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Quantum Mechanics

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Theory to describe objects at very small scale, the scale of the atom. ... Photoelectric effects have many practical applications (read the book) ... – PowerPoint PPT presentation

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Title: Quantum Mechanics


1
Quantum Mechanics
  • Chapter 9

Great Idea At the subatomic scale, everything is
quantized. Any measurement at that scale
significantly alters the object being measured
2
The World of the Very Small
  • Quantum mechanics
  • Theory to describe objects at very small scale,
    the scale of the atom.
  • Quantum Latin word for bundle
  • Bundles vs. continuous
  • In very small scale, objects come in small
    bundles but not continuous form
  • In the world of sub-microscopically small,
    everything comes in quantized bundles
  • - Does not match our intuition with large
    (macroscopic) scale objects

3
Measurement Observation in the Quantum World
  • Measurement of objects involves interaction with
    objects. In the quantum world, it alters the
    status of the objects.
  • Components of Measurement
  • Sample
  • Source of energy
  • Detector
  • Example A radar antenna sends out microwaves
    that interact with flying airplanes, are
    reflected.
  • Interactions with objects in measurement can be
    ignored in macroscopic measurement but cannot in
    quantum world.

4
The Heisenberg Uncertainty Principle
  • In 1927, Werner Heisenberg (1901-1976) put the
    idea into precise mathematical form.
  • Uncertainty Principle
  • At the quantum scale, any measurement
    significantly alters the object being measured
  • Cannot know both position and velocity of an
    object with infinite accuracy at the same time.
    Every measurement involves trade-offs.
  • Equation
  • h plank constant,
    Joule-seconds

5
Uncertainty in the Newtonian World
  • (1) Small uncertainty with large object
  • A moving automobile with a mass of 1000 kgs is
    located in an intersection that is 5 meters
    across. How precisely can you know how fast the
    car is traveling?
  • (2) Large uncertainty with small object
  • An electron with a mass of is in an
    atom within the area about on a side. To what
    accuracy can we measure the velocity of the
    electron?

6
Uncertainty in the Newtonian World(continued)
  • Problem 1 (p 195)
  • A bass ball (mass 0.145 kg) is thrown towards
    home-plate at a speed between 45.0 and 45.1 per
    second. How accurate can we determine its
    position?
  • Problem 3
  • A gold at0m (mass kg travels at a speed
    between 20.0 and 20.1 meters per second. How
    accurately can we determine its position? Is this
    accuracy in position attainable?

7
Probabilities
  • Newtonian View
  • Determine velocity and position exactly at any
    time
  • Quantum View
  • Cannot determine velocity and position exactly at
    the same time.
  • Probability is used.

8
Wave Particle Duality
  • Double-Slit Test to determine whether something
    is a particle or wave

9
Double-Slit cont.
  • When electrons or photons pass through a two-slit
    over time
  • Wave particle duality

10
Photoelectric Effects
  • When photons (light particles) of sufficient
    energy strikes some materials, electrons may come
    out from the materials. This is caller
    photoelectric effect.
  • Einstein pointed out that this is due to the
    particle nature of photons. He won Nobel Prize in
    1921 for this work.
  • Photoelectric effects have many practical
    applications (read the book).

11
Wave-Particle Duality Bohr Atom
  • Electrons wave-particle duality helps explain
    why only certain orbits are allowed in atoms
    (Bohr model).
  • Particles force, velocity, distance
  • Wave standing waves on a string or bend into a
    circle
  • The only orbits allowed in the atoms are those
    that satisfy both.

12
Quantum Weirdness
  • Richard Feynman I can safely say that nobody
    understands quantum mechanics. Do not keep
    saying to yourself, But how can it be like
    that? Nobody knows how it can be like that.
  • In the quantum world, quantum mechanics is used
    to predict the state of the atomic-scale objects
    change with time. The state is described by
    probability.
  • Einstein I cannot believe that God plays dice
    with the universe.
  • Neil Bohr Albert, stop telling God what to do.

13
Quantum Entanglement
  • Quantum Entanglement
  • Dice
  • Photons emitted at same time
  • Despite distance, remain entangled
  • Quantum teleportation
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