aphy201 4/29/2015 1 27.1 the electron cathode rays are light waves or particles?

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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