Yet more

RE: the last quiz, second problem.

You have only 3 possible energy levels. Photons can only be emitted when you have an electron changing from a higher level to a lower one. Here we have

E1 = 1.2eV
E2 = 2.4eV
E3 = 4.8eV

The only photons that can be emitted must correspond to differences between energy levels:

E3-E2 = 2.4eV
E2-E1 = 1.2eV
E3-E1 = 3.6eV

So three types of photons can be emitted: when an electron jumps from level 3 to 2, from 2 to 1, or from 3 to 1. Given these energies, convert to joules:

E=(2.4eV)(1.6e-19 J/eV) = 3.84e-19 J

Once you have that, use E = hc/(wavelength) and solve for wavelength. You should get about 1e-6, 5.17e-7, and 3.44e-7 meters.

Another question

Could you direct me (like HW so and so) to an example of the kind of refraction problem I should be looking at? They range from easy to REALLY hard and I don't know. HELP.

Study problems more like the easier ones - the quiz 9 problems, for example. Not the pathological ones like atmospheric refraction or prisms.

A couple of other examples that are useful to look at:

PH102 Fall 2007, Exam 2, problems #9 & 10
PH102 Fall 2007 Final, problem #9
PH102 Spring 2008 Exam 2 #6 & #7

If you understand these problems, you basically have things under control.

Collected questions so far this evening.

So far, two questions this evening. Here they are, along with my responses. If I get any more interesting questions by email this evening, I'll try to post the answers here for all to see.

1. Quick question on #7 from fall 2007 final: In part b do you use the E=mc^2 equation?

On #7 you do want to use E=mc^2 - I reused this question on a homework from Spring 2008 (HW12, here). The result is as silly as you would expect - half of the mass of the bullet would have to be turned directly into energy for this to work, which can't be done with any known technology.

2. An FM radio transmitter has a power output of 130kW and operates at a frequency of 98.3MHz. How many photons per second does the transmitter emit?

This is one like the HW question, where you need to convert power and energy. In fact, it is from spring 2008 HW11, here.

First, 130kW means 130e3=1.3e5 Joules per second, since a watt is a joule per second.

The frequency 98.3Mhz means 98.3e6 Hz. If the transmitter has this frequency, than means each photon emitted has an energy of

E = hf = (6.6e-34 J*sec)*(98.3e6 sec^-1) = 6.5e-26 Joules (per photon)

So, if the transmitter puts out 1.3e5 joules per second, and each photon is worth 6.25e-26 joules, that means the transmitter must put out

# photons = (1.3e5 J/sec) / (6.5e-26 J/photon) = 2e30 photons per second.

Basically, find the total energy being emitted per second (just the power given) and divide by how much energy a single photon has to figure out how many photons per second must be coming out. In this case, the intermediate step of converting to electron volts isn't really necessary - the power and energy per photon both need to have the same units, so you can convert to eV or just use Joules.

Exam II and a partial solution

Here. Answers for all problems, solutions for some.

Final Exam

So, I've made it up now, and I estimate based on your past performance that most of you can finish it in 90 minutes or so. Some of you will finish in under 60 minutes, just barely. One update: you can choose 6 of 9 questions, not 6 of 8. One more choice than promised.

Here are some of my random thoughts on the final. It goes to 11.

(1) Relax. There is no reason to be more freaked out about this exam than any other, percentages and so forth notwithstanding. You should be less scared: broadening the scope of the material necessarily means that the difficulty level for any given problem goes down.

(2) There is only one way to study for this thing: read and solve problems. You will be given all of the required formulas, memorizing them will not help, nor will memorizing tricks or shortcuts. All problems will involve the systematic application of simple principles and simple relationships, there are no tricks. Cramming is not useful.

(3) Cut your losses on chapters you don't think you can master in the time remaining. There are only 6 problems you have to solve. Nailing 5 out of 6 can still get you a solid B without even attempting a 6th.

(4) Look at the Fall 2007 final exam and the second PH106 exam from Fall 2008 (only the parts without calculus).

(5) No problems I have asked you before will appear on the final. No problems directly from the notes will appear on the final. However, about half of the problems will seem eerily familiar.

(6) Relax. You will need to have your wits about you, the problems on the final will involve logically solving problems step-by-step using simple rules. They will not involve remembering an arcane phrase or formula buried in the text, nor will they have many convoluted twists and turns. Most of them involve only 1 or 2 steps.

(7) Run the numbers on your grade. The final is worth 25% of the total grade ... it can only change your overall grade by so much. Not as much as you think. I will scale the average on the final if necessary, such that it is at least 75% for the class. It is unlikely to be necessary.

(8) I like crystals and mass spectrometers, and quantization of angular momentum. Capacitors have been done to death.

(9) Study the homework solutions.

(10) Seriously, relax. Get some rest Wednesday night, you'll be better off well-rested.

(11) Answers to exam II will be posted soon. Full-blown solutions are probably beyond my grasp at this point, given that it is already nearly 3am.

The last lab

Our last lab will be the measurement of atomic light emission spectra. It will prove that, although the Bohr model of the atom is provably wrong, and a bit silly at times, there is some truth contained in it.

More to the point: it is better than anything classical we've come up with, so its a good start. That there are more sophisticated models doesn't change the fact that our simple model works for a few things, and nicely at that.

I will be at the lab starting about 3:30 or 3:40, at which time I will probably have a few additional hints about the final.

HW 8 Solutions

The key for the crossword puzzle is now up.

Answers for exam 2 will be up once I finish with some grading later tonight.

Traffic for this site

Nicely periodic. Can you spot the weekends without looking at the time axis?

Neat optical trick

Your brain interprets color in funny ways.

Last HW and extra credit

I'll accept them both through this Friday, 7 Aug.

Updated Grades

I have uploaded current grades to eLearning, which now include HW7. If I made changes to lab or exam grades last week, they should now appear.

If I made changes to your lab grades today (or promised to) they do not appear yet, but will show up in tomorrow's update.

Have a look and make sure that everything shows up correctly.

HW 9 is out

Here. You can turn it in at the start of the final exam.

Recall that 1eV = 1.6e-19 J for #3.

Sections covered on the final

The final will have about 8 problems, you will have to do about 6 of them. You will get a basic formula sheet as in previous exams, and you may bring 2 sheets front & back of your own.

The following sections from the textbook are fair game for the final (ref. 8th ed.)

15.1-6, 15.9; 16.1-4, 16.6-10; 17.1-6; 18.1-5; 19.1, 3-4, 6-9
20.1-4, 6-8; 21.1-3 (no rms), 7, 10-12; 22.1-5
23.1-6; 26.1-6; 27.1-2, 5-6; 28.1-3

The following sections will not be on the final

15.7,8; 16.5; 17.7,8; 18.6-8; 19.2,5,10
20.5; 21.4-6, 8,9,13; 22.6-7; 23.7
24, 25 all; 26.7; 27.3-4, 7-8; 28.4-7; 29 all

To start with, you will do well to study the example problem in the textbook, and after that probably the end of chapter problems in the course notes. More example problems will be posted tomorrow.

Lab for Mon 4 Aug 2009

We'll measure Planck's constant.

I've got your extra credit right here

A small paper. You will need to watch a movie to complete the assignment; I'm sorry.

HW8 is out

Here you go. Have fun, it is due Monday 3 Aug by the end of the day.

Why, yes, it is a thinly-veiled attempt to get you to read the chapters ;-)

New internets

Here are the slides I used Friday to discuss wave behavior.

Also, the notes now have 3 (incomplete) chapters on optics.