Announcements 10/12/11
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Prayer
Term projects: Proposals due a week from Saturday,
emailed to me with your proposal in body of email.
Groups of 2 are encouraged. Groups of 3 with permission.
Just send one email per group, but CC your partners on
the email.
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.”
Guidelines: “At least 15 hours of work per person (not
including time spent at the hardware store, etc.)”
http://www.physics.byu.edu/faculty/colton/courses/phy123resources/project/guidelines.htm
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Some ideas to get you thinking:
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Some actual projects:
http://www.physics.byu.edu/faculty/colton/courses/phy123resources/project/ideas.htm
http://www.physics.byu.edu/faculty/colton/courses/phy123resources/project/actualprojects.
htm
Logarithm Review
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Log10(x) is the inverse of 10y
→ if x = 10y then y = log10(x)
a. I.e. “10 to the what equals 22?”
answer: 1.3424
calculator: log10(22)
Review of “Laws of Logs”:
– 1. log(ab) = log(a) + log(b)
– 2. log(an) = n log(a)
log10(100) = ? Translation: 10 to what equals 100?
ln(100) = ?
(“ln” = loge = log2.71828…)
Translation: e to what number =100? (4.605…)
Ambiguity: “log(100)”…could be either log10 or ln
Question: log10(1,000,000) = ?
Question: If log(3) = 0.477, what is log(300)?
Thought Question
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A +3 dB increase is just about a factor of 2
in intensity. How many dB represents a
factor of 4 increase in intensity?
a. +4
b. +6
c. +8
d. +9
e. +10
What if you need to solve for I?
Other Power and Intensity Scales
Power or Intensity
 sound
a. dB
β = 10 log(I/I0)
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microwaves/rf
a. dBm β = 10 log(P/P0)
I0 = 10-12 W/m2
P0 = 1 mW
electronics/electrical circuits
a. dB
β = 10 log(P2/P1) (ratio only)
Thought Question
dBm: β = 10 log(P/P0)
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P0 = 1 mW
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. 1003 mW
b. 1006 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
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 (0:32)
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 _______
new=(vbelt-vs)/fs
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Doppler, cont.
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Combine both effects:
v  vo
f f
v  vs
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What does  mean?
Stokes Flash video
a. http://stokes.byu.edu/doppler_script_flash.html (1:50)
Distant light source
a. Traveling toward you
b. Traveling away from you
 See HW 19.4 for equation
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Astronomy
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Edwin Hubble, 1929: Distance to galaxies is proportional
to their speed
a. Distance measured through Cepheid variable star
observations, “standard candle”
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 distance to far
away objects… 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 (2:47)
http://stokes.byu.edu/boomray_script_flash.html (2:53)
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
Interference
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Path length
a. Constructive
b. Destructive
Video: Two outdoor speakers
(1:16)
Demo: Moire pattern
transparencies
Demo: Hearing test
Demo: 2-speaker interference
Ripple Tank
image: wikipedia