Chapter 27- Atomic/Quantum Physics

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Chapter 27- Atomic/Quantum Physics
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The Sun

• http//soho.nascom.nasa.gov/
• Why do we see the sun as yellow instead of green
  or blue or pink?

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Blackbody Spectrum
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Photon Theory of Light

• Light is transmitted as tiny particles called
  photons
• The amount of energy in a photon depends on its
  frequency

h Plancks Constant 6.626 x 10-34 Js f
frequency of light c speed of light ?
wavelength p momentum
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The Photoelectric Effect

• When light shines on a metal surface, electrons
  are emitted from the surface

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Photocells (p.829)

• When the photocell is in the dark, the ammeter
  reads 0 (no current)
• When light with a high enough frequency shines on
  the current flows in the circuit

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Photocells

• KEmax of the emitted electrons can be found by
  reversing the voltage and making the C electrode
  negative
• The electrons are repelled by C, but the fastest
  electrons will still make it across
• There is a minimum voltage, Vo, called the
  stopping voltage. No current will flow if the
  voltage is less than the stopping voltage
• Kemax e Vo

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Photoelectric Effect

• Einsteins Theory Predicts
• Increasing the intensity of the light does not
  make the electrons go faster.
• This is because although more photons are
  striking the surface, they have same energy
• Increasing the frequency of the light beam
  increases the energy of the photons which changes
  the maximum KE of the ejected electrons

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Photoelectric Effect

• The work function, Wo, is the minimum amount of
  energy necessary to get an electron off the
  surface of the metal
• fo is the cutoff frequency. If the light beams
  frequency is below that, then no electrons will
  be emitted

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Compton Effect

• A.H. Compton scattered xrays from various
  materials
• Found out that the scattered light had a lower
  frequency than incident light
• Since frequency decreases, wavelength increases
• Used conservation of momentum to determine that
  the photon transfers some of its energy to the
  electron

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de Broglie Wavelength

• Light sometimes behaves like a wave and sometimes
  like a particle
• Louis de Broglie came up with the idea that
  particles might also have wave properties

De Broglie wavelength of a particle
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de Broglie Wavelength

• The wavelength of large objects is very small
• For a 0.20 kg ball travelling at 15 m/s

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de Broglie Wavelength

• Determine the wavelength of an electron that has
  been accelerated through a potential difference
  of 100 V

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Davisson-Germer Experiment

• The spacing of atoms in a crystals is on the
  order of 10-10 m, so one could be used as a
  diffraction grating
• In 1927, Davisson and Germer scattered electrons
  from the surface of a metal crystal. The
  wavelength they got matched the predicted de
  Broglie wavelength

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Bohr Model of the Atom

• Electrons orbit the nucleus in circular orbits
  called stationary states
• When an electron jumps from one state to another,
  light is either absorbed or emitted
• The energy required to go between states is a
  fixed amount

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Bohr Model of the atom

• If an electron jumps from a higher state to a
  lower state, it emits a single photon of light

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Energy Level Diagram (p. 847)

• n 1is ground state, n2,3,4.. Are excited states
• To completely free an electron in the ground
  state, youd need to put in 13.6 eV of energy
  (ionization energy of Hydrogen)

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Energy Level Diagram

• How much energy to go from ground to n2?
• How much energy to go from n2 to n4?

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Energy Level Diagrams

• What are the possible transitions for an electron
  in excited state n3?
• 3?1
• 3?2
• 2?1

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Emission Spectra

• A materials emission spectrum show the
  wavelengths of the photons emitted when electrons
  jump to lower energy states

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Absorption Spectra

• The absorption spectrum of a material shows that
  gases can absorb light at the same frequencies at
  which they emit

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Absorption/Emission Spectra