Atomic and Nuclear Physics Chapters 38-40
Mar 26, 2015
Atomic and Nuclear Physics
Chapters 38-40
Wave-Particle Duality of Light
Young’s Double Slit Experiment (diffraction) proves that light has wave properties So does Interference and Doppler Effect
Photoelectric Effect proves that light has properties of particles
Max Planck
From Planck’s work on Blackbody Radiation, he proposed that the energy of light is quantized
Quantization is an idea that energy comes in bundles or discrete amounts Energy is quantized
This idea disagreed with established (traditional) physics
Photoelectric Effect
Light shining on a photo-sensitive metal plate will emit electrons.
Photoelectric Effect
Frequency must be above a minimum (threshold) frequency
Brighter light (higher intensity) produces more electrons, but with the same energy
Light with higher frequency will emit electrons with higher energy
Photoelectric Effect
Photoelectric Effect
Law of Conservation of Energy must be followed Energy must be related to frequency
Law of Conservation of Momentum must also be followed Light has momentum
Photoelectric Effect
Einstein used Planck’s work to explain Photoelectric Effect (Nobel Prize 1921)
Proposed that discrete bundles of light energy are photons
Energy is proportional to Frequency E=hf
h, Planck’s Constant 6.63 x 10-34 J*s
Photoelectric Effect
Conservation of Energy
Energy of Photon = Energy of ejected electron + work needed to eject electron (work function, Φ)
hf KEMAX
Photoelectric Effect
Photoelectric Effect
Maximum Kinetic Energy is measured by how much voltage (stopping voltage) is needed to stop electron flow
KEMAX = qV
1electron stopped by 1 Volt = 1.6 x 10-19J 1electron stopped by 1 Volt = 1eV
Compton Effect
1923 Arthur Compton uses photon model to explain scattering of X-rays
Determines equation for momentum of a photon
Compton Effect
X-ray photon strikes an electron at rest
After the collision both the electron and X-ray photon recoil (move) in accordance with Laws of Conservation of Momentum and Energy
The photon transfers some momentum to the electron during collision.
Compton Effect
Change in wavelength of photon must be related to momentum
Magnitude of Photon Momentum:
h
c
hfp
de Broglie Wavelength
1923, graduate student, Louis de Broglie suggested that if light waves could exhibit properties of particles, particles of matter should exhibit properties of waves
Used same equation as momentum of photon
p
h
Davisson-Germer Experiment
Verified de Broglie’s idea of matter waves
Directed beam of electrons at crystal of nickel
Electrons showed diffraction pattern
Proof that particles have wave properties
Schrödinger’s Cat
Thought Experiment about basis of quantum mechanics
Place cat, vial of poison, Geiger counter with radioactive sample in a seal box.
After 1 hour the cat is either alive or dead Can’t know without interrupting the
experiment (opening the box) The cat is considered BOTH alive and dead
Atomic Models
Dalton’s Model, early 1800’s Hard uniform sphere
Plum Pudding Model, 1904 After discovery of electron by J.J.
Thomson
Rutherford Model, 1909 After Geiger Marsden Experiment
Atomic Models
Bohr Model, 1913 Dense positive nucleus Electrons moving in certain energy levels (orbits)
Quantum Mechanical Model
More detailed view of the Bohr Model
Schrödinger Wave Equation and Heisenberg Uncertainty provides region of high probability where electron COULD be. Orbital
Modern Model
Energy Level Transitions
Electron energy is quantized
Electrons can move between energy levels with gains(absorption) or losses(emission) of specific amounts of energy.
Energy Level Transitions
Line Spectra
Emission Spectra Shows only the light that is emitted from an
electron transition
Absorption Spectra Shows a continuous color with certain
wavelengths of light missing (absorbed)
Energy Level Transitions
Energy Level Transitions
Examples: Calculate energy
needed for transition from n=1 to n=6 13.22eV
Calculate energy released by transition from n=5 to n=2 2.86eV
What wavelength of light is this? 434 nm
Nuclear Physics
Nucleus – center of atom Contains nucleons, protons and neutrons
Proton, p Positively charged particle, 1e m=1.6726 x 10-27 kg
Neutron, n Neutral particle m=1.6749 x 10-27 kg
Atomic Mass Unit
Based on Carbon-12 atom 1u = 1.6605 x 10-27 kg
Proton mass = 1.00728 u
Neutron mass = 1.00867 u
Nuclear Reactions
Fission and Fusion
Energy produced comes from mass being converted into energy (Mass Defect, Δm)
Mass-Energy Conversion
E=mc2
1 u = 1.4924 x 10-10 J
1 u = 9.31 x 108 eV = 931 MeV
Fundamental Forces
Strong Force Force that holds nucleons (protons and neutrons)
together Short range
Weak Force Associated with radioactive decay Short Range
Fundamental Forces
Gravitational Force Attractive only Long distance range (think planets)
Electromagnetic Force Attractive and repulsive force on charged particles Long range (think stars)
Classification of Matter
Matter is broken down into 2 types Hadrons and Leptons
The Quark Family, also called Hadrons, are broken down into 2 types Baryons and Mesons
Quarks
Six quarks Up, Down, Top, Bottom, Strange, and Charm
Up, Charm, and Top all have +2/3 charge Down, Strange, and Bottom all have -1/3 charge
Baryons
Baryons are comprised (made of) three quarks
The total charge for any baryon is the net charge of the three quarks together
Examples: uud = +2/3, +2/3, -1/3 = +1 = proton udd = +2/3, -1/3, -1/3 = 0 = neutron
Mesons
Mesons are comprised of a quark and its antiquark
Antimatter Particles that have the same mass but opposite
charge of their matter partner Have same symbol as matter but with added bar
above symbol Up quark, u up antiquark, ū
Leptons
Leptons are separated into six flavours Electron, Muon, and Tau all have -1 charge Electron neutrino, muon neutrino, and tau
neutrino all have 0 charge
Annihilation
When matter and antimatter particles collide, they annihilate each other and produce energy
E=mc2
kg J (use equation) u eV (use conversion on Reference Tables)