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Quantum Theory
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Quantum Theory

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simple intro to quantum theory, includes Bohr, deBroglie, emission spectrum, photoelectric effect

simple intro to quantum theory, includes Bohr, deBroglie, emission spectrum, photoelectric effect

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  • 1. Quantum Theory and the nature of the atom Lisa Allen Stonington High School
  • 2. Waves: basic math
    • C= speed of light
    • distance / time = 3.0 x 10 8 m/s
    •  = wavelength or distance /wave
    • That wavelength can be expressed in any unit, but to work with c, it must be in meters!
    • V = frequency or Hertz, or waves/sec or just sec-1. In physics, they use f instead.
    • (This is another way to write 1/sec)
  • 3. Overview
    • So c=  v tells us that
    • Distance = distance x waves Second wave second
    • c =  x v
    • OR
    • Speed = distance/time
  • 4. Max Planck
    • Studied the radiation of hot objects
    • Continuous versus discrete energy
    • Analogy: digital versus analog?
    • If packets of energy are discrete, there must be some minimum size for one of these packets. That packet is called a quantum.
  • 5. Quantum Leap, and other dumb misuses of the term in pop culture...
    • If a quantum is the smallest possible size for a packet of energy, it must be small, right? So why is a “Quantum Leap” huge?
    • The term “Quantum physics” or “quantum mechanics” refers to the mathematical consequences of a quantized view of energy, and the specific math of the intensely tiny insides of atoms.
    • The regular laws of physics don’t work very well on this tiny scale. We will discuss why, but will not take on the killer math. (you’re welcome!)
  • 6. Richard Feynman, 1965 Nobel Laureate in Physics, for work in quantum electrodynamics
    • “ I think I can safely say that nobody understands quantum mechanics.”
  • 7. How do you find the energy of a quantum?
    • E = hv
    • Where E = energy of a quantum in joules
    • h = Planck’s constant or 6.626 x 10 -34 Js
    • v = frequency in Hertz or sec-1
    Quantum Leap by Cheryl Lavender
  • 8. Try a few of these problems
    • What is the wavelength of light with a frequency of 8 x 10 14 Hz?
    • What color is that light?
    • What is the frequency of red light?
    • What is the energy of a quantum of green light?
    • What color light has a quantum size of 4.97 x 10 -19 joules?
  • 9. The Photoelectric Effect
    • Blue light shines on metal, electrons come off. Even a little blue light will do this.
    • Red light shines on metal, nothing happens. Line up a dozen red lights, still nothing.
    • This undermines the wavelength theory. Why?
  • 10. Why is Einstein in this chapter?
    • Einstein was obsessed with light. He recognized that Planck’s discrete packets of energy could explain the photoelectric effect.
    • Einstein called the quanta of light PHOTONS
    • Before Einstein, only the wave nature of light was recognized.
  • 11. The Bubblegum machine analogy
    • If this machine only accepts dimes, then feeding a hundred nickels into it won’t get you a gumball.
    • You need the right amount of energy in one KICK to get an electron loose!
  • 12. Emission Spectra
  • 13. Demonstration
    • Materials: Spectroscopes, exciter, gas tubes
    • Compare sunlight through spectroscope with “naked eye” light
    • Compare fluorescent light through spectroscope with sunlight through spectroscope
    • Observe hydrogen gas tube with spectroscopes
    • How does sunlight differ from hydrogen’s emission spectrum?
  • 14. Why do we see lines and not a smooth rainbow?
  • 15. Niels Bohr: a little bit right!
  • 16. Bohr’s explanation of hydrogen’s spectrum
    • Electrons exist at certain distances from the nucleus.
    • Excited electrons absorb energy to jump further from the nucleus.
    • When electrons fall back closer to the nucleus, they give off energy equal to that which they absorbed going up.
    • E=hv, so the color tells us the energy!
  • 17. The famous LADDER ANALOGY
    • An electron can only exist at certain distances from the nucleus.
    • Intermediate distances are like being between rungs of a ladder. They just don’t support electrons.
  • 18. Why isn’t Bohr the final word on the subject?
    • Atoms other than hydrogen have more than one electron. Their spectra are more complicated.
    • Some atoms have electrons whose ground state isn’t the first possible energy level.
    • The chemical behavior of atoms didn’t substantiate this view completely
  • 19. Louis de Broglie
    • Considered Bohr’s “quantized orbits”
    • Concluded that electrons must have a wave nature.
    • Note standing wave patterns only exist when  is a whole number ratio with the medium
  • 20. Now where do we stand?
    • Light has a particle nature.
    • Electrons have a wave nature.
  • 21. But wait, there’s more!
    • Werner Heisenberg: the Heisenberg Uncertainty Principle
    • Erwin Schrödinger: Schrödinger’s wave equations
  • 22. The Dumb Heisenberg Joke...
    • A cop stopped Heisenberg for speeding. Heisenberg rolled down the window. The cop asked him, “Do you know how fast you were going?”
    • Heisenberg answered, “No, but I know where I am!”
  • 23. Schrödinger’s wave equations
    • We are not doing them! The math is too complex and not necessary.
    • We will only consider the solutions without looking at the equations. The solutions, or quantum numbers, form a simple pattern. The equation above is just for describing hydrogen’s emission spectrum! Imagine how ugly it gets for bigger atoms!