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Physics 451
Quantum mechanics
Fall 2012
Karine Chesnel
Announcements
• Today: Review - Monday: Practice testBe prepared to present the solution of your chosen problem during class (~ 5 to 10 min)
Test 1 next weekMo Sep 24 – Th Sep 27
Phys 451
EXAM I
• Time limited: 3 hours• Closed book• Closed notes• Useful formulae provided
Review lectures,Homework
and sample test
Phys 451
EXAM I
1. Wave function, probabilities and expectation values
2. Time-independent Schrödinger equation
3. Infinite square well
4. Harmonic oscillator
5. Free particle
Phys 451
Review I
What to remember?
Phys 451
Review IQuantum mechanics
1. Wave function and expectation values
2( , ) ( , )x t x t Density of probability
Normalization: 2
( , ) 1x t
Review IQuantum mechanics
1. Wave function and expectation values
*x x dx
“Operator” x
*p i dxx
“Operator” p
* ,Q Q x i dxx
Quiz 9a
22
2m x
22 21
2m x
m x
2 2
22m x
What is the correct expression for the operator T= Kinetic energy?
2 / 2x m2
1
2m x
A.
B.
C.
D.
E.
Review IQuantum mechanics
1. Wave function and expectation values
2x p
Heisenberg’s Uncertainty principle
Variance: 22 2
22 2
x
p
x x
p p
Review IQuantum mechanics
2. Time-independent Schrödinger equation
2 2
22i V
t m x
Here ( )V xThe potential is independent of time
General solution: ( , ) ( ) ( )x t x t “Stationary state”
Review IQuantum mechanics
2. Time-independent Schrödinger equation
2 2
2
1 1
2
d di V
dt m dx
Function of time only
Function of space only
E
( )iEt
t e
Solution:/( , ) ( ) iEtx t x e Stationary state
Review I
^
H E
Quantum mechanics
2. Time-independent Schrödinger equation
Q is independent of time
0d x
p m v mdt
for eachStationary state
^ 2
n nn
H c E
1
( , ) ( , )n nn
x t c x t
/( , ) ( ) niE tn nx t x e
where
A general solution is
Review IQuantum mechanics
3. Infinite square well
x0 a
The particle can only exist in this region
22
2
dk
dx
2mEk
with
Review IQuantum mechanics
3. Infinite square well
2 2 2
22n
nE
ma
Quantization of the energy
2sinn
nx
a a
x0 a
Ground state 1 1,E
Excited states
2 2,E
3 3,E
Review IQuantum mechanics
3. Infinite square well
^
n n nH E
*m n nm
* 2( ) ( ,0) sin( ) ( ,0)n n
n xc x x dx x dx
a a
/ /
1 1
( , ) ( ) sin( )n niE t iE tn n n
n n
xx t c x e c n e
a
^ 2
1n n
n
H c E
x0 a
Quiz 9b
The particle is in this sinusoidal state.What is the probability of measuring the energy E0 in this state?
A. 0
B. 1
C. 0.5
D. 0.3
E. 1
9
Review IQuantum mechanics
4. Harmonic oscillator
x
V(x)
1
2a ip m x
m
1
2H a a
or1
2H a a
2 2 21 1( )
2 2V x kx m x
• Operator position 2
x a am
• Operator momentum 2
mp i a a
Review IQuantum mechanics
4. Harmonic oscillator
Ladder operators:
0
1
!
n
n an
nnE
Raising operator: 11n na n nE
a1n
Lowering operator: 1n na n nE
a
1n
1
2nE n
Quantum mechanics
5. Free particle
22
2
dk
dx
with2mE
k
2
2
pE
m
Review I
/( , ) ( ) i kx t i kx tiEtx t x e Ae Be
( )1( , ) ( )
2i kx tx t k e dk
Wavepacket
Quantum mechanics
Free particle
Method:
( ,0)x1. Identify the initial wave function
1( ) ( ,0)
2ikxk x e dx
2. Calculate the Fourier transform
( )1( , ) ( )
2i kx tx t k e dk
3. Estimate the wave function at later times
Review I
Quantum mechanics
5. Free particle
( )1( , ) ( )
2i kx tx t k e dk
Dispersion relation ( )k2
2
k
m
here
Physical interpretation:
• velocity of the each wave at given k: phasevk
• velocity of the wave packet: group
dv
dk
here
2phase
kv
m
group
kv
m
Review I