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)