announcements 2/11/11 prayer exam 1 ends on tuesday night lab 3: dispersion lab – computer...
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Announcements 2/11/11 Prayer Exam 1 ends on Tuesday night Lab 3: Dispersion lab – computer
simulations, find details on class websitea. “Starts” tomorrow, due next Saturday…
but we won’t talk about dispersion until Monday, so I recommend you do it after Monday’s lecture.
Taylor’s Series review:a. cos(x) = 1 – x2/2! + x4/4! – x6/6! + …b. sin(x) = x – x3/3! + x5/5! – x7/7! + …c. ex = 1 + x + x2/2! + x3/3! + x4/4! + …d. (1 + x)n = 1 + nx + …
Reminder What is ? What is k?
Reading Quiz What’s the complex conjugate of:
a.
b.
c.
d.
1 3
4 5
i
i
1 3
4 5
i
i
1 3
4 5
i
i
1 3
4 5
i
i
1 3
4 5
i
i
Complex Numbers – A Summary What is “i”? What is “-i”? The complex plane Complex conjugate
a. Graphically, complex conjugate = ? Polar vs. rectangular coordinates
a. Angle notation, “A” Euler’s equation…proof that ei = cos +
isina. must be in radiansb. Where is 10ei(/6) located on complex
plane?
What is the square root of 1… 1 or -1?
Complex Numbers, cont. Adding
a. …on complex plane, graphically? Multiplying
a. …on complex plane, graphically?b. How many solutions are there to x2=1? c. What are the solutions to x5=1?
(xxxxx=1) Subtracting and dividing
a. …on complex plane, graphically?
Polar/rectangular conversion Warning about rectangular-to-polar
conversion: tan-1(-1/2) = ?a. Do you mean to find the angle for (2,-1)
or (-2,1)?
Always draw a picture!!
Using complex numbers to add sines/cosines
Fact: when you add two sines or cosines having the same frequency (with possibly different amplitudes and phases), you get a sine wave with the same frequency! (but a still-different amplitude and phase)
a. “Proof” with Mathematica… (class make up numbers)
Worked problem: how do you find mathematically what the amplitude and phase are?
Summary of method:Just like adding vectors!!
Using complex numbers to solve equations
Simple Harmonic Oscillator (ex.: Newton 2nd Law for mass on spring)
Guess a solution like
what it means, really: (and take Re{ … } of each side)
2
2
d x kx
mdt
( ) i tx t Ae
( ) cos( )x t A t
A few words about HW 16.5…
Complex numbers & traveling waves Traveling wave: A cos(kx – t + )
Write as:
Often:
…or – where “A-tilde” = a complex number, the
phase of which represents the phase of the wave
– often the tilde is even left off
( ) i kx tf t Ae
( ) i kx tif t Ae e
( ) i kx tf t Ae
Reflection/transmission at boundaries: The setup
Why are k and the same for I and R? (both labeled k1 and 1) “The Rules” (aka “boundary conditions”)
a. At boundary: f1 = f2
b. At boundary: df1/dx = df2/dx
Region 1: light string Region 2: heavier string
in-going wave transmitted wave
reflected wave
1 1( )i k x tIA e
1 1( )i k x tRA e
2 2( )i k x tTA e
1 1 1 1( ) ( )1
i k x t i k x tI Rf A e A e 2 2( )
2i k x t
Tf A e
Goal: How much of wave is transmitted and reflected? (assume k’s and ’s are known)
x = 0
1 1 1 1 1cos( ) cos( )I I R Rf A k x t A k x t 2 2 2cos( )T Tf A k x t