15- Modern Physics

Slideshow: Unit 15 Modern Physics Slideshow (updating for 2024-25 revision), Nuclear & Modern Physics (pre-2024 version)

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

Must watch: Quantum Physics video: https://youtu.be/Usu9xZfabPM 

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 a complete fundamental charge

  • 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 than neutral atoms

    • 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)

Helpful/interesting videos

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.