Building Blocks of the Atom

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Building Blocks of the Atom
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Faraday's Law of Electrolysis
mass of liberated substance
number of coulombs in a mole
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Thomson's "cathode ray" tube
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V, L, and d are all measurable characteristics of
the apparatus
If you can measure vx, then you can determine e/m.
A magnetic field can be used to just cancel the
deflection and determine vx.
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A modern application of charge/mass spectroscopy
The top quark.
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Millikan's Oil Drop experiment
Determining the electron charge e separately.

• Spray small droplets of oil which quickly reach
  terminal velocity due to air resistance.
• Small number of droplets fall between two plates
  into to a region of constant electric field.
  Velocity of fall can be estimated by measuring
  the time to fall a distance d.
• Ionizing radiation then charges the droplet,
  introducing an electric force.
• Charge is quantized. By measuring the velocity
  of a number of particles with the field on and
  off and assuming that the electric charges must
  be multiples of each other, e can be determined.

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v terminal velocity
a radius of drop n viscosity of oil
Find a by observing drop in free-fall
p density of oil o viscosity of air
Turn on field Reverse field direction
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Data from Millikan's experiments
Rise times
80.708s 22.335s 22.390s 22.368s 140.566s
79.600s 34.748s 34.762s 29.286s 29.236s
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The discovery of the atomic nucleus Rutherford
Back Scattering
what Rutherford expected from the plum pudding
model
the clever experiment
the (surprise!) result
It was almost as incredible as if you had fired
a 15-inch shell at a piece of tissue paper and it
came back and hit you."
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Real World example materials characterization

• When aiming a beam at a thin sample two processes
  occur
• Particles loose energy due to the materials
  stopping power
• There are head on collisions at high angles due
  to Rutherford backscattering
• scattering depends on mass/size and charge of
  nucleus

Particles loose energy in glancing collisions and
interactions with electrons-particles scattered
from deep in material are scattered with less
energy.
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TaSi film on Si substrate
Si
TaSi

• The height of the peak indicates the density of
  atoms and the ion energy identifies the species
  and the depth from the surface.
• High energy edges come from scattering at the
  materials surface
• Ta more likely to scatter
• Width of Ta peak or width of Si step gives
  thickness of substrate
• Measuring peak height and normalizing for the
  scattering spectra for that element give
  elemental concentrations

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Evolution of our views of the atom

• positive charge is concentrated at the center of
  the atom in an area 1/1000th the size of the
  atom
• the mass of the electron is very small compared
  to the mass of the atom (one thousand times less
  than the hydrogen atom