Probability Current Density: In general for 3 space dimensions the probability density
P(r, t)=ψ*ψ dτ where dτ is a volume element. Integrating P over all space gives us the total probability N ( if not normalized to 1).
Differentiating the integrated probability density w.r.t the time t and replacing the time derivatives of ψ* and ψ from the Schroedinger equation and its complex conjugate we have
∂/∂ t ∫V P dτ= -i /2m ∫V [ψ∇2 ψ* - ψ*∇2ψ]
using the Green theorem on the RHS we can write the integrand as divergence of a vector field ∇.S where S= -/m Im (ψ∇&psi*). Thus we have the equation
∂/∂ t P = ∇.S which is a equation of continuity for the
probability and indicates that probability is conserved. Correspondingly S is called the Probability Current Density ( in analogy with the current density in EM theory or the velocity current in fluid mechanics). So we see that Probability
can flow. In particular for free particle we see that S=P v where
v is the velocity of the particle. This is just like J=ρ v in EM theory.
As an exercise do this for 1 dimensions Ref Gasiorowicz ( in 1 dim there is no Green Theorem and one will have to use total derivatives to reduce the integral).
Friday, February 11, 2011
Thursday, February 10, 2011
Schroedinger Equation II
The postulates of Quantum Mechanics:
I. Dynamical variables or observables ( measurable quantities ) are associated with actions of Linear Operators. ( Note that some variables like spin are intrinisically quantum mechanical without any classical analogs).
** We will adopt the covention of representing operators by boldface
Assuming ψ(x,t)= A ei(kx-ωt) for a free particle it is easy to see
by direct operation that the momentum operator p=-i ℏ(∂/∂ x)
and that for the energy operator E=-i ℏ(∂/∂ t)
II. The Measurement of a dynamical variable or observable A that yields the value a leaves the system in a quantum state given by the wave function Φa
or A Φa=a Φa is an eigenvalue equation.
For example pψ=pψ.
III) Expectation Value: The average value of an observable for a system in the quantum state given by the wave function ψ is given as
⟨ C ⟩=∫ψ*C&psi dx
The average value is understood in the sense of an ensemble average where
simultaneous measurements of the observable is made at time t on a large number of exact replica of the system with identical initial quantum state specified by ψ(x,0).
IV) The time development of the wave function ψ(x,t) is given by the time dependent Schroedinger's equation.
I. Dynamical variables or observables ( measurable quantities ) are associated with actions of Linear Operators. ( Note that some variables like spin are intrinisically quantum mechanical without any classical analogs).
** We will adopt the covention of representing operators by boldface
Assuming ψ(x,t)= A ei(kx-ωt) for a free particle it is easy to see
by direct operation that the momentum operator p=-i ℏ(∂/∂ x)
and that for the energy operator E=-i ℏ(∂/∂ t)
II. The Measurement of a dynamical variable or observable A that yields the value a leaves the system in a quantum state given by the wave function Φa
or A Φa=a Φa is an eigenvalue equation.
For example pψ=pψ.
III) Expectation Value: The average value of an observable for a system in the quantum state given by the wave function ψ is given as
⟨ C ⟩=∫ψ*C&psi dx
The average value is understood in the sense of an ensemble average where
simultaneous measurements of the observable is made at time t on a large number of exact replica of the system with identical initial quantum state specified by ψ(x,0).
IV) The time development of the wave function ψ(x,t) is given by the time dependent Schroedinger's equation.
Schroedinger Equation and the Probability Wave Function I
http://www.youtube.com/watch?v=6Q4_nl0ICao
The above youtube link shows the thought experiment described in the class and also available in the first chapter of Feynman Lectures III. It clearly tells us that the waves describing matter are actually probability waves such that the modulus squared of this probability wave amplitude describes the probability of the particles location and behaves just like the intensity of a normal ( linear) wave ( like an EM Wave).
This may be summarized as
For normal waves Intensity I ~ |amplitude|2
For probability waves Probability P ~ |probability wave amplitude|2
So we may describe the matter waves by a Probability Wave function/Wave Function
( considering only 1 space dimension x for simplicity)
Ψ(x,t)=A ei(kx-ωt) where A is a constant. This wave function
is expected to satisfy a linear wave equation so that the superposition principle holds and must be consistent with the following relations.
p=ℏk; , E=ℏω=P2/2m + V(x)
or E=ℏ 2K2/2m + V(x)=ℏω
From these relations and the form of the wave function Ψ(x,t) it is obvious that
the equation must be second order in space derivatives ( for the k2 and
first order in the time derivative ( for ω). So we can assume a form of the equation to be
[ α∂2/∂ x2 + V(x)]ψ (x,t)=β(∂/∂ t) ψ(x,t)
Using the form of ψ(x,t) this gives -αk2 +V =∓iβω this has two solutions β=± i ℏ. Assuming the + sign ( - is equivalent) we have the Schroedinger equation in 1 space dimension as
[ -ℏ2/2m ∂2/∂ x2 + V(x)]ψ (x,t)=i ℏ(∂/∂ t) ψ(x,t)
This can easily be generalised to 3 dimensions as
[ -ℏ2/2m ∇2 + V(x)]ψ (r ,t)=i ℏ(∂/∂ t) ψ(r ,t)
This is refered to as the Time Dependent Schoredinger Equation
It can be shown that the solution to this equation ψ is a complex valued function of space and time. Hence ψ is NOT MESURABLE.
The Probability density of location of the particle between x to x + dx is then proportional to P(x,t)~ ψ2. P(x,T) must be real and positive semi definite that is either positive or zero.
The state of the system is given by the wave function ψ and knowing the state
at time t=0 the Schroedinger equation predicts the state at time t=t0. However notice that since all Physics is contained in |ψ|2 the knowledge of all physical characteristic of the system at t=0 does not determine the function ψ(x, 0).
Quoting Max Born "Motion of the particles in QM conforms to the laws of probability however probability itself is propagated in CAUSAL FASHION through the Schroedinger equation.
The total probability is given as the volume integral over all space ∫|psi;|2 d x and it must be finite so that the integral must converge. This requires the wave function to be a SQUARE INTEGRABLE or an L2 function. The total probability may be normalized to 1.
Notice now that the probabilities corresponding to two wave functions are individually P1 and P2 but when they are superposed their
probability
P12=|psi;1 +psi;2|2=P1 + P2 + Cross Terms. The cross terms gives rise to the interference pattern just like the cross terms in the intensity of superposed EM waves like light in a Youngs Double slit experiment described at the beginning.
Monday, February 7, 2011
Bohr Atom Model
http://hyperphysics.phy-astr.gsu.edu/hbase/bohr.html#c4
http://hyperphysics.phy-astr.gsu.edu/hbase/hyde.html
Click the relevant blobs for the webpages related to the index in the link below
http://hyperphysics.phy-astr.gsu.edu/hbase/bohrcn.html
TO BE UPDATED
Matter Waves
The host of experimental data and the theoretical ideas of Planck and Einstein conclusively established that radiation has a DUAL behavior. Namely it behaves both as
a wave in interference and diffraction and as a particle in photoelectric and compton effect. Inspired by this and guided by the close correspondence of the Fermats principle of optical path in geometrical optics ( particle like behavior) and the
Least Action principle in Classical Analytical Mechanics, de Broglie postulated that
Matter must also exhibit wave like behaviour.
Since for radiation E=hν and p=h/λ de Broglie postulated that the wavelength of matter waves was also given as λDB=h/p where p is the momentum of the particle. A quick calculation with standard values show that for macroscopic systems and velocities λDB is negligibly small, for a 1.0 kg mass moving at 10 m/s the λDB =6.6 x 10 -35 m which is too small to be detectable. But for a 100 eV energy electron it was of the order of 1 Angstrom.
From Optics we know that when λ >> a where a is the aperture dimension there is no difraction and geomterical optics holds. For diffraction λ ~ a.
So obviously to have observable diffraction effects λDB ~ a which is 1 angstorm. Such gratings are offered by the ordered periodic arrays of atomic layers in a crystal. The inter layer spacing is of the order of 1 angstorm.
So an electron beam scattered from such a crystal should exhibit diffraction.
This experiment was first performed by Davisson and Germer
the results of the experiment are summarized here
http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/davger2.html#c1
The results clearly showed that the wave particle duality was valid also for matter just as in radiation and obeyed the same formula for both the momentum and energy namely E=hν and p=h/λ
Bohr's Complementarity Principle : This principle asserts that the wave and the particle behaviours for both matter and radiation are complementary. In a single experiment either the wave nature or the particle nature is manifest but never together.
So radiation and matter are neither simply waves nor simply particles. They are more general objects. In advanced applications the view of radiation and matter that emerges is a general description in terms of quantized fields which displays both particle (quanta) and wave ( field) properties. Particles are described as the field quanta
The link between the particle and the wave picture is provided by an interpretation of the wave particle duality based on probability. In the wave picture the Intensity I ~ Eav2 where Eav is the average electric field ( amplitude) over 1 cycle. In the photon picture I ~ Navhν where Nav is the average number of photos crossing unit area/unit time perpendicular to the propagation vector of the EM wave. In EM theory Eav2 is proportional to the energy density. Einstein interpreted this as the average photon density which owing to the statistical nature of emission was related to the probability measure for a photon to cross unit area per unit time. So I~Eav2 ~Navhν .
Born borrowed this probability interpretation later to apply to the de Broglie's matter waves also. Since probability and intensity had a simmilar behaviour and the intensity is related to the square of an amplitude, one thinks of a probability wave
amplitude whose square is related to the probability. This wave is a probability wave
whose amplitude squared is related to the probability. This probability wave should satisfy a linear wave equation simmilar to the wave equation satisfied by a standard
EM wave for the superposition principle to be valid.
a wave in interference and diffraction and as a particle in photoelectric and compton effect. Inspired by this and guided by the close correspondence of the Fermats principle of optical path in geometrical optics ( particle like behavior) and the
Least Action principle in Classical Analytical Mechanics, de Broglie postulated that
Matter must also exhibit wave like behaviour.
Since for radiation E=hν and p=h/λ de Broglie postulated that the wavelength of matter waves was also given as λDB=h/p where p is the momentum of the particle. A quick calculation with standard values show that for macroscopic systems and velocities λDB is negligibly small, for a 1.0 kg mass moving at 10 m/s the λDB =6.6 x 10 -35 m which is too small to be detectable. But for a 100 eV energy electron it was of the order of 1 Angstrom.
From Optics we know that when λ >> a where a is the aperture dimension there is no difraction and geomterical optics holds. For diffraction λ ~ a.
So obviously to have observable diffraction effects λDB ~ a which is 1 angstorm. Such gratings are offered by the ordered periodic arrays of atomic layers in a crystal. The inter layer spacing is of the order of 1 angstorm.
So an electron beam scattered from such a crystal should exhibit diffraction.
This experiment was first performed by Davisson and Germer
the results of the experiment are summarized here
http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/davger2.html#c1
The results clearly showed that the wave particle duality was valid also for matter just as in radiation and obeyed the same formula for both the momentum and energy namely E=hν and p=h/λ
Bohr's Complementarity Principle : This principle asserts that the wave and the particle behaviours for both matter and radiation are complementary. In a single experiment either the wave nature or the particle nature is manifest but never together.
So radiation and matter are neither simply waves nor simply particles. They are more general objects. In advanced applications the view of radiation and matter that emerges is a general description in terms of quantized fields which displays both particle (quanta) and wave ( field) properties. Particles are described as the field quanta
The link between the particle and the wave picture is provided by an interpretation of the wave particle duality based on probability. In the wave picture the Intensity I ~ Eav2 where Eav is the average electric field ( amplitude) over 1 cycle. In the photon picture I ~ Navhν where Nav is the average number of photos crossing unit area/unit time perpendicular to the propagation vector of the EM wave. In EM theory Eav2 is proportional to the energy density. Einstein interpreted this as the average photon density which owing to the statistical nature of emission was related to the probability measure for a photon to cross unit area per unit time. So I~Eav2 ~Navhν .
Born borrowed this probability interpretation later to apply to the de Broglie's matter waves also. Since probability and intensity had a simmilar behaviour and the intensity is related to the square of an amplitude, one thinks of a probability wave
amplitude whose square is related to the probability. This wave is a probability wave
whose amplitude squared is related to the probability. This probability wave should satisfy a linear wave equation simmilar to the wave equation satisfied by a standard
EM wave for the superposition principle to be valid.
Monday, January 17, 2011
Compton Effect
More experimental evidence of the quantized nature of radiation and the photon picture
was provided by the famous Compton effect observed when monochromatic X-rays of wavelnegth λ were scattered from a target material like graphite. At high angles of scattering an extra peak at wavelength λ' greater than the original
wavelength was observed in the intensity vs wavelength plot.
http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/compdat.html#c1
This was termed as the Compton shift Δλ=λ-λ' was dependent on the scattering angle θ
The classical picture of radiation as an EMW predicted that the free electron in the material would be set into forced oscillations by the oscillating electric field of the EMW and would re radiate at the same wavelength and frequency as the incident radiation. There was no explanation for a peak in the Intensity for a shifted wavelength λ' from Classical Theory of EMW.
Compton viewed this effect as Relativistic Collison between an energetic photon
with E=hν and a stationary electron with rest mass m0.
Relativistic collison theory is distinct from collisons in classical mechanics
due to the rest mass energy and energy and momentum are on the same footing.
(In fact the 3 mom and energy are the 4 components of the relativistic momentum 4-vector in 3 space+ 1 time dimensional Minkowski space)
The operative relation for energy is the famous equation of Einstein
E=[ m0c2]γ where
1/γ=(1-v2/c2)1/2
and m0 is the rest mass of the particle.
The energy momentum formula is
E2=c2p2+ m02C4
For photons with rest mass ZERO E=cp=hν and hence p=h/λ where p is the photon momentum
Now we apply momentum and energy conservation along x and y directions and obtain the two key formula
E0-E1=K where E are the energy of the incident and scattered photons and K is the kinetic energy of the electron
or
c(p0-p1=K ......(1) as E=Cp for the photon and
p2=p20+ p21 -2 p0 p1Cos θ.....(2)
We also have the formula
K2/C2 - 2Km0=p2=p2......(3)
we substitute eqn. (2) in (3) and using (1) we have
1/p1 - 1/p0 = (1/m0c) (1-Cos θ )........(4)
multiplying (4) by h and using p=h/λ we have
λ1 - λ0 = λcomp (1-Cos θ)
=Δλ
where λcomp= (h/m0c) is called the Compton Wavelength and is measured for the electron to be 0.0243 Angstorms.
This analysis explains the shifted wavelength in the Compton effect. The other peak at the original incident wavelength may be understood as scattering from tightly bound electrons. If the incident photon energy is low this situation is like that of the incident photon colliding with the atom as a whole ( electron is tightly bound to the atom) so the mass in the Compton wavelength in the scattering formula is
the mass of the atom M which is far larger than that of the electron. Hence the Compton wavelength is vanishingly small and the Compton shift goes to ZERO.
was provided by the famous Compton effect observed when monochromatic X-rays of wavelnegth λ were scattered from a target material like graphite. At high angles of scattering an extra peak at wavelength λ' greater than the original
wavelength was observed in the intensity vs wavelength plot.
http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/compdat.html#c1
This was termed as the Compton shift Δλ=λ-λ' was dependent on the scattering angle θ
The classical picture of radiation as an EMW predicted that the free electron in the material would be set into forced oscillations by the oscillating electric field of the EMW and would re radiate at the same wavelength and frequency as the incident radiation. There was no explanation for a peak in the Intensity for a shifted wavelength λ' from Classical Theory of EMW.
Compton viewed this effect as Relativistic Collison between an energetic photon
with E=hν and a stationary electron with rest mass m0.
Relativistic collison theory is distinct from collisons in classical mechanics
due to the rest mass energy and energy and momentum are on the same footing.
(In fact the 3 mom and energy are the 4 components of the relativistic momentum 4-vector in 3 space+ 1 time dimensional Minkowski space)
The operative relation for energy is the famous equation of Einstein
E=[ m0c2]γ where
1/γ=(1-v2/c2)1/2
and m0 is the rest mass of the particle.
The energy momentum formula is
E2=c2p2+ m02C4
For photons with rest mass ZERO E=cp=hν and hence p=h/λ where p is the photon momentum
Now we apply momentum and energy conservation along x and y directions and obtain the two key formula
E0-E1=K where E are the energy of the incident and scattered photons and K is the kinetic energy of the electron
or
c(p0-p1=K ......(1) as E=Cp for the photon and
p2=p20+ p21 -2 p0 p1Cos θ.....(2)
We also have the formula
K2/C2 - 2Km0=p2=p2......(3)
we substitute eqn. (2) in (3) and using (1) we have
1/p1 - 1/p0 = (1/m0c) (1-Cos θ )........(4)
multiplying (4) by h and using p=h/λ we have
λ1 - λ0 = λcomp (1-Cos θ)
=Δλ
where λcomp= (h/m0c) is called the Compton Wavelength and is measured for the electron to be 0.0243 Angstorms.
This analysis explains the shifted wavelength in the Compton effect. The other peak at the original incident wavelength may be understood as scattering from tightly bound electrons. If the incident photon energy is low this situation is like that of the incident photon colliding with the atom as a whole ( electron is tightly bound to the atom) so the mass in the Compton wavelength in the scattering formula is
the mass of the atom M which is far larger than that of the electron. Hence the Compton wavelength is vanishingly small and the Compton shift goes to ZERO.
Monday, January 10, 2011
Photoelectric Effect.
The Black Body radiation spectrum forced upon us the idea of quantized oscillators
radiating EMW and a departure from the concepts of Classical Physics. Another phenomena was soon discovered which seemed to defy a classical explanation in terms of EMW. This was the photoelectric effect which demonstrated that incident radiation on the (metal) surface are able to eject electrons. The electrons are called photo electrons and may constitute a current called a photo current
In 1886 Hertz did his famous experiment on this effect
http://physics.info/photoelectric/circuit.html
* Image from Flickr "Robbo Coach's" photostream
Several interesting features may be noted
1. The current I is NOT ZERO till a "reverse voltage is applied". This shows that
the electrons are ejected with some kinetic energy. The current saturates at some value of the forward voltage and this is dependent on the intensity of the incident radiation.
2. The cut off voltage is independent of Intensity of the radiation but depends on the frequency.
3. Millikan showed that the stopping potential varied linearly with the frequency and there was a "cut off frequency" ν0 beyond which no electrons are emitted.
*Graph
An explanation of this phenomena based on the Classical Wave picture of EMW fails
due to the following reasons
1. Kinetic energy of photo electrons in particular the maximum kinetic energy Kmax
=e V0 where V0 is the stopping potential should depend on the intensity as Intensity is proportional to the Electric Field amplitude squared in an EMW and this is the field that energizes the electron in the wave picture. But experiment shows otherwise.
2.The effect should occur for any frequency provide the incident radiation is intense enough to energize the electron. Experiment shows a cut off frequency specific to the material below which no photo electrons are ejected.
3. Effect should show a time lag as the wave energy is diffused over a spatial extent. Experiment shows that the effect is instantaneous.
These discrepancies inspired Einstein to apply the idea of Quantization due to Planck to EM radiation. Einstein postulated that EM radiation is composed of a large number of discrete packets of localized energy which behave like particles. The particles were called photons and their energy was quantized as E=hν where ν was the frequency of the radiation.
Einstein postulated that in the photoelectric effect 1 photon was absorbed by 1 electron to be energized and ejected provided the energy was adequate for the electron to overcome the surface atomic attractions.
So the kinetic energy of the ejected electrons would be K=hν-W where W is the energy for overcoming the surface barriers.
The maximum kinetic energy Kmax=hν -W0 where the second term was called the work function of the material and referred to the minimum energy for electrons to overcome the surface atomic attractions.
The photon picture was able to explain the experimental results
1. Intensity of radiation was proportional to the number of photons or their density. However each electron could absorb only 1 single photon. Hence the maximum kinetic energy was independent of the intensity.
2. At threshold when an electron is Just ejected with zero kinetic energy ( corresponding to the stopping potential Kmax=0 and hν0=W0 where ν0 is the cut off frequency
and no emission was possible below this.
3. As the energy of radiation was highly localized in the photon picture the absorption is instantaneous and hence there was no time lag as observed experimentally but was predicted by the classical wave picture.
From the equation Kmax=eV0=hν -W0
its is seen that the stopping potential V0 varies linearly with the frequency and from the slope of the graph of the stopping potential vs frequency
we can fix the Planck's constant h knowing e and m for the electron, which comes out to be close to the measured value.
radiating EMW and a departure from the concepts of Classical Physics. Another phenomena was soon discovered which seemed to defy a classical explanation in terms of EMW. This was the photoelectric effect which demonstrated that incident radiation on the (metal) surface are able to eject electrons. The electrons are called photo electrons and may constitute a current called a photo current
In 1886 Hertz did his famous experiment on this effect
http://physics.info/photoelectric/circuit.html
* Image from Flickr "Robbo Coach's" photostream
Several interesting features may be noted
1. The current I is NOT ZERO till a "reverse voltage is applied". This shows that
the electrons are ejected with some kinetic energy. The current saturates at some value of the forward voltage and this is dependent on the intensity of the incident radiation.
2. The cut off voltage is independent of Intensity of the radiation but depends on the frequency.
3. Millikan showed that the stopping potential varied linearly with the frequency and there was a "cut off frequency" ν0 beyond which no electrons are emitted.
*Graph
An explanation of this phenomena based on the Classical Wave picture of EMW fails
due to the following reasons
1. Kinetic energy of photo electrons in particular the maximum kinetic energy Kmax
=e V0 where V0 is the stopping potential should depend on the intensity as Intensity is proportional to the Electric Field amplitude squared in an EMW and this is the field that energizes the electron in the wave picture. But experiment shows otherwise.
2.The effect should occur for any frequency provide the incident radiation is intense enough to energize the electron. Experiment shows a cut off frequency specific to the material below which no photo electrons are ejected.
3. Effect should show a time lag as the wave energy is diffused over a spatial extent. Experiment shows that the effect is instantaneous.
These discrepancies inspired Einstein to apply the idea of Quantization due to Planck to EM radiation. Einstein postulated that EM radiation is composed of a large number of discrete packets of localized energy which behave like particles. The particles were called photons and their energy was quantized as E=hν where ν was the frequency of the radiation.
Einstein postulated that in the photoelectric effect 1 photon was absorbed by 1 electron to be energized and ejected provided the energy was adequate for the electron to overcome the surface atomic attractions.
So the kinetic energy of the ejected electrons would be K=hν-W where W is the energy for overcoming the surface barriers.
The maximum kinetic energy Kmax=hν -W0 where the second term was called the work function of the material and referred to the minimum energy for electrons to overcome the surface atomic attractions.
The photon picture was able to explain the experimental results
1. Intensity of radiation was proportional to the number of photons or their density. However each electron could absorb only 1 single photon. Hence the maximum kinetic energy was independent of the intensity.
2. At threshold when an electron is Just ejected with zero kinetic energy ( corresponding to the stopping potential Kmax=0 and hν0=W0 where ν0 is the cut off frequency
and no emission was possible below this.
3. As the energy of radiation was highly localized in the photon picture the absorption is instantaneous and hence there was no time lag as observed experimentally but was predicted by the classical wave picture.
From the equation Kmax=eV0=hν -W0
its is seen that the stopping potential V0 varies linearly with the frequency and from the slope of the graph of the stopping potential vs frequency
we can fix the Planck's constant h knowing e and m for the electron, which comes out to be close to the measured value.
Sunday, January 9, 2011
Black Body Radiation III
The Planck Distribution
------------------------
We saw that the Rayleigh Jeans law fails to explain the black body spectrum at higher frequencies. Planck in trying to explain this discrepancy noticed that the average total energy for the modes was kT at low frequencies but must fall to zero at high frequencies to describe the experimental curve. This required a high frequency cut off for the oscillators in violation of the equipartition theorem which assumes that the oscillators may have any energy from 0 to infinity with equal a priori probability resulting in an average total energy per mode independent of frequency.
Planck departed from this hypothesis and postulated that the oscillators have
discrete energies which are equally spaced that is E=0, ΔE. 2 ΔE..... and ΔE=hν was proportional to the frequency and the proportionality constant h was refered to as the Planck's constant.
As ΔE=hν the energy of the oscillators E=nhν with n=0,1,2,3.....
An average energy computed from this discrete values using the Boltzmann
distribution as in the Classical Equipartition Theorem ( using summations over discrete values instead of integrals) yielded the Plancks distribution formula for the average energy
Eav=[hν/ e(hν/kT) -1]
It is obvious from the formula that the desired high frequency cut off for the oscillators is obtained as E goes to zero with large values of frequencies and E
goes to (kT) ( expanding the exponential to first order) for small values of frequency.
Multiplying this by the Jeans number of modes in an infinitessimal frequency interval we obtain the energy density of the cavity radiation to be
ρ(ν)dν=(8πν2/c3 [hν/ e(hν/kT) -1] dν
This is the Planck Black body Spectrum and completely matches the experimental black body curve ( upto a proportionality constant).
http://www.youtube.com/watch?v=cW4vmr3hb2o&feature=related
------------------------
We saw that the Rayleigh Jeans law fails to explain the black body spectrum at higher frequencies. Planck in trying to explain this discrepancy noticed that the average total energy for the modes was kT at low frequencies but must fall to zero at high frequencies to describe the experimental curve. This required a high frequency cut off for the oscillators in violation of the equipartition theorem which assumes that the oscillators may have any energy from 0 to infinity with equal a priori probability resulting in an average total energy per mode independent of frequency.
Planck departed from this hypothesis and postulated that the oscillators have
discrete energies which are equally spaced that is E=0, ΔE. 2 ΔE..... and ΔE=hν was proportional to the frequency and the proportionality constant h was refered to as the Planck's constant.
As ΔE=hν the energy of the oscillators E=nhν with n=0,1,2,3.....
An average energy computed from this discrete values using the Boltzmann
distribution as in the Classical Equipartition Theorem ( using summations over discrete values instead of integrals) yielded the Plancks distribution formula for the average energy
Eav=[hν/ e(hν/kT) -1]
It is obvious from the formula that the desired high frequency cut off for the oscillators is obtained as E goes to zero with large values of frequencies and E
goes to (kT) ( expanding the exponential to first order) for small values of frequency.
Multiplying this by the Jeans number of modes in an infinitessimal frequency interval we obtain the energy density of the cavity radiation to be
ρ(ν)dν=(8πν2/c3 [hν/ e(hν/kT) -1] dν
This is the Planck Black body Spectrum and completely matches the experimental black body curve ( upto a proportionality constant).
http://www.youtube.com/watch?v=cW4vmr3hb2o&feature=related
Black Body Radiation II
In the last 2 sessions we have established that the Cavity Radiation spectrum is identical to the black body radiation spectrum and this is expressed as
ρ(ν)dν=constant. R(ν)dν at some temperature T deg K at thermal equilibrium.
The cavity radiation is due to the energized atomic oscillators and at ε is a standing wave configuration ( assuming no dissipation at walls)
To calculate the energy density of the cavity radiation in an infinitesimal frequency interval dν we need
(i) The number of modes in that interval
(ii) The average total energy in each mode
so ρT(ν)= # of modes X average energy/volume
To count the modes for simplicity we assume a cubic cavity and we will ses that the final result is independent of the geometry of the cavity.
As a warm up exercise we first compute the modes for 1 dimension ( line cavity of length L). We solve the 1 d wave equation with the boundary condition of nodes at the walls at x=0 and x=L. The boundary conditions restricts the modes to be such that the wave vector k=(nπ/L).
from standard relations between frequency and wave vectors its easy to see that the number of modes in the frequency interval dν is dN=(2L/C) X dν
This is easily generalized to 3d with a cube of side L and
assuming independent propagation in (x, y,z) directions due to parallel walls and
n2=n2x + n2y
+ n2z
Using the same construction as 1d we mark the n's on a 3 d lattice and we see that
the number of modes = volume in lattice space. To find the number of modes in the frequency interval dν it is required to find the volume between two spheres of radius N and N + dN in the first octant as all
n > 0.
N= (1/8)(4π/3)n3=(π/6)(2L/c)3 ν3
so that dN=N(ν)dν=(4π/c)3 L3ν2dν X 2
for the 2 states of polarization of EMW
so the density is dN/L3 = (8π/c3)ν2 dν
This is to be multiplied by the average total energy of each mode which may have any energy continuously between 0 and infinity.
Since the standing waves are in Ε with each other the equipartition theorem may be applied to find the average total energy to be E=kT.
(potential and kinetic)
So ρ(ν)dν=kT (8π/c3)ν2 dν
This is the famous Rayleigh Jeans law and shows a quadratic dependence on the frequency. It is quite obvious that the law predicts an indefinite increase in the
energy density with frequency. It describes the black body spectrum at low frequencies but breaks down at high frequencies ( ultraviolet direction). Hence this came to be known as the "Ultraviolet Catastrophe"
It actually marks the failure of classical physics comprising of Mechanics and
EM.
ρ(ν)dν=constant. R(ν)dν at some temperature T deg K at thermal equilibrium.
The cavity radiation is due to the energized atomic oscillators and at ε is a standing wave configuration ( assuming no dissipation at walls)
To calculate the energy density of the cavity radiation in an infinitesimal frequency interval dν we need
(i) The number of modes in that interval
(ii) The average total energy in each mode
so ρT(ν)= # of modes X average energy/volume
To count the modes for simplicity we assume a cubic cavity and we will ses that the final result is independent of the geometry of the cavity.
As a warm up exercise we first compute the modes for 1 dimension ( line cavity of length L). We solve the 1 d wave equation with the boundary condition of nodes at the walls at x=0 and x=L. The boundary conditions restricts the modes to be such that the wave vector k=(nπ/L).
from standard relations between frequency and wave vectors its easy to see that the number of modes in the frequency interval dν is dN=(2L/C) X dν
This is easily generalized to 3d with a cube of side L and
assuming independent propagation in (x, y,z) directions due to parallel walls and
n2=n2x + n2y
+ n2z
Using the same construction as 1d we mark the n's on a 3 d lattice and we see that
the number of modes = volume in lattice space. To find the number of modes in the frequency interval dν it is required to find the volume between two spheres of radius N and N + dN in the first octant as all
n > 0.
N= (1/8)(4π/3)n3=(π/6)(2L/c)3 ν3
so that dN=N(ν)dν=(4π/c)3 L3ν2dν X 2
for the 2 states of polarization of EMW
so the density is dN/L3 = (8π/c3)ν2 dν
This is to be multiplied by the average total energy of each mode which may have any energy continuously between 0 and infinity.
Since the standing waves are in Ε with each other the equipartition theorem may be applied to find the average total energy to be E=kT.
(potential and kinetic)
So ρ(ν)dν=kT (8π/c3)ν2 dν
This is the famous Rayleigh Jeans law and shows a quadratic dependence on the frequency. It is quite obvious that the law predicts an indefinite increase in the
energy density with frequency. It describes the black body spectrum at low frequencies but breaks down at high frequencies ( ultraviolet direction). Hence this came to be known as the "Ultraviolet Catastrophe"
It actually marks the failure of classical physics comprising of Mechanics and
EM.
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