Rutherford Model of the Atom

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Rutherford Model of the Atom

• Why Not?

• To explain this we need to examine light.

Artists Impression
2
Black Body Radiation (YF 40-9)

• Treating light classically predicts Intensity of
  emitted radiation given by
• E sAT4
• Solved by Plank
• DE nhf
• Energy only transferred in quantized amounts.

Classical theory
Observed distribution
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Photoelectric Effect (YF 40-3)
Light Frequency f

• Light of frequency, f, is shined on a metal
  surface.
• Currant as function of Voltage

4
Photoelectric Effect II

• Need to explain
• Threshold frequency.
• Threshold depends on metal.
• Instant emission of electrons if threshold
  frequency obtained.
• Current proportional to intensity.
• Maximum kinetic energy of emitted electrons
  depends on frequency.

Maximum Kinetic Energy of electrons
f
-W
5
Sample Photoelectric Effect Problem

• In the photoelectric effect what is the relation
  between the threshold frequency, f0, and the work
  function ??
• The threshold of wavelength of for producing
  photoelectrons from a metal is 172 nm. What is
  the work function of the surface in eV?

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Photons

• EinsteinLight is not a wave, but a particle
  with energy hf
• PlankEnergy can only be transferred between
  matter and light in quanta.

• Photoelectric Effect
• Kmax hf F
• The photon has to give all of its energy to the
  electron, not part of it.

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Side Trip Relativity (YF 39)

• E2 (mc2)2 (pc)2
• This equation gives the total energy of a
  particle in motion.Note, is p 0, then the
  energy is due to the mass of the object.

• Note if m 0, i.e. the object is massless, then
  the object has momentum given by
• P E/c hf/c
• The photon is massless.

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A word about Units.

• In atomic we tend to use units that are
  appropriate for the distance and energy scales of
  the atom
• Develop additional units later.

• Electron Volts1eV 1.602 10-19J
• Angstrom1Å 10-10m
• Planks Constanth 6.63 10-34 Js 4.14 10-15
  eV s

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Compton Scattering and the Photon

• If light is a particle, then it must act like
  one.
• Scatter photons off electrons

• It can be shown (by ugly algebra, and a few
  assumptions)
• See YF 40-8 for a discussion

?
electron
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Summary Light

• Waveproved by 200 years of scientific work
• Interference
• Diffraction
• Refraction
• Maxwells equations
• Radio etc

• ParticleNew necessary feature at turn of
  century
• Photoelectric Effect
• Planks theory
• Compton Effect

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Hydrogen Line Spectra (YF 40-4)

• Balmer Series of Hydrogen
• Each of the spectral lines represents light of a
  particular wavelength.

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Additional Hydrogen Line Spectra