RADIATION AND THE STRUCTURE OF THE ATOM

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RADIATIONAND THE STRUCTURE OF THE ATOM
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NOTE!!!

• Equipment for the prac exam is setup in the first
  year lab.

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OUTLINE

• Experimental observation
• Emission and Absorbtion of light
• Photoelectric effect
• Atomic line spectrum and quantized energy levels
• Rutherfords experiment
• Models
• Rutherfords model
• Bohrs models
• Application -Laser

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Emission and Absorption of Light
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EMISSION SPECTRA OF ELEMENTS
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Light Quanta (Photons)

• An Electromagnetic wave can be viewed as a
  particle (photon) which has an amount of energy
  which is proportional to its frequency.

where h 6.62 x 10-34 Js
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PHOTOELECTRIC EFFECT

• The Photoelectric effect is the emission of
  electrons when light strikes a surface.

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Photon electron interaction in metal

• Photon absorbed by electrons in metal.

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• Applying the right voltage across the two
  parallel plates results in a de-acceleration of
  the emitted electrons.

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• The Voltage needed to just stop the electrons
  from reaching the other end is called the
  stopping voltage.

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The photoelectric effect
Yields
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• Plot V0 vs f should yield a straight line which
  passes through f.

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Momentum of photon

• From special relativity

• Since photons are massless

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• Furthermore

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Solar Sailing
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The Discovery of the Atomic Nucleus

• Before 1910 the following was known about the
  atom.
• Size of atoms (10-10m) ltlt wavelength of visible
  light.
• Electron charge / electron mass was known.
• Almost all the mass of the atom had to be carried
  by the positive charges.

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• The distribution of the and charges inside
  the atom was not known.
• Thomsons model of the atom.

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Rutherfords Experiment
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Rutherfords Model of the Atom
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• Most of the atom is empty space.
• Most of the atoms mass and charge is located
  at the center of the atom.

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Next weeks program

• Bohrs model
• How a laser works
• X-ray production
• Wave-particle duality
• Quantum Physics

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Exercise

• A radiostation broadcasts at 89.3 MHz with a
  radiated power of 43.0 kW.
• What is the magnitude of the momentum of each
  photon?
• How many photons does the radiostation emit each
  second?

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Exercise

• For a certain cathode material in a
  photoelectric-effect experiment you measure a
  stopping potential of 1.0V for light for
  wavelength 600nm, 2.0 V for 400 nm, and 300nm for
  300nm. Determine the work function for this
  material and the value of Plancks constant.

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Things to consider

• Unique spectral lines for each element.
• Each spectral line has a particular frequency gt
  particular photon energy
• Heavy positively charge nucleus in the center of
  the atom arounded by electrons.

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• Attraction between negative electrons and
  positived nucleus.
• Rutherfords proposal

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Bohrs model

• Electrons move around the nucleus at stable
  orbits without emitting radiation.
• Electron in one of these stable orbit has a
  definite energy.
• Energy is radiated only when electrons make
  transitions from high energy orbit to a low
  energy orbit.

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• Energy is emitted as photons with energy

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Quantifying the energy spectrum

• Bohr postulate that the angular momentum of an
  electron revolving around a nucleus is quantized
  in units of h/2p.

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• Newtons 2nd law yields

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• The smallest radius is obtained by setting n 1,
  is called the bohr radius.

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• Kinetic energy of moving electrons

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• Potential energy of electron bound to nucleus

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Total energy of electron n-th orbital
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Energy level diagram

• The posible energies which electrons in the atom
  can have is depicted in an energy level diagram.