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27.1 The Electron Cathode rays are light waves or
particles?
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27.1 The Electron
1897 – J. J. Thomson measures e/m directly using electric and magnetic fields.
Newton’s 2nd Law: F = ma Centripetal Motion: a = v2/r Magnetic Force: FB = evB Electric Force: FE = eE
Millikan oil-drop experiment
rBE
me
2
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27.2 Planck’s Quantum Hypothesis Light from hot dense objects -
Blackbody radiation
λpT = 2.9 x 10-3 m K
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27.2 Planck’s Quantum Hypothesis
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27.2 Planck’s Quantum Hypothesis
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27.2 Planck’s Quantum Hypothesis Max Planck (1900) theorized that atoms in
molecules have oscillations with discrete or quantized energies given by En = nhf where h = 6.6 x 10-34 Js
A quantized world is grainy and coarse which is not noticeable in our everyday experience
Example: a mass oscillating on a spring
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27.2 Planck’s Quantum Hypothesis
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27.3 Photon Theory of Light
Einstein suggested that light is quantized or made of photons
A photon is emitted with an energy E = hf when a molecule’s energy state changes from a higher level to a lower level.
Photoelectric Effect
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27.3 Photon Theory of Light Classical wave theory of light cannot explain
why …
Higher intensity light should eject electrons of higher KE. A single photon is absorbed to eject the electron
There is no relationship between KE and frequency.KE increases linearly with increasing
frequency The photoelectric effect should be observed at any
frequency.No electrons are ejected below fo for the
material The electrons require time to absorb enough energy
to be ejected from the metal.The electrons are ejected in less than 10-9
s
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27.5 The Compton Effect 1923: X-rays scattered off electrons in
graphite
)cos1( cm
h
o
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27.6 Photon Interactions
Photoelectric effect, excitation of atoms, Compton scattering, pair production/annihilation
Minimum energy is 1.02 MeV
Gamma ray photons