## Thursday, September 30, 2010

### Public talk this evening

We had a good turn out, I saw quite a few of you there. I though it was a great talk, particularly relevant to what we've been covering lately (down to the fact that he used some of the same images/videos I've posted here). I really liked the football throwing explanation of particle tunneling ... it is important to know your audience.* Thanks for making a good showing for Dr. Orzel!

If you couldn't make either of his lectures today, you can still do the extra credit assignment. The first chapter of his book (and a dramatic reading of chapter 3) are available here. Chapter 1 discusses the wave particle duality (which we spent quite a bit of time on). In lieu of attending the talk, you can read it and write me a page about what you learned from it according to the previous rules. It is a good read, and I highly recommend the whole book. (And no, I don't get anything for saying that, I met Dr. Orzel for the first time today.)

*That's not to say that my brown bottle explanation of tunneling this morning was making a comment about what you do in your spare time, but it seemed like an everyday thing you could all at least imagine, even if lacking the requisite first-hand experience. I clearly need more football-based explanations.

## Wednesday, September 29, 2010

### PH253: Wave functions

Hyperphysics is good stuff. Good example calculations/solutions and explanations of Schrodinger's equation.

### Everyday life is basically understood

Interesting couple of posts.

One.
Two.

The basic argument is that while everyday phenomena can be hideously complex (turbulence, for example), the basic laws governing them are completely understood. There is no everyday example of something that violates quantum mechanics or relativity, you have to go well beyond the everyday to find situations where we don't even know the basic rules that apply.

Of course, knowing the basic laws (i.e., equations) is one thing. Solving them is completely another, plenty to keep scientists gainfully employed for some time to come.

### PH253: Exam 1 scaling

Long story short, it didn't really work out to simply add points to everyone's score or take the best 4 of 5. I suppose this is a common theme if you read the news, but either of those benefited the grade-rich quite nicely, but did little for those of you who really needed a boost. Luckily, our problem has a nice solution.

### PH253: online grades

I now finally have the system set up so you can log in and see your grades (HW1-3 and Exam 1 right now). Since I am still young enough to be idealistic, I'm using an open source system called Moodle. Apparently you still have to pay if you want a sensible name. Instructions below the jump ...

### PH253: Extra credit possibility

I posted last week that we'll have a special physics lecture Thursday evening. I think it will be really fun, and directly relevant to our course material. So here's the deal
1. You attend the lecture.
2. You write me 1 page (8.5x11 in paper, 1.5 inch margins, 12pt font, single spaced) covering
• One new thing you learned from the lecture
• One way the lecture tied in to PH253
• One way the lecture tied in to your major/field of study
• One way the lecture tied in to everyday life
Do this, and I give you as much as +2% on your overall grade at the end of the semester, depending on quality of your writeup.*

To give you an idea of what 2% means:

• There will be about 10 HW sets. Each is worth about 1% of your grade. This is 2 free HW sets. Typing up a single page should take a minuscule amount of time compared to my HW sets :-)
• Each exam is 30% of your grade, each question on the exam about 6%. This is like adding 3 points to your lowest exam question.

The speaker has a very good blog here, and a highly entertaining and interesting book you might check out.

*If you are a non-native English speaker, this will be taken into account. English grammar and spelling is very silly.

## Tuesday, September 28, 2010

Some random plots related to the exam: average score by question, percentage of you choosing that question, and grade distributions for raw scores (5 questions) and with the proposed scaling (best 4 out of 5). Click on the picture for a larger version.

More details to come as I think about this a bit more ...

### PH253: Exam 1 solution

Here you go. UPDATE: typo in #1 fixed - I missed a couple of primes.

## Friday, September 24, 2010

### PH253: HW5, misc

Here is the exam you took yesterday. Your HW5 is to solve the two problems you didn't choose on the exam, due Monday before midnight. My suspicion is that a few of you get together in a group, you've probably solved them all amongst you ...

Also, I'll have your exams back on Tuesday at the start of class.

A recent tour-de-force experiment by NIST managed to demonstrate gravitational time dilation on ordinary length scales. Neat stuff. (We didn't really cover gravitational time dilation, part of general relativity, but there is a small bit on it in your textbook.)

## Wednesday, September 22, 2010

here.

### PH253: random exam hints

Just sent this as an email to one of you:

I think the best thing to study right now would be 1) the practice problems I put out - I will use one or two of them directly on the exam, and 2) this semester's and last semester's homework problems, focusing on the shorter problems (not the really lengthy and mathematical ones). The exam questions will be easier than most HW problems, but covering similar topics.
The book doesn't have many good problems for 2.4.1-2, I have to admit. The problems on uncertainty will be like numbers 9 & 10 on the practice problems - using a de Broglie wavelength and the uncertainty principle together, but in a more or less straightforward way. If you can do those two problems, you are more or less OK on 2.4.1-2.
For Compton scattering, just read the notes I put out, and that should be enough.
For relativity, the sample problems are good.
For the Photoelectric effect, there is really only the one equation - it will have to be something like given wavelengths and voltages, find the work function ... not much else to ask.
For photons & the quantum hypothesis, questions will be like something like the HW problem finding the number of photons per second put out by a radio transmitter.

For radiation, it will be a simple example like given an acceleration, find the power radiated by a charge, or the energy radiated in a time t (E = Pt).

### Today's schedule, approximately

I have meetings from about 1-3, so today is a bit less open than I had hoped. Here is when (and where) I'm likely to be available:

11-12:30 Gallalee 110
~3-5:30 Bevill 2012

I'll be reading email regularly this evening for last minute questions as well. I will try to be in Gallalee by about 10:00 tomorrow for any truly last-minute questions.

## Tuesday, September 21, 2010

### Special colloquium next week!

Next week, on Thursday, 30 September at 7:30pm, we are having a very interesting public talk in 227 Gallalee:

"What Every Dog Should Know About Quantum Physics" by Dr. Chad Orzel of Union College

Chad Orzel is the author of a popular physics blog called Uncertain Principles: http://scienceblogs.com/principles/

and the author of a book titled "How to Teach Physics to Your Dog"
http://dogphysics.com/book_info.html

Chad Orzel's bio can be found here:

Here is an abstract for Chad's public talk:

Quantum physics, the science of extremely small things like atoms and subatomic particles, is one of the best tested theories in the history of science, and also one of the most bizarre. Many of its predictions -- particles that behave like waves, cats that are alive and dead at the same time, objects that pass through barriers as if they weren't even there -- seem more like science fiction than science fact. This talk will explain the reality behind some of the stranger aspects of quantum physics, and why it is so important that even dogs should know about it.

## Monday, September 20, 2010

### PH253: Exam 1 practice problems

Here you are. It is unlikely that there will detailed solutions before Thursday, but I will try.

Remember, the exam has 7 problems and you must do 5 of them. I will provide a formula sheet with all the necessary equations & constants (an example of which will show up tonight or tomorrow), what you put on the sheet you're allowed to bring in is up to you.

It will be important that you manage your time -- don't get stuck on any one problem too long, since you have the freedom to skip two of them. If you really don't know how to go about a problem, don't freak out: just pick another one. Choose your problems carefully, and start with the ones you think are easiest.

### PH253: Exam 1 coverage

Reminder: Exam 1 is this Thursday during the normal class period.

Sections of the textbook covered on Exam I:

• 1.2.1, 1.2.2 EM Waves
• 1.3 Special relativity
• 1.5.1-3 Appendix to special relativity
• 2.1.1-3, 2.1.5-6 Quantum hypothesis & radiation
• 2.2.1-3 Photoelectric
• 2.3.1-3 Photons / Compton
• 2.4.1, 2.4.2 de Broglie, uncertainty

Things not in the textbook on Exam I (from notes provided):

• Power radiated by an accelerated charge (no derivations, just the Larmor formula)
• More detailed discussion of Compton scattering

Remember that you can bring in one standard 8.5x11in sheet of paper with formulas, etc., front & back. If you write on only one side, you can bring 2 sheets. Bring a calculator.

A few more practice problems will show up this evening.

### PH255: Writing a scientific paper

Lab reports are a real pain, and I know many of you in PH255 have been struggling with how to go about writing a lab report. Here's how I usually go about writing a paper:

## Thursday, September 16, 2010

### PH253: EXAM 1 DATE MOVED

Exam 1 has been moved to THURSDAY. It is not on Tuesday. Full stop.

## Wednesday, September 15, 2010

### PH253: HW4 hints

I'll be updating these through the evening as I finish them up. The link will be persistent. I'll post here when they are complete.

UPDATE: barring any typos I find later, the hints are about as complete as they are going to get.

### PH253: HW4 typo

Fixed a typo on number 2; both uncertainty relationships should be > h/4pi, not > 4pi. Same link as before.

## Monday, September 13, 2010

### Black body radiation & climate change

Interesting interview/discussion on how black body radiation provides feedback for earth's climate.

## Sunday, September 12, 2010

### PH253: HW4 is out

Here you are. As usual, we will do a number of them in class. A couple of quick notes in advance of more substantial hints:

• For number 3, you will need to look up the intensity pattern for single-slit diffraction. Presume the electron must be confined within the central, most intense region, which gives you an estimate of position uncertainty.
• I've asked number 6 before.
• Number 7 is a famous thought experiment, it can be found in the wikipedia article on the uncertainty principle.
• Number 8 is a straight math problem (math we'll need soon). Don't let the the unfamiliar terms stump you, all you need to do is integration and algebra for parts a-c.
UPDATE: fixed a few typos, in particular on number 8.

## Friday, September 10, 2010

### PH253: Textbook

I loaned my textbook to someone until theirs came in. If your copy has come in, it would be nice to have my book back soon ... a colleague is asking to borrow it.

## Thursday, September 9, 2010

### Double slit applet

A nice applet to illustrate the crazy double slit stuff we talked about today.  You can use photons, electrons, neutrons, and helium atoms, and even use multiple sources or vary the incident wavelength. Note the different modes - high intensity, single particles, and lasers. If you like, add a detector on one or both of the slits and watch the interference go away ... neat stuff.

Also, the actual experiment with single electrons!

### How to read science papers without reading every word

Nice read. Applies equally well to sections of a textbook, I think.

### PH253: Compton scattering notes

Here are some notes on Compton scattering. And by 'notes' I really mean deriving the Compton equation, the photon and electron energies, etc., and not so much on the discursive side. They have not been heavily proofread yet, but I believe everything is substantially correct.

Consider this your mathematical guide for Compton scattering (chock full of goodies related to the homework), with your textbook filling in the basic background.

I would like to be able to keep up with notes on the same level as the blackbody notes for every topic, but it is unrealistic to do on the fly ... I'll provide what I can.

## Wednesday, September 8, 2010

### PH253: HW3 hints

I'm starting a file of HW3 hints. Right now, there are hints for the first three problems, I will be updating it tonight and tomorrow to add more. Probably by class time tomorrow it will be as complete as it's going to get ... the URL will not change as I update the file. If you like, check here to look at the time stamp and see if it has been updated since you lacked looked.

UPDATE: the hints are probably about as complete as they are going to get. A little something for each of the 9 problems.

### PH253: partial HW2 solutions

I've got solutions to problems 1-5. Hope to have problems 6 & 7 finished tomorrow. Let me know if you find any typos or places that need clarification.

UPDATE: there is a solution for problem 6 now. Only problem 7 remains, I will try to get a solution out Wed.

## Tuesday, September 7, 2010

### PH253: HW3 is out

Shorter than the last two, I think. I asked a couple of these on HW last year, and we will cover a few of them almost directly in lecture.

We should cover the bulk of the material on Compton scattering and the photoelectric effect on Tuesday, on Thursday we'll go over any remaining questions you might have.

## Friday, September 3, 2010

### PH253: radiation notes

I've fixed sum typos in various places, the latest version is at the same url.

## Thursday, September 2, 2010

### PH253: homework deadline extended

You can turn in HW2 any time up until this coming Tuesday's class, 11am on 7 Sept 2010. I'll be posting more hints over the weekend, and will be around Bevill this afternoon and tomorrow afternoon.

(Also, some typos in the notes came up today, I'll fix them this afternoon/evening and post when it is done.)

## Wednesday, September 1, 2010

### PH253: HW 2.1

For problem 1 on HW2,  you'll find this interesting. (Note the 'dot' notation to signify derivatives with respect to time.)

### PH253: key points from the last lecture

The last lecture covered a lot of ground, but in the end there are really only two main things to take away from the last lecture:

1) Accelerating charges radiate electromagnetic energy (a.k.a., light). The total radiated power P in terms of the acceleration a for a charge q is
P = \frac{q^2a^2}{6\pi\epsilon_o c^3}
This is only valid for velocities small compared to c - even though we correctly transformed the field according to relativity, we used the classical expression for acceleration. This expression is known as the Larmor formula.

2) Since accelerating charges are radiating and therefore losing energy, they also experience a 'radiation reaction' force or a recoil due to the emission of radiation. This force is analogous to viscous drag or friction, and is given by
F = \frac{q^2}{6\pi\epsilon_o c^3}\frac{d^3x}{dt^3} = \frac{q^2}{6\pi\epsilon_o c^3}\frac{da}{dt}
Again, this is valid for low speeds compared to c, and is typically called the Abraham-Lorentz force. This force is unusual in many respects - for one, it represents the charge effectively acting on itself, and it depends on the rate of change of acceleration or the third derivative of position.