Announcements 2/18/11 Prayer Exam 1: a. a.Graded, available for pick up in HW boxes b. b.Median: 82.5; mean: Five scores of c. c.My apologies for length—median time was 3:05 hours instead of the ~2:10 hours that I expected. d. d.Solutions have been posted to class web page – – You can ask me questions about your exam, but if you haven’t yet looked at the posted solutions to try to figure out what you did wrong, I will send you away. Extra time on Lab 3: now due Tues Feb 22. Labs 4 & 5 starting tomorrow, due next Sat as usual Monday Feb 21 is President’s Day holiday. Tues Feb 22 is a virtual Monday
Term Projects! Term project proposals due a week from tomorrow, ed to me directly, text in body of & figures (if needed) as attachments. a. a.Syllabus: “The term project is an opportunity for you to propose and conduct a simple experiment or to theoretically, mathematically, or computationally investigate an aspect of the course in more depth.” b. b.Groups of 2 are encouraged. Groups of 3 with permission. c. c.At least 15 hours of work per person. d. d.Guidelines/Grading info: winter11/project/guidelines.htm winter11/project/guidelines.htm e. e.Ideas: winter11/project/ideas.htm winter11/project/ideas.htm f. f.Some actual projects: winter11/project/actualprojects.htm winter11/project/actualprojects.htm
Thought Question How much power is 0 dBm? a. a.0 mW b. b.1 mW c. c.10 mW d. d.-10 mW e. e.masspacity dBm: β = 10 log(P/P 0 ) P 0 = 1 mW
Thought Question How much power is 30 dBm? a. a.1 mW b. b.3 mW c. c.30 mW d. d.100 mW e. e.1000 mW
Thought Question How much power is 33 dBm? a. a.1002 mW b. b.1003 mW c. c.1100 mW d. d.2000 mW e. e.3000 mW
Thought Question You have a 1 volt amplitude sine wave. You want to go up 3 dB in power. How many volts do you need? (Recall: Power ~ amplitude 2 ; true for voltages, sound, and light waves as well as waves on a string) a. a.1 V b. b. 2 V c. c. 3 V d. d.2 V e. e.3 V
Spherical Waves
Plane Waves
Doppler Effect Demo: Doppler Speaker What happens if the source is moving? What happens if the observer is moving? Key point: Frequency is _______________when the source and observer approach each other, ______________ when they go away from each other Stokes Come, Come, Ye Saints recording a. a.
The Pie Factory v belt Spacing between pies = ? a. a. = v/f v s source speed v o observer speed v speed of sound (v belt ) If observer moves toward source (pie maker), she would measure the same ___________ but the pies are coming at her at a faster ________ If source moves toward observer, the __________ shrinks, but the pie _______ doesn’t change wavelength speed f = v new / old wavelength speed f = v old / new
Doppler, cont. Stokes Flash video a. a. Combine both effects: What does mean?
Distant light source a.Traveling toward you b.Traveling away from you See HW 19.4 for equation
Astronomy How far away is a galaxy? Hard question Edwin Hubble, 1929: Distance away proportional to speed a. a.Distance measured through supernovae observations b. b.How did he measure speed? – – Doppler shift of spectral lines! That’s now a standard technique for today’s astronomers when they want to measure distances… just measure Doppler shift. Hubble’s Law and the Big Bang a. a.(Yes, it’s OK for LDS to believe in the Big Bang…)
Sonic Boom Happens with all types of waves whenever the sources is traveling faster than the speed of the wave… …so, what is “sonic boom” of water waves?
θ Sonic boom manifested by condensation of water in air Sonic Boom sin = v sound /v source = 1/“Mach number”
Sonic boom of bullet in flight (holographic interferometry?) Sonic Boom How fast is the bullet traveling? a.Mach # = 1/sin
Reading Quiz Which term did not appear in the discussion of wave interference for today’s reading assignment? a. a.constructive b. b.destructive c. c.masspacity d. d.path length e. e.superposition
Interference Path length a. a.Constructive b. b.Destructive Video: Two outdoor speakers Demo: Moire pattern transparencies Demo: Hearing test Demo: 2-speaker interference a. a.Speed of sound = ? Ripple Tank image: wikipedia