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Announcements 2/18/11

Announcement regarding the availability of graded Exam 1, reminder about upcoming term project proposals, and notification of President's Day holiday.

cynthiaperkins
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Announcements 2/18/11

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  1. Announcements 2/18/11 • Prayer • Exam 1: • Graded, available for pick up in HW boxes • Median: 82.5; mean: 76.9. Five scores of 100+. • My apologies for length—median time was 3:05 hours instead of the ~2:10 hours that I expected. • 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

  2. Term Projects! • Term project proposals due a week from tomorrow, emailed to me directly, text in body of email & figures (if needed) as attachments. • 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.” • Groups of 2 are encouraged. Groups of 3 with permission. • At least 15 hours of work per person. • Guidelines/Grading info: http://www.physics.byu.edu/faculty/colton/courses/phy123-winter11/project/guidelines.htm • Ideas: http://www.physics.byu.edu/faculty/colton/courses/phy123-winter11/project/ideas.htm • Some actual projects: http://www.physics.byu.edu/faculty/colton/courses/phy123-winter11/project/actualprojects.htm

  3. Thought Question dBm: β = 10 log(P/P0) P0 = 1 mW • How much power is 0 dBm? • 0 mW • 1 mW • 10 mW • -10 mW • masspacity

  4. Thought Question • How much power is 30 dBm? • 1 mW • 3 mW • 30 mW • 100 mW • 1000 mW

  5. Thought Question • How much power is 33 dBm? • 1002 mW • 1003 mW • 1100 mW • 2000 mW • 3000 mW

  6. 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 ~ amplitude2; true for voltages, sound, and light waves as well as waves on a string) • 1 V • 2 V • 3 V • 2 V • 3 V

  7. Spherical Waves

  8. Plane Waves

  9. 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 • http://stokes.byu.edu/bells.wav

  10. The Pie Factory vbelt • Spacing between pies = ? •  = v/f • vs source speed • vo observer speed • v speed of sound (vbelt) • If observermoves toward source (pie maker), she would measure the same ___________ but the pies are coming at her at a faster ________ • If sourcemoves toward observer, the __________ shrinks, but the pie _______ doesn’t change f = vnew/lold f = vold/lnew wavelength speed wavelength speed

  11. Doppler, cont. • Stokes Flash video • http://stokes.byu.edu/doppler_script_flash.html • Combine both effects: • What does  mean?

  12. Doppler shift of light • Distant light source • Traveling toward you • Traveling away from you • See HW 19.4 for equation

  13. Astronomy • How far away is a galaxy? Hard question • Edwin Hubble, 1929: Distance away proportional to speed • Distance measured through supernovae observations • 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 • (Yes, it’s OK for LDS to believe in the Big Bang…)

  14. Sonic Boom • http://stokes.byu.edu/boom_script_flash.html • http://stokes.byu.edu/boomray_script_flash.html • 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?

  15. θ Sonic Boom Sonic boom manifested by condensation of water in air sinq = vsound/vsource = 1/“Mach number”

  16. Sonic Boom Sonic boom of bullet in flight (holographic interferometry?) • How fast is the bullet traveling? • Mach # = 1/sinq

  17. Reading Quiz • Which term did not appear in the discussion of wave interference for today’s reading assignment? • constructive • destructive • masspacity • path length • superposition

  18. Ripple Tank image: wikipedia Interference • Path length • Constructive • Destructive • Video: Two outdoor speakers • Demo: Moire pattern transparencies • Demo: Hearing test • Demo: 2-speaker interference • Speed of sound = ?

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