see it, you scratch your head wondering why the ring would rotate, but once you understand why, you've learned something important.
When it rains it pours: HW6 solutions
Problem 6 does not have a full solution, but based on the general velocity transformation we went over in class you probably got it ...
Anyway: HW6 solutions are out.
Anyway: HW6 solutions are out.
Homework 5 solutions
Very late ... but here they are. You'll get your graded HW5 back tomorrow.
Tomorrow, we'll continue with magnetic induction, with a heavy emphasis on the homework problems. Friday, Prof. Harrell will give a lecture on magnetic materials. Monday, we'll finish up induction and Maxwell's equations so we can start ac circuits on Wednesday.
Tomorrow, we'll continue with magnetic induction, with a heavy emphasis on the homework problems. Friday, Prof. Harrell will give a lecture on magnetic materials. Monday, we'll finish up induction and Maxwell's equations so we can start ac circuits on Wednesday.
Tuesday, October 20, 2009
Relativity
If you find relativity interesting, and you've had a bit of math, you'll probably find these lecture notes very nice. In fact, they've been turned into a book by Prof. Carroll, which has been well-received (it is what we use for our grad relativity course).
There is also a "non-nonsense" introduction to general relativity, the first bits of which should be familiar. Don't be scared by the tensors later on, the barrier is mostly the notation.
Anyway: good book, free preview online. Can't beat that.
There is also a "non-nonsense" introduction to general relativity, the first bits of which should be familiar. Don't be scared by the tensors later on, the barrier is mostly the notation.
Anyway: good book, free preview online. Can't beat that.
Monday, October 19, 2009
Saturday, October 17, 2009
Relativistic time dilation
... how the last minute of a football quarter seems to take half an hour watching it on TV, in spite of what the on-field clock says. (The second quarter will not seem to end ...)
Friday, October 16, 2009
Homework
Back to serious stuff. On number 9, remembering your calculus is a big help. For instance, note
and use the Lorentz transformations along with velocity addition. I'll spell this out more tomorrow in recitation.
For number 6, the x component of the velocity is just what you think it is. However: the y component of the velocity in the second reference frame would be
We'll go over the rest tomorrow, but you will find many of the other questions in my PH102 notes or previous PH102 homework sets. I'll give some hints on where to look ... but start with the problems at the end of Ch. 1.
\frac{dv_x^{\prime}}{dt^{\prime}} = \frac{dv_x^{\prime}/dt}{dt^{\prime}/dt}
and use the Lorentz transformations along with velocity addition. I'll spell this out more tomorrow in recitation.
For number 6, the x component of the velocity is just what you think it is. However: the y component of the velocity in the second reference frame would be
u_y^{\prime}=\frac{dy^{\prime}}{dt^{\prime}} Since y=y^{\prime}the numerator is trivial. However, there is still time dilation, so you'll need to use the Lorentz transformation to relate dt and dt'. Again, remember the calculus trick above, and the main point is this: the velocity along the direction of relative motion follows the addition formula we derived, but along the orthogonal direction, there is still a transformation because while distance is uncontracted, time is still dilated.We'll go over the rest tomorrow, but you will find many of the other questions in my PH102 notes or previous PH102 homework sets. I'll give some hints on where to look ... but start with the problems at the end of Ch. 1.
Randomness
Since we're about to finish up relativity, I've been thinking a lot about what I think is the 'correct' way to cover mechanics, E&M, and relativity. In doing some reading, I ran across this quote, which made me think even more that our usual approach is severely lacking:
Just something I find interesting: despite what you might hear in intro courses or popular accounts, E&M played a big role in inspiring relativity, it was not merely the speed of light and the aether.
Problem is, the history is in a way conceptually out of order, and harder to teach (IMHO). My evidence being how relativity is often presented in intro-physics sequences: as a total non sequitur, just sort of shoved in there. Moreover, in spite of the preserved historical ordering of topics, the motivation given usually starts out with the Michelson-Morley experiment and the aether, and quietly ignores EM forces in different reference frames. Thus, we keep the historical ordering, but throw out crucial parts of the original (and exceedingly insightful) motivation!
I prefer to follow the Mechanics -> Relativity -> Electromagnetism ordering, which I guess I've made obvious now. Relativity is hard conceptually, but I find students have a harder time with electromagnetism at first, particularly magnetism. Having relativity under your belt at least makes the magnetic field seem less arbitrary, which is reassuring I think. Introducing relativity after mechanics and E&M, while historically accurate, sometimes makes it seem like an ugly hack, which it wasn't at all. If you are going to do relativity after E&M anyway, why not cover the E&M aspect too? Purcell's book does a wonderful job.
On the other hand, doing relativity after mechanics is harder to motivate sometimes, and one has to resort to strange little thought experiments to find anything wrong with Newtonian physics. Thinking about relativity right after a mechanics course, though, has your brain in the right mode and the kinematics fresh in your mind.
Of course, I suppose it is just difficult either way, reality is a harsh mistress. And Poincare probably deserves more credit than he gets.
"The influence of the crucial Michelson-Morley experiment on my own efforts has been rather indirect. I learned of it through H.A. Lorentz's decisive investigations of the electrodynamics of moving bodies (1895) with which I was acquainted before developing the special theory of relativity . . . What led me more or less directly to the special theory of relativity was the conviction that the electromotive force acting on a body moving in a magnetic field was nothing else than an electric field. - Albert Einstein
Just something I find interesting: despite what you might hear in intro courses or popular accounts, E&M played a big role in inspiring relativity, it was not merely the speed of light and the aether.
Problem is, the history is in a way conceptually out of order, and harder to teach (IMHO). My evidence being how relativity is often presented in intro-physics sequences: as a total non sequitur, just sort of shoved in there. Moreover, in spite of the preserved historical ordering of topics, the motivation given usually starts out with the Michelson-Morley experiment and the aether, and quietly ignores EM forces in different reference frames. Thus, we keep the historical ordering, but throw out crucial parts of the original (and exceedingly insightful) motivation!
I prefer to follow the Mechanics -> Relativity -> Electromagnetism ordering, which I guess I've made obvious now. Relativity is hard conceptually, but I find students have a harder time with electromagnetism at first, particularly magnetism. Having relativity under your belt at least makes the magnetic field seem less arbitrary, which is reassuring I think. Introducing relativity after mechanics and E&M, while historically accurate, sometimes makes it seem like an ugly hack, which it wasn't at all. If you are going to do relativity after E&M anyway, why not cover the E&M aspect too? Purcell's book does a wonderful job.
On the other hand, doing relativity after mechanics is harder to motivate sometimes, and one has to resort to strange little thought experiments to find anything wrong with Newtonian physics. Thinking about relativity right after a mechanics course, though, has your brain in the right mode and the kinematics fresh in your mind.
Of course, I suppose it is just difficult either way, reality is a harsh mistress. And Poincare probably deserves more credit than he gets.
A note from the Russian club
The Russian Club will be hosting Dr. Anthony Vanchu, Director of the Johnson Space Center Language Education Center. He will be giving a lecture entitled "Teaching NASA Astronauts to Speak Russian, or How I Came to Love the FGB" on Thursday, October 22, at 6:30 pm in ten Hoor room 125. The Russian Club would like to invite you and your students to attend this lecture and encourages you to spread the word of this upcoming opportunity.
-Jonathan Williams
Vice President
UA Russian Club
Wednesday, October 14, 2009
Friday
Friday, we'll worry about energy & momentum in relativity, and if there is time, work out some reasonably general transformations of the E&B fields between reference frames. Basically, finish reading the first chapter of the ph102 notes, and you'll be ready for that. See here for an overview of much of what I did today with E&B, along with some additional notes on radiation & accelerated charges.
You might also find Lecture 12 here useful. For that matter, the rest of those notes are great too. If there is a model for how I've been trying to plan the course thusfar (if it is not apparent, I am planning things to a degree!), it would be 8.022 at MIT.
We'll make sure to spend some time on the homework problems that are still nagging you by then too ...
Also, Drew inadvertently reminded me about something: every time I make a crack about engineers, you should remind me that my undergrad degree is in Materials Science & Engineering ;-)
You might also find Lecture 12 here useful. For that matter, the rest of those notes are great too. If there is a model for how I've been trying to plan the course thusfar (if it is not apparent, I am planning things to a degree!), it would be 8.022 at MIT.
We'll make sure to spend some time on the homework problems that are still nagging you by then too ...
Also, Drew inadvertently reminded me about something: every time I make a crack about engineers, you should remind me that my undergrad degree is in Materials Science & Engineering ;-)
Tomorrow
Our Arduino boards for the project have arrived. We can start playing with them tomorrow. If you want to bring your laptop to make things easier, that might be good, otherwise, we'll use the PCs in the modern physics laboratory, which are not locked down by security software ...
Next week, we'll cover magnetic induction, which will explain the basics of how RFID works and allow you to get started more seriously on the project.
Tomorrow, we'll work on deriving magnetic fields from electrostatics + special relativity, and review the special relativity we covered on Monday. Friday, we'll finish of what we need of relativity by talking about relativistic energy, momentum, and force.
Next week, we'll cover magnetic induction, which will explain the basics of how RFID works and allow you to get started more seriously on the project.
Tomorrow, we'll work on deriving magnetic fields from electrostatics + special relativity, and review the special relativity we covered on Monday. Friday, we'll finish of what we need of relativity by talking about relativistic energy, momentum, and force.
Monday, October 12, 2009
This week
Owing to popular demand, this week we'll cover relativity. This brings up two key points:
(1) Your next homework is out, and covers relativity. All problems are due this coming Friday.
(2) You'll want to read Ch. 1 of these notes, which cover relativity at the level we require. I'll try to have printed copies of that chapter for you tomorrow.
Monday's lecture should bring us all the way through time dilation and length contraction, while Wednesday's lecture will cover Lorentz transformations, spacetime diagrams, and energy & momentum.
Friday, we'll derive E from B and apply what we've learned to tie E&M together. For that, you should read section 6.1.2 in the notes linked above, which derives E from B for the special case of a point charge moving parallel to a current-carrying wire at constant velocity.
(1) Your next homework is out, and covers relativity. All problems are due this coming Friday.
(2) You'll want to read Ch. 1 of these notes, which cover relativity at the level we require. I'll try to have printed copies of that chapter for you tomorrow.
Monday's lecture should bring us all the way through time dilation and length contraction, while Wednesday's lecture will cover Lorentz transformations, spacetime diagrams, and energy & momentum.
Friday, we'll derive E from B and apply what we've learned to tie E&M together. For that, you should read section 6.1.2 in the notes linked above, which derives E from B for the special case of a point charge moving parallel to a current-carrying wire at constant velocity.
Friday, October 9, 2009
Misc
First, I ordered two of these for us to use in the RFID project. We'll probably have to bring in our own laptops to play with them, though, since the lab PCs are heavily locked down.
Second, if you have a preference for covering relativity, or just moving forward, please express it before Monday. If a quorum decides for relativity, we'll start that on Monday.
Third, there is really a new homework set coming out, due a week from today, but its content depends a little bit on what you choose to do next ...
Second, if you have a preference for covering relativity, or just moving forward, please express it before Monday. If a quorum decides for relativity, we'll start that on Monday.
Third, there is really a new homework set coming out, due a week from today, but its content depends a little bit on what you choose to do next ...
Wednesday, October 7, 2009
Midterm Grades
Reminder ... your midterm grade is just the average of your two exam scores. I am not including homework, quizzes, or labs.
Quick Poll
Looking at the rest of the semester's schedule, I'd like to give you a (small) choice in how to proceed.
We could probably cover special relativity, quick and dirty, in two lectures. This would tie together E & B very nicely, and fill in a nagging gap in our curriculum thus far. Plus, it is just cool.
In order to squeeze this in, some things would have to be left for you to mostly read on your own, likely geometric optics. The geometric optics we cover is pretty simple, I have notes on the subject, and we would still spend a recitation period for Q&A to make sure you 'got it.'
So here's the question: would you be interested in taking a detour to cover relativity, at the expense of a bit more reading, or shall we stay the course? One argument in favor is that we can derive B from E and make things less mysterious, something you probably will not see again as an undergraduate. One argument against is that there would be (relatively easy) final exam material that we would not cover extensively in class.
Leave a comment with your preference, if you have one ... you have until the weekend to decide collectively. If you choose not to cover relativity, you will see it in PH253, though possibly without the connection to electricity and magnetism.
We could probably cover special relativity, quick and dirty, in two lectures. This would tie together E & B very nicely, and fill in a nagging gap in our curriculum thus far. Plus, it is just cool.
In order to squeeze this in, some things would have to be left for you to mostly read on your own, likely geometric optics. The geometric optics we cover is pretty simple, I have notes on the subject, and we would still spend a recitation period for Q&A to make sure you 'got it.'
So here's the question: would you be interested in taking a detour to cover relativity, at the expense of a bit more reading, or shall we stay the course? One argument in favor is that we can derive B from E and make things less mysterious, something you probably will not see again as an undergraduate. One argument against is that there would be (relatively easy) final exam material that we would not cover extensively in class.
Leave a comment with your preference, if you have one ... you have until the weekend to decide collectively. If you choose not to cover relativity, you will see it in PH253, though possibly without the connection to electricity and magnetism.
Warm-up project ...
If you read the last links, you'll realize that RFID seems really complicated. It is, and we probably shouldn't be messing with it in a 100-level class ;-) However, given that you are all exceedingly clever, I think we'll be OK.
As a sort of warm-up project, while we're figuring out how RFID workst, we'll first consider how shoplifting tags work. Not, and I cannot stress this enough, how to defeat them, but how they are implemented.
Two common types of anti-shoplifting measures, implemented in DVDs and books for example, essentially use magnetic induction and resonant circuits, which we'll get to next week. If you can figure out how these work, the basic idea behind RFID is not so far off. Another topic we'll touch on along the way to RFID is wireless power transmission, or how to charge your phone without cables. Ostensibly less potential for misuse, similar physics.
The main point here is that I think these things are worth understanding, highly relevant, and involve some nice applied physics. We will not delve into the nefarious uses of what we learn, just as we did not really discuss practical weaponry in mechanics. Learning enough to figure out what's going on around us is a Good Thing, learning how to use that knowledge is another.
Vonnegut had an interesting take on this in Timequake (see the top of the page), which I thought made a good point if a bit too extreme. We'll focus on physics, and leave the philosophizing to more qualified departments.
Anyway: just fair warning, I'm not going to help you figure out how to read other people's IDs, but I will help you figure out that the relevant technology is not in fact magic, but merely clever, Clarke's 3rd law notwithstanding.
As a sort of warm-up project, while we're figuring out how RFID workst, we'll first consider how shoplifting tags work. Not, and I cannot stress this enough, how to defeat them, but how they are implemented.
Two common types of anti-shoplifting measures, implemented in DVDs and books for example, essentially use magnetic induction and resonant circuits, which we'll get to next week. If you can figure out how these work, the basic idea behind RFID is not so far off. Another topic we'll touch on along the way to RFID is wireless power transmission, or how to charge your phone without cables. Ostensibly less potential for misuse, similar physics.
The main point here is that I think these things are worth understanding, highly relevant, and involve some nice applied physics. We will not delve into the nefarious uses of what we learn, just as we did not really discuss practical weaponry in mechanics. Learning enough to figure out what's going on around us is a Good Thing, learning how to use that knowledge is another.
Vonnegut had an interesting take on this in Timequake (see the top of the page), which I thought made a good point if a bit too extreme. We'll focus on physics, and leave the philosophizing to more qualified departments.
Anyway: just fair warning, I'm not going to help you figure out how to read other people's IDs, but I will help you figure out that the relevant technology is not in fact magic, but merely clever, Clarke's 3rd law notwithstanding.
Wednesday's class
Wednesday, we'll cover magnetism in a bit more depth. Specifically, we'll look at some tricks for how to solve for the field due to an arbitrarily-shaped wire with the Biot-Savart law. A good reference is this paper (you can only download the paper from on campus), which shows a cute trick for calculating the field from any wire whose geometry you can express in polar coordinates.
We'll also delve deeper into Ampere's law, and figure out how to find the field from solenoids and current sheets, which will let us derive some boundary conditions on the magnetic field at current sheets, similar to what we did for the electric field near sheets of charge.
Finally, we'll look at the most general equations we have for magnetostatics (i.e., only steady currents) and contrast the current situation to electrostatics.
Once we're done with the lecture part of class, we'll start in on the project for the rest of the semester: figuring out how to read RFID. There will be many sub-projects: coding, building circuitry, antenna design, and more. It will not be easy, but I think it will be a lot of fun, illustrate how one must be careful with this technology (morally speaking), and require all of you to pool your diverse expertise to work on a large project. Tomorrow, your task will be to figure out (1) what is RFID anyway, (2) what basic physics is involved in it, and (3) come up with more specific information-gathering missions and delegate them.
If all goes well, at the end of the semester you will know how to read my campus ID while it is still in my wallet. Of course, we'll have to be very careful about what we do with the knowledge we gain: knowing is one thing, but doing is another. There are rules.
We'll also delve deeper into Ampere's law, and figure out how to find the field from solenoids and current sheets, which will let us derive some boundary conditions on the magnetic field at current sheets, similar to what we did for the electric field near sheets of charge.
Finally, we'll look at the most general equations we have for magnetostatics (i.e., only steady currents) and contrast the current situation to electrostatics.
Once we're done with the lecture part of class, we'll start in on the project for the rest of the semester: figuring out how to read RFID. There will be many sub-projects: coding, building circuitry, antenna design, and more. It will not be easy, but I think it will be a lot of fun, illustrate how one must be careful with this technology (morally speaking), and require all of you to pool your diverse expertise to work on a large project. Tomorrow, your task will be to figure out (1) what is RFID anyway, (2) what basic physics is involved in it, and (3) come up with more specific information-gathering missions and delegate them.
If all goes well, at the end of the semester you will know how to read my campus ID while it is still in my wallet. Of course, we'll have to be very careful about what we do with the knowledge we gain: knowing is one thing, but doing is another. There are rules.
Tuesday, October 6, 2009
Exam II
Here's the exam you took today. Note that two of the magnetism problems are example problems from Griffiths (Ex. 5.9), the third is from Purcell. Two of the circuit problems are from Purcell, the third is based on a Serway problem (ph106 textbook).
I hope to have solutions out on Wednesday, and you will get your graded exams back during Wednesday's class if all goes well. So far, it seems the results will be very good. The exam was much easier than the homework, and your performance seems to be commensurate ...
I hope to have solutions out on Wednesday, and you will get your graded exams back during Wednesday's class if all goes well. So far, it seems the results will be very good. The exam was much easier than the homework, and your performance seems to be commensurate ...
Monday, October 5, 2009
Partial HW4 solution
Here you are.
I will try to finish up a bit more of this tonight yet ...
UPDATE: only number 3 is missing a solution now.
I will try to finish up a bit more of this tonight yet ...
UPDATE: only number 3 is missing a solution now.
Sunday, October 4, 2009
Monday's exam
The exam will be low-key, and you should have plenty of time. Here's the basic format:
I think you can probably solve three problems and be done in less than 90 minutes, some of you certainly in about 60 minutes. They are not hard problems compared to the homework, and a few of them are drawn from PH102 homework in fact.
If you're cramming now, I would suggest reviewing the solution to the field from a finite segment of current-carrying wire, and the battery charging problem from Hw4. HW4 partial solutions will be out in a couple of minutes.
- six questions total
- three questions on circuits (dc; resistors and batteries only)
- three questions on the magnetic field from wires (straight or circular; superposition)
- answer any three of the six
I think you can probably solve three problems and be done in less than 90 minutes, some of you certainly in about 60 minutes. They are not hard problems compared to the homework, and a few of them are drawn from PH102 homework in fact.
If you're cramming now, I would suggest reviewing the solution to the field from a finite segment of current-carrying wire, and the battery charging problem from Hw4. HW4 partial solutions will be out in a couple of minutes.
Friday, October 2, 2009
HW5
Quick google search turned this up.
Better: see homework 12 solutions here.
I don't think my method of solving the problem in class was that great, I'll try to post some quick notes on the dipole problem this afternoon.
Better: see homework 12 solutions here.
I don't think my method of solving the problem in class was that great, I'll try to post some quick notes on the dipole problem this afternoon.
Thursday, October 1, 2009
Homework
Maybe I'm just loopy from preparing my tenure dossier, but here are some thoughts on homework.
1) By way of Drew: "Problems worthy / of attack / prove their worth / by fighting back."
2) I showed some grad students and faculty your homework, and they now fear you. You will be rocking PH331 and ECE340 when the time comes, because you're doing their homework already ;-)
1) By way of Drew: "Problems worthy / of attack / prove their worth / by fighting back."
2) I showed some grad students and faculty your homework, and they now fear you. You will be rocking PH331 and ECE340 when the time comes, because you're doing their homework already ;-)
Subscribe to:
Posts (Atom)