Course Announcements
« PHYS221 Spring 2008
Final exam scores and toughest problems Fri 02May2008 10:53AM
Final exam scores are posted.
The toughest problems:
1. The mean (average) lifetime of a muon in its rest frame is 2.2 microseconds. A beam of muons is moving through the lab with speed 0.994c. How far on average does a muon travel through the lab before it decays?
Only 23% selected right answer 6 km, while 56% selected 660m, which is simply 0.994c x 2.2 us. Note that while it passes 2.2 us in muons frame, its clock runs slower than the lab clock since it moves in respect to it. One has to calculate time in lab frame and then calculate the distance. Alternatively, one can calculate the length of path passed through the lab in muons frame (660m), and then get the "proper" path length using respective equation for length contraction (L0 will be the unknown - length in the rest frame of the path, i.e. lab). This is almost identical to a problem introduced during the recitation. See lecture slides Lec25-chap26-Relativity
2. A red and green laser are each rated at 1 mW of output and emit light at 640 nm and 532 nm, respectively. Which laser produces more photons per second?
Only 44% selected red, 38% selected green, and rest selected "the same". Qualitative reasoning: red photon has longer wavelength or lower frequency, which means that one red photon has less energy than the green one (E=hf). Thus one needs more red photons to account for the same energy (1 mJ per second). Numerical solution: one can calculate the number of photons per second (1 mJ)/(hc/lambda) as we did in class.
3. What is the smallest energy photon that can be absorbed by a hydrogen atom in its ground state?
Only 51% selected right answer (10.2 eV), while 42% selected 13.6eV. Note that the 13.6 eV is energy required to ionize atom, since this is the electron binding energy in n=1 state. There are many states between ionization level (no interaction, n=infinity and E=0). The closest to ground state is n=2, i.e. 13.6eV/2^2. A photon of 10.2 eV will promote electron from ground state n=1 to the nearest excited state n=2.
Final exam score and bonus HW
Thu 01May2008 7:33PM1. Final exam results are still being processed. They will be posted on Friday sometime before noon.
2. Please check you final exam score as soon as it appears. If you have any questions you must address them on Friday or Saturday. The grades will be finalized, posted and submitted to registrar on Monday.
3. Sevedents asked me about the Bonus HW. This is how it works. You receive 100 points toward your total score if you get perfect score on all other HW except Bonus. The bonus HW score is scaled the same way as the rest of HW and its score is added on top of your total score. The scaled (HW score + bonus HW score) can be viewed in the Course grade components section. Equation used is:
Sum(HW1...HW14)/1120*100%
I.e. maximum extra credit points for Bonus HW = 90/1120*100=8
Final exam: chapters Thu 17Apr2008 1:38PM
Final exam is not comprehensive, and it will cover:
chapter 25 - x-ray diffraction only
chapter 26 - Relativity
chapter 27 - Early quantum physics and photon
chapter 28 - Quantum Physics
Grading
Tue 15Apr2008 12:50PMThe grades for Phys221 (% out of 740pts total):
A: 88%
B: 78%
C: 64%
D: 50%
Exam 2 Tue 08Apr2008 12:20PM
Exam time: April 10, Thursday, 7PM-9PM
Material: Chapters 20, 22, 23, 25
Exam place: You must take exam in a room you are assigned to as follows:
Room PHYS 114: lab sections 8-13
Room CL 50: lab sections 1-7 and 15-16
* Sample exam is posted in Exam section of the course web page
* There will be no lecture on the day of the exam
* Bring your ID, pencil and a calculator. You can also have one standard page size crib sheet. You can write anything you wish on it, we will be not collecting crib sheets nor checking for contents.
Exam 2 sample posted Sun 06Apr2008 12:05PM Exam 2 sample posted in Exam section of Phys221 web page
Exam 1 results Fri 29Feb2008 5:34PM
The results of the first exam are posted on CHIP. The scores are normalized to 150 points total, each problem was worth 7.5 points (20x7.5=150). You can view your exam and your answers to all questions using CHIP - reference to Exam 1 is the first in your homework list.
Note: the Exam 1 is posted on CHIP for viewing only, not for correcting errors.
Correction: One of the problems (Capacitor 1) was graded incorrectly by a computer, i.e. wrong answer was marked as a correct one. This was fixed on Friday and scores were recalculated. In most of the cases the scores went up after this correction, though there were few cases when they went down..
Exam 1 Thu 21Feb2008 3:15PM
Exam time: February 26, Tueasday, 7PM-9PM
Material: Chapters 16-19 (lectures 1-13)
Exam place: You must take exam in a room you are assigned to as follows:
Room PHYS 114: lab sections 8-13
Room CL 50: lab sections 1-7 and 16-16
* Sample exam is posted in Exam section of the course web page
* There will be no lecture on the day of the exam
* Bring to exam your ID, pencil and a calculator. You can also have one standard page size crib sheet. You can write anything you wish on it, we will be not checking it.
Some i>Clickers available in local bookstores Thu 17Jan2008 10:33AM Follett’s at the Village have 18 i>clickers available. Also, University Bookstore on State Street have 20 i>clickers.
Things to do as soon as possible
Mon 07Jan2008 5:07PM1. Obtain textbook
2. Obtain lab manual
3. Obtain iClicker audience response remote (not CPS pad!)
4. Register on CHIP
5. Register iClicker on CHIP