chapter 40: intro to quantum problems with...
TRANSCRIPT
Chapter 40: Intro to Quantum
Problems with Classical Physics
•Nature of Light- particle or wave?
•Nature of Spectra? Emission Lines?
•Blackbody Radiation? UV Catastrophe?
•Atoms exist? Structure of Atoms?
•Photoelectric Effect?
•Compton Effect?
Black Body Radiation and the
Dawn of Quantum Mechanics
https://www.youtube.com/watch?v=B7pACq_xWyw
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Double Slit is VERY IMPORTANT because it is evidence
of waves. Only waves interfere like this.
Thomas Young 1804
sind m
REVIEW! Derive Fringe Equations
• For bright fringes
• For dark fringes
bright ( 0 1 2 ), ,λL
y m md
dark
1( 0 1 2 )
2, ,
λLy m m
d
Double Slit for Electrons
shows Wave Interference!
Key to Quantum Theory!
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James Clerk Maxwell 1860s
Light is an electromagnetic wave.
The medium is the Ether.
8
0
13.0 10 /
o
c x m s
Heinrich Rudolf Hertz
•1857 – 1894
•German physicist
•First to generate and detect electromagnetic waves in a laboratory setting in 1887.
Section 34.2
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The Electromagnetic Spectrum
Maxwell-Boltzmann Distribution: 1877
• The observed speed distribution of gas molecules in thermal equilibrium is shown at right
• NV is called the Maxwell-Boltzmann speed distribution function
• The distribution of speeds in N gas molecules is
• The probability of finding the molecule in a particular energy state varies exponentially as the negative of the energy divided by kBT
2
3 / 2
/ 22
B
42
Bmv k ToV
mN N v e
k T
nV (E ) = noe –E /kBT
Ludwig Boltzmann
1844 – 1906
• Temperature ~ Ave KE of each particle
• Particles have different speeds
• Gas Particles are in constant RANDOM motion
• Equipartition of Energy: Average KE of
each particle is: 3/2 kT
• Pressure is due to momentum transfer
Speed ‘Distribution’ at
CONSTANT Temperature
is given by the
Maxwell Speed Distribution
23/ 2 1/ 2 rmskT KE mv
k =1.38 x 10-23 J/K Boltzmann’s Constant
Equipartition of Energy
• Each translational degree of freedom contributes an equal amount to the energy of the gas
• Each degree of freedom contributes
½kBT to the energy of a system, where
possible degrees of freedom are those
associated with translation, rotation
and vibration of molecules
• Theory breaks down for black body
radiation – degrees of freedom get
frozen out.
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Kirkoff’s Rules for Spectra: 1859
Bunsen
German physicist who developed the spectroscope and the science of
emission spectroscopy with Bunsen.
Kirkoff
* Rule 1 : A hot and opaque solid, liquid or highly compressed gas emits a continuous spectrum.
* Rule 2 : A hot, transparent gas produces an emission spectrum with bright lines.
* Rule 3 : If a continuous spectrum passes through a gas at a lower temperature, the transparent
cooler gas generates dark absorption lines.
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Continuous vs Discrete
This is a continuous spectrum of colors: all colors are present.
This is a discrete spectrum of colors: only a few are present.
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Compare absorption lines in a source with emission lines found in the laboratory!
Kirchhoff deduced that elements were present in the atmosphere of the Sun
and were absorbing their characteristic wavelengths, producing the absorption
lines in the solar spectrum. He published in 1861 the first atlas of the solar
spectrum, obtained with a prism ; however, these wavelengths were not very
precise : the dispersion of the prism was not linear at all.
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Kirkoff’s Rules
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Anders Jonas Ångström 1869
Ångström measured the wavelengths on the
four visible lines of the hydrogen spectrum,
obtained with a diffraction grating, whose
dispersion is linear, and replaced
Kirchhoff's arbitrary scale by the
wavelengths, expressed in the metric
system, using a small unit (10-10 m) with
which his name was to be associated.
Line color Wavelength
red 6562.852 Å
blue-green 4861.33 Å
violet 4340.47 Å
violet 4101.74 Å
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Balmer Series: 1885Johann Balmer found an empirical equation that
correctly predicted the four visible emission
lines of hydrogen
H 2 2
1 1 1
2R
λ n
RH is the Rydberg constant
RH = 1.097 373 2 x 107 m-1
n is an integer, n = 3, 4, 5,…
The spectral lines correspond to different
values of n
Johannes Robert Rydberg generalized
it in 1888 for all transitions:
Hα is red, λ = 656.3 nm
Hβ is green, λ = 486.1 nm
Hγ is blue, λ = 434.1 nm
Hδ is violet, λ = 410.2 nm
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When an object it heated it will
glow first in the infrared, then the
visible. Most solid materials break
down before they emit UV and
higher frequency EM waves.
Frequency ~ Temperature
Long
Short
All objects radiate energy continuously
in the form of electromagnetic waves
due to thermal vibrations of their
molecules.
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Blackbody Radiation
• A black body is an ideal system that
absorbs all radiation incident on it
• The electromagnetic radiation emitted by a
black body is called blackbody radiation
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Why this shape? Why the drop?
Stefan’s Law: 1879
Rate of radiation of a Black Bodyis the rate of energy transfer, in Watts
– σ = 5.6696 x 10-8 W/m2 . K4
– A is the surface area of the object
– e is a constant called the emissivity• e varies from 0 to 1
• The emissivity is also equal to the absorptivity
– T is the temperature in Kelvins
– With his law Stefan determined the temperature of the Sun’s surface and he calculated a value of 5430C. This was the first sensible value for the temperature of the Sun.
– Boltzmann was his student and derived Stefan’s Law from Thermodynamics in 1884 and extended it to grey bodies.
Jožef Stefan
(1835–1893)
4P e T A
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Blackbody Experiment Results
The total power of the radiation emitted from the
surface increases with temperature
– Stefan’s law: P = AeT4
– P is the power and is the Stefan-Boltzmann constant: = 5.670 x
10-8 W / m2 . K4 (0<e < 1, for a blackbody, e = 1)
Intensity is Power/Area so: I = eT4
The peak of the wavelength distribution shifts to
shorter wavelengths as the temperature increases
– Wien’s displacement law
(T must be in kelvin):
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Sun Power
The radius of our Sun is 6.96 × 108 m, and its
total power output is 3.77 × 1026 W. (a)
Assuming that the Sun’s surface emits as a black
body, calculate its surface temperature. (b) Using
the result of part (a), find λmax for the Sun.
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Sun vs Earth Black Body Emission Curves
Sun emits most strongly in the visible, the Earth in the Infrared which is trapped
by the Atmosphere and warms the planet– green house effect.
The heating effect of a medium such as glass or the Earth’s
atmosphere that is transparent to short wavelengths but opaque
to longer wavelengths: Short get in, longer are trapped!
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Sun vs Earth Peak Wavelength
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A brass plate at room temperature (300 K) radiates 10 W of
energy. If its temperature is raised to 600 K, the
wavelength of maximum radiated intensity
A. Increases.
B. Decreases.
C. Remains the same.
D. Not enough information to tell.
QuickCheck 37.2
Slide 37-31
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A brass plate at room temperature (300 K) radiates 10 W of
energy. If its temperature is raised to 600 K, the
wavelength of maximum radiated intensity
A. Increases.
B. Decreases.
C. Remains the same.
D. Not enough information to tell.
QuickCheck 37.2
Slide 37-32
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Intensity of Blackbody Radiation
• The intensity increases with increasing temperature
• The amount of radiation emitted increases with increasing temperature
– The area under the curve
• The peak wavelength decreases with increasing temperature
• Combining gives the Rayleigh-Jeans law:
I = P/A = T4
I , ~4
1λ T
λ
Stefan’s law: P = AeT4 Wein’s Law:
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Problems with the Wein’s World
• At short wavelengths, there
was a major disagreement
between the Rayleigh-Jeans
law and experiment
• This mismatch became
known as the ultraviolet
catastrophe
– You would have infinite
energy as the wavelength
approaches zero
I , ~4
1λ T
λ
https://www.youtube.com/watch?v=FXfrncRey-4
https://www.youtube.com/watch?v=7BXvc9W97iU
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Solves Black Body Mystery. Atomic Energy is quantized.
It comes in chunks of Planck’s constant, h.
34
, n= 0,1,2,3,...
6.626 10
E nhf
h x Js
2
5
2
1I ,
Bhc λk T
πhcλ T
λ e
Max Planck: Father of Quantum
• Introduced the concept of “quantum of action” in 1900 to solve the black body mystery
• In 1918 he was awarded the Nobel Prize for the discovery of the quantized nature of energy
The possible frequencies and energy states of a wave on
a string are quantized.
2
vf n
l
Strings are Quantized
Energy is Quantized
• An energy-level diagram
shows the quantized energy
levels and allowed
transitions
• Energy is on the vertical axis
• Horizontal lines represent
the allowed energy levels
• The double-headed arrows
indicate allowed transitions
Equipartition of Energy
• Each translational degree of freedom contributes an equal amount to the energy of the gas
• Each degree of freedom contributes
½kBT to the energy of a system, where
possible degrees of freedom are those
associated with translation, rotation
and vibration of molecules
• Theory breaks down for black body
radiation – degrees of freedom get
frozen out.
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Planck’s Two Assumptions
• The energy of an oscillator can have only certain discretevalues En= nhƒ, n=interger
– This says the energy is quantized
– Each discrete energy value corresponds to a different quantum state
• The oscillators emit or absorb energy when making a transition from one quantum state to another
– The entire energy difference between the initial and final states in the transition is emitted or absorbed as a single quantum of radiation
2
5
2
1I ,
Bhc λk T
πhcλ T
λ e
The Turn Around in Planck’s
Model
• The average energy of a wave is the average
energy difference between levels of the oscillator,
weighted according to the probability of the wave
being emitted
• This weighting is described by the Boltzmann
distribution law and gives the probability of a state
being occupied as being proportional to
BE k Te
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Planck had seen the turn over before:
Maxwell-Boltzmann Distribution: 1877• The observed speed
distribution of gas molecules in thermal equilibrium is shown at right
• NV is called the Maxwell-Boltzmann speed distribution function
• The distribution of speeds in N gas molecules is
• The probability of finding the molecule in a particular energy state varies exponentially as the negative of the energy divided by kBT
2
3 / 2
/ 22
B
42
Bmv k ToV
mN N v e
k T
nV (E ) = noe –E /kBT
Ludwig Boltzmann
1844 – 1906
Planck’s Wavelength
Distribution Function
• Planck generated a theoretical expression
for the wavelength distribution
– h = 6.626 x 10-34 J.s
– h is a fundamental constant of nature
2
5
2
1I ,
Bhc λk T
πhcλ T
λ e
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
Solves Black Body Mystery. Atomic Energy is quantized.
It comes in chunks of Planck’s constant, h.
34
, n= 0,1,2,3,...
6.626 10
E nhf
h x Js
2
5
2
1I ,
Bhc λk T
πhcλ T
λ e
Max Planck: Father
of Quantum
• In 1900 Planck developed a theory of blackbody radiation that leads to an equation for the intensity of the radiation which is in complete agreement with experimental observations.
• From his radiation equations Stefan’s Law is Derived!!
• Introduced the concept of “quantum of action” in 1900
• In 1918 he was awarded the Nobel Prize for the discovery of the quantized nature of energy
34
, n= 0,1,2,3,...
6.626 10
E nhf
h x Js
2
5
2
1I ,
Bhc λk T
πhcλ T
λ e
Intensity of Blackbody RadiationP40.61
The total power per unit area radiated by a black body at a temperature T is the area under the I(λ, T)-versus-λ curve, as shown in Figure 40.3. (a) Show that this power per unit area is
where I(λ, T) is given by Planck’s radiation law and σ is a constant independent of T.
This result is Stefan’s law.
To carry out the integration, you should make
the change of variable x = hc/λkT and use the
fact that
4
0λ λ, TdTI
0
43
151
xe
dxx
2
5
2
1I ,
Bhc λk T
πhcλ T
λ e
Particle Wave Duality
What is a particle? What is a wave?
The Photoelectric Effect• In 1886 Hertz noticed, in the
course of his investigations, that a
negatively charged electroscope
could be discharged by shining
ultraviolet light on it.
• In 1899, Thomson showed that the
emitted charges were electrons.
• Around 1900, Phillip Lenard built
an apparatus which produced an
electric current when ultraviolet
light was shining on the cathode.
• This phenomenon is called the
photoelectric effect.
The Problem with Waves:
Increasing the intensity of a low frequency
light beam doesn’t eject electrons. This
didn’t agree with wave picture of light
which predicts that the energy of waves
add so that if you increase the intensity of
low frequency light (bright red light)
eventually electrons would be ejected –
but they don’t! There is a cut off
frequency, below which no electrons will
be ejected no matter how bright the beam!
Also there is no time delay in the ejection
of electrons as the waves build up!
The PROBLEM with the
Photoelectric Effect
The Problem with Waves:
Increasing the intensity of a low frequency
light beam doesn’t eject electrons. This
didn’t agree with wave picture of light
which predicts that the energy of waves
add so that if you increase the intensity of
low frequency light (bright red light)
eventually electrons would be ejected –
but they don’t! There is a cut off
frequency, below which no electrons will
be ejected no matter how bright the beam!
Also there is no time delay in the ejection
of electrons as the waves build up!
The Photoelectric EffectProof that Light is a Particle
https://phet.colorado.edu/en/simulation/legacy/photoelectr
ic