QUANTUM AND NUCLEAR PHYSICS
Mark LesmeisterPearland ISD Physics
Acknowledgements
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PARTICLES OF LIGHT
Infrared Videos: Bats Emerging from Caves
Videos from Boston University Center for Ecology and Conservation Biology, http://www.cs.bu.edu/fac/betke/research/bats/videos.html
Blackbody Radiation
All objects emit electromagnetic radiation.
The frequencies of this radiation and the amount emitted depend on the temperature of the object.
“Room temperature” objects emit infrared frequencies.
Very hot objects (like glowing coals) emit visible radiation.
The universe, which has a temperature of 3 Kelvin, is awash with microwave radiation.
Source: Darth Kule, “Black-body.svg”, http://en.wikipedia.org/wiki/File:Black_body.svg. Retrieved May 14, 2010. Graphic is public domain.
Quantization of Energy In the 19th
century, experiments were conducted to measure the blackbody radiation emitted at each wavelength of light.
Source: Darth Kule, “Black-body.svg”, http://en.wikipedia.org/wiki/File:Black_body.svg. Retrieved May 14, 2010. Graphic is public domain.
Quantization of Energy The wave-
theory of EM radiation did not agree with the experimental results for blackbody radiation.
Source: Darth Kule, “Black-body.svg”, http://en.wikipedia.org/wiki/File:Black_body.svg. Retrieved May 14, 2010. Graphic is public domain.
Quantization of Energy
In 1900, Max Planck developed a new theory that assumed energy isn’t emitted in any amount, but only in packets, called “quanta”.
This “quantum” theory agreed with the blackbody experiments.
Experimental data points based on a graph from Wilson,Buffa and Lou, College Physics, Pearson Education Inc., 2010
Planck’s Equation
Planck’s equation relates the energy of a quantum of light with the frequency of light.
Energy = Planck’s constant x frequency
In atoms, energy is measured in electron volts (eV) 1 eV = 1.60 x 10-19 J h=4.14 x 10-15 eV-s
hfE
Photon Practice 1
A certain radio station broadcasts a radio wave of 100 MHz and 30,000 W power.
How many photons does the station emit per second?
Answer: 4.5 x 1029 photons/second
Photon Practice 2 Two monochromatic light beams, one red and
one green, have the same intensity and cover the same area. How does the energy of each photon and the number of photons in each beam compare?
Photon Energy # per Second A) Same Same B) Greater for red Less for red C) Greater for redGreater for red D) Less for red Less for red E) Less for red Greater for red
THE PHOTOELECTRIC EFFECT
The Photoelectric Effect When light strikes a
metal surface, it will give off electrons.
The Photoelectric Effect If light is a wave,
then a light beam that has greater intensity (greater energy) should cause the electrons given off to have more energy.
A more intense light beam causes more electrons to be emitted, but they all have the same energy.
The Photoelectric Effect Einstein explained
the photoelectric effect by assuming that light was made of particles.
A more intense light beam has more photons, but each carries the same energy.
Thus, more intense light would produce more electrons, with the same energy.
The Photoelectric Effect
Quantum Theory of Light and the Photoelectric Effect When a photon strikes a metal plate, it
transfers energy to the electrons in the plate.
If an electron acquires enough energy, it can be ejected from the plate. The energy necessary to eject an electron is
called the “work function” of the metal. The kinetic energy of the electrons will be
the energy of the photon minus the work function. ionWork Funct hfKE
Photoelectric Effect: Practice
A certain beam of light has photons with 5 eV of energy. When this light strikes a metal plate, electrons with 3 eV of kinetic energy are released. What is the work function of the metal?
Answer: 2 eV
The Cutoff Frequency
The Cutoff Frequency
The cut-off frequency is the frequency that produces photons of just enough energy to be emitted, i.e. equal to the work function.
Photons with less energy will not be able to eject electrons.
Work Function = h fCutoff
Wave Particle Duality In some
experiments, such as blackbody radiation and the photoelectric effect, light acts like a particle (called a photon).
In others, like double slit interference and thin film interference, light acts like a wave.
Light never appears as both in the same experiment.
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THE QUANTUM THEORY OF LIGHT AND THE ATOM
Review- Electrical Potential Energy An atom’s electrons
are attracted to its nucleus.
It requires energy to separate them.
The farther away an electron is from the nucleus, the greater the energy.
Energy Levels
The electrons in an atom occupy discrete energy levels. These levels
are determined by the type of atom involved.
An electron gains or loses energy when transitioning between levels.
Energy Levels
We use an energy level diagram to represent the levels.
Absorption When an atom absorbs a photon of light, an
electron “jumps” from a lower energy level to a higher energy level. Only photons with the right amount of
energy will cause an electron to jump to a higher level.
Absorption When an atom absorbs a photon of light, an
electron “jumps” from a lower energy level to a higher energy level. Only photons with the right amount of
energy will cause an electron to jump to a higher level.
Emission Electrons at higher levels can jump back
to a lower energy level by emitting a photon. The frequency of the emitted photon is
determined by the energy that the electron gave up.
Energy Levels Electrons at
higher levels can jump back to a lower energy level by emitting a photon. The frequency
of the emitted photon is determined by the energy that the electron gave up.
Emission Spectra Since the energy levels are different for
each type of atom, the wavelengths of light given off by the excited electrons jumping back down are different for each type of atom.
We can thus identify the atoms by the colors of light they emit.
The pattern of colors is called an emission spectrum.
Emission Spectra
H
Fe
Source: Adrignola, “Emission spectrum-H.svg”Yttrium91, “Emission spectrum-Fe.svg”Wikipedia Commons, public domain
Absorption Spectra
When light containing all wavelengths passes through a gas, the same wavelengths of light that appear in that gas’ emission spectrum will be absorbed by the gas.
Source: Maureen Gebruiker, “Fraunhofer lines.svg” Wikipedia Commons, public domain
CONSEQUENCES OF QUANTUM MECHANICS
Double Slit Experiment for Electrons
We can carry out a double slit experiment for electrons just like Thomas Young did for light.
But electrons are particles, so they won’t produce an interference pattern, right?
Double Slit Experiment for Electrons
Source: Belsazar, Double-slit experiment results Tanamura 2.jpg, Wikipedia Commons, Creative Commons Attribution-Share Alike 3.0 Unported license.
Double Slit Experiment for Electrons
Electrons interfere with each other, just like waves.
If we cover up one slit, the wave-like behavior goes away.
The wave-particle duality of matter Particles of matter, such as
electrons, atoms, etc., can also behave like a wave in some experiments.
These matter waves are called de Broglie waves. The wavelength of these waves is
given by the equation l = h/p . The frequency is given by the
equation f=E/h
NUCLEAR PHYSICS
The Nucleus
An atomic nucleus consists of protons and neutron.
When we want to specify a specific isotope, we write the mass number and atomic number.
He42
neutrons ofnumber
protons ofnumber number atomic
nucleons ofnumber number mass
N
Z
A
NZA
Units of Mass and Energy in Nuclear Physics In nuclear
physics, mass is usually stated in terms of atomic mass units, or u. 1 u = 1.660 X 10-
27 kg Energy is usually
stated in electron-volts, or eV.
Particle
m (kg) m (u)
Proton 1.673 x 10-
27
1.007276
Neutron
1.675 x 10-
27
1.008665
Electron
9.109 x 10-
31
.000549
1 eV= 1.602 X 10-19 J
Mass-Energy Equivalence
Energy and mass are equivalent.
Einstein’s equation shows how much energy a quantity of mass corresponds to.
2
2
light) of (speed mass Energy
mcE
Mass-Energy Equivalence
Energy and mass are equivalent.
Einstein’s equation shows how much energy a quantity of mass corresponds to.
Particle m (kg) E (MeV)
Proton 1.673 x 10-27 938.3
Neutron 1.675 x 10-27 939.6
Electron 9.109 x 10-31 0.5110
1 u = 931.5 MeV
The Strong Force
The electric force would cause the protons in a nucleus to repel each other.
The strong force is an attractive that overcomes the electric repulsion over small distances.
Both protons and neutrons attract by the strong force.
Neutrons help stabilize the nucleus.
Elements with Z>83 do not have stable nuclei.
Fundamental Forces
Force Strength* Range Field Particle
Strong Nuclear 1 ~ 1 fm = 10-15 m
gluon
Electromagnetic
10-2 Infinite (1/r2) photon
Weak Nuclear 10-13 < 10-3 fm W+,W-, and Z
Gravity 10-38 Infinite (1/r2) graviton
* Since the forces involve different quantities and vary with distance, their strength is not a simple comparison. The answer on the homework reverses the weak nuclear and electromagnetic force relative strength.
Binding Energy The total energy
or mass of a stable nucleus is less than the mass of the individual nucleons.
The difference is called the binding energy, Ebind
=+ Binding Energy
Mass Defect Since mass and
energy are equivalent, the difference can also be expressed in terms of mass.
The difference in mass is called the mass defect, Δm
=+ Binding Energy
Nuclear Physics Questions 1 & 2 Determine
the mass defect and binding energy of deuterium.
Determine the mass defect and binding energy of helium.
Particle m (u) E (MeV)
Proton 1.007825 938.3
Neutron 1.008665 939.6
D 2.014102
He 4.002602
1 u = 931.5 MeV
Binding Energy and Nuclear Reactions Nuclear reactions
involve the nuclei of atoms.
If a rearrangement of protons and neutrons in a nucleus or nuclei results in a greater binding energy, the reaction will release energy.
Energy
Nuclear Physics Question 3 Determine
the energy “released” when 2 deuterium combine to form a helium nucleus.
Particle m (u) E (MeV)
Proton 1.007825 938.3
Neutron 1.008665 939.6
D 2.014102
He 4.002602
1 u = 931.5 MeV
Nuclear stability
The attraction of the strong force results in lower energy/nucleon for light nuclei.
Because the range of the strong force is limited, beyond a certain size, binding energy/nucleon increases. Source: Fastfission, Wikipedia,
public domain
Fission In nuclear fission, heavy nuclei split
into lighter nuclei.
Fission This may result in a chain reaction.
A nuclear reactor uses a controlled chain reaction.
The first “atomic” bombs were fission bombs.
Fusion Light nuclei can
undergo fusion. Fusion is the
nuclear reaction that powers the sun.
Researchers are trying to develop a fusion reactor that releases more energy than it uses.
Source: Wykis, “Deuterium-tritium fusion.svg” Wikipedia Commons, public domain
Nuclear Physics Question 3
Correct statements about the binding energy of a nucleus include which of the following:
I) It is the energy needed to separate the nucleus into its individual nucleons.
II) It is the energy liberated when the nucleus is formed from the original nucleons.
III) It is the energy equivalent of the apparent loss of mass of its nucleon constituents.
A) I only. B) III only. C) I and II only. D ) II and III only. E) I, II, and III
NUCLEAR RADIATION
What is your radiation dose?
Calculate Your Radiation Dose | Radiation Protection | US EPA
Nuclear Decay An unstable
nucleus can break apart into other particles, releasing energy.
This process occurs naturally in hundreds of types of nuclei.
+ + Energy
Nuclear Radiation Three types of radiation are emitted in
radioactive decay.
Particle Symbol Composition Charge Effect on Parent
Alpha 2 protons, 2 neutrons (He nucleus)
+2 Less mass, new element
Beta electron
positron
-1
+1
~same mass, new element
Gamma photon 0 Energy loss
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cs,
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25
-3,
p.
90
3
Uses of nuclear radiation Carbon dating Chemical tracing Radiation therapy Food irradiation Sterilization of
medical equipment
Non-destructive testing
Smoke detectors Security scanning Nuclear power
n10
Carbon Dating
Some atmospheric carbon is C-14, produced by cosmic rays.
N147C14
6H11
HCNn 11
146
147
10
Carbon Dating
This carbon-14 naturally decays to nitrogen with beta decay.
eNC 01
147
146
C146-N
147
Radioactive Decay Question 1
H)(deuteron a (E)
He).( nucleus helium a (D)
electron.an (C)
neutron. a (B)
proton. a (A)
plusRn into decays Ra
21
42
22286
22688