7- Modern Physics

Textbook: Lightweight coverage of chapters 25, 26, 27, 28 & 29

Objectives

  • Structure of particles
    • Fundamental Particles have no internal structure
      • Electrons, neutrinos, photons and quarks are fundamental (not made of other things)
      • Neutrons and Protons are both made of 3 quarks.
        • Quarks are fundamental and have a partial charge, but combine to be either neutral or
    • supporting ideas not directly tested
      • Know how protons and neutrons and electron numbers can be found from atomic number and mass number.
      • know about radioactive decay emissions from nucleus
      • know about how half half life works.
    • Atomic structure
      • ions have different properties
      • Energy level structure of atoms
      • discrete energy states of atoms (bohr model) lead to spectra
        • emission spectrum can be used to determine the elements in a source of light.
      • Nucleon interactions are dominated by the nuclear strong force, which only works at extremely small scales.
  • gravitational mass and inertial mass
    • differentiate inertial mass and gravitational mass, but explain that we have found the values to be the same.
    • E = mc2 indicates that we should have a mass-energy conservation law as opposed to just our mass conservation law. (from special relativity)
      • mass and energy are two forms of the same entity/property
      • mass and energy can be converted from one form to the other.
      • Significant energy can be produced by nuclear processes.
  • Wave particle duality
    • wavelike particles, like electrons, can exhibit diffraction
    • we only see the wave properties of matter at small scales
    • λ=h/f to find the de broglie wavelength of a particle.
    • light (classically a wave) shows particle behavior.
      • light has quantized energy and has momentum only explained by photon (particle) behavior, while still exhibiting wave behaviors
      • energy transfer occurs when photons are absorbed or emitted by atoms or nuclei.
  • Relativistic explanations work where classical physics breaks down.
    • Classical physics does not work to predict behaviors at very high rates of speed (like 75% c)
    • measurements of length and time depend on speed (observers disagree on length and time measurements based on FOR)

Equations

Common Misconceptions

Models of the Atom

  • There is only one correct model of the atom.
  • Electrons in an atom orbit nuclei like planets orbit the sun.
  • Electron clouds are pictures of orbits.
  • Electrons can be in any orbit they wish.
  • Hydrogen is a typical atom.
  • The wave function describes the trajectory of a electron.
  • Electrons are physically larger than protons.
  • Electrons and protons are the only fundamental particles.
  • Physicists currently have the "right" model of the atom.
  • Atoms can disappear (decay).

Wave Particle Duality

  • Light is one or the other--a particle or a wave--only.
  • Light can be a particle at one point in time and a wave at another point in time.
  • Particles can't have wave properties.
  • Waves can't have particle properties.
  • The position of a particle always can be exactly known.
  • A photon is a particle with a wave inside.
  • Photons of higher frequency are bigger than photons of lower frequency.
  • All photons have the same energy.
  • Intensity means that the amplitude of a photon is bigger.
  • The Uncertainty Principle results from the limits of measuring devices.
  • Laser beams are always visible by themselves.
  • Sometimes you feel like a wave, sometimes you don't.

Michelson Morely Experiment

  • A null result means experiment was a failure.
  • The aether exists because something must transmit light.
  • Relativistic effects (length contraction) is the reason why no difference in the speed of light
  • was observed.

Special Relativity

  • Velocities for light are additive like for particles.
  • Postulates cannot be used to develop a theory.
  • Length, mass, and time changes are just apparent.
  • Time is absolute.
  • Length and time only change for one observer.
  • Time dilation refers to 2 clocks in 2 different frames.
  • Time dilation and length contractions have not been proven in experiments.
  • There exits a preferred frame of reference in the universe.
  • A mass moving at the speed of light becomes energy.
  • Mass is absolute, that is, it has the same value in all reference frames.

Curved Space and Black Holes

  • Space is not something.
  • Black holes are big.
  • Light always travels in straight lines.
  • Black holes exert a greater gravitational force on distant objects than the star from which it
  • was formed.
  • Observations made in a gravitational field are different that those made in a system undergoing
  • constant acceleration.
  • Things in space make sounds.
  • If the Sun were to become a black hole, the Earth would get sucked into it.

Wave Nature of Light

  • Velocities for light are additive like for particles.
  • Postulates cannot be used to develop a theory.
  • Length, mass, and time changes are just apparent.
  • Time is absolute.
  • Length and time only change for one observer.
  • Time dilation refers to 2 clocks in 2 different frames.
  • Time dilation and length contractions have not been proven in experiments.
  • There exits a preferred frame of reference in the universe.
  • A mass moving at the speed of light becomes energy.
  • Mass is absolute, that is, it has the same value in all reference frames.