Announcements 2/18/11
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Prayer
Exam 1:
a. Graded, available for pick up in HW boxes
b. Median: 82.5; mean: 76.9. Five scores of 100+.
c. My apologies for length—median time was 3:05
hours instead of the ~2:10 hours that I expected.
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.
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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!
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Term project proposals due a week from tomorrow, emailed to
me directly, text in body of email & figures (if needed) as
attachments.
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. Groups of 2 are encouraged. Groups of 3 with permission.
c. At least 15 hours of work per person.
d. Guidelines/Grading info:
http://www.physics.byu.edu/faculty/colton/courses/phy123winter11/project/guidelines.htm
e.
Ideas: http://www.physics.byu.edu/faculty/colton/courses/phy123-
f.
Some actual projects:
winter11/project/ideas.htm
http://www.physics.byu.edu/faculty/colton/courses/phy123winter11/project/actualprojects.htm
Thought Question
dBm: β = 10 log(P/P0)
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How much power is 0 dBm?
a. 0 mW
b. 1 mW
c. 10 mW
d. -10 mW
e. masspacity
P0 = 1 mW
Thought Question
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How much power is 30 dBm?
a. 1 mW
b. 3 mW
c. 30 mW
d. 100 mW
e. 1000 mW
Thought Question
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How much power is 33 dBm?
a. 1002 mW
b. 1003 mW
c. 1100 mW
d. 2000 mW
e. 3000 mW
Thought Question
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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)
a. 1 V
b. 2 V
c. 3 V
d. 2 V
e. 3 V
Spherical Waves
Plane Waves
Doppler Effect
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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. http://stokes.byu.edu/bells.wav
The Pie Factory
vbelt
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Spacing between pies = ?
a.  = v/f
vs source speed
f = vnew/old f = vold/new
vo observer speed
v speed of sound (vbelt)
If observer moves toward source (pie maker), she
wavelength but the pies are
would measure the same ___________
speed
coming at her at a faster ________
wavelength
If source moves toward observer, the __________
speed doesn’t change
shrinks, but the pie _______
Doppler, cont.
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Stokes Flash video
a. http://stokes.byu.edu/doppler_script_flash.html
Combine both effects:
v  vo
f f
v  vs
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What does  mean?
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Distant light source
a. Traveling toward you
b. Traveling away from
you
See HW 19.4 for equation
Astronomy
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How far away is a galaxy? Hard question
Edwin Hubble, 1929: Distance away proportional to
speed
a. Distance measured through supernovae observations
b. How did he measure speed?
– Doppler shift of spectral lines!
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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. (Yes, it’s OK for LDS to believe in the Big Bang…)
Sonic Boom
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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?
Sonic Boom
Sonic boom
manifested by
condensation
of water in air
θ
sinq = vsound/vsource = 1/“Mach number”
Sonic Boom
Sonic boom of
bullet in flight
(holographic
interferometry?)
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How fast is the bullet traveling?
a. Mach # = 1/sinq
Reading Quiz
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Which term did not appear in the discussion of
wave interference for today’s reading
assignment?
a. constructive
b. destructive
c. masspacity
d. path length
e. superposition
Interference
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Path length
a. Constructive
b. Destructive
Video: Two outdoor speakers
Demo: Moire pattern
transparencies
Demo: Hearing test
Demo: 2-speaker interference
a. Speed of sound = ?
Ripple Tank
image: wikipedia