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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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### Transcript

• 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!