physics 1202: lecture 31 today’s agenda announcements: extra creditsextra credits –final-like...
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Physics 1202: Lecture 31Today’s Agenda
• Announcements:
• Extra creditsExtra credits–Final-like problemsFinal-like problems
–Team in class Team in class
• HW 9 this FridayHW 9 this Friday
• Modern physics
Modern Physics
Quantization• Physical quantities come in small but finite quantities
– Quantum (or quanta for many of them)
– Not continuous
• Atomic Spectra:a) Emission line spectra for hydrogen, mercury,
and neon;b) Absorption spectrum for hydrogen.
Blackbody and temperature• Peak gives main
color
Black Body Radiation
Intensity of blackbody radiation
Planck’s expression
h = 6.626 10-34 J · s : Planck’s constant
Assumptions: 1. Molecules can have only discrete values of energy En;
2. The molecules emit or absorb energy by discrete packets - photons
Max Planck (1899):
Quantum energy levels
Energy
E
0
1
3
4
5
2
n
hf
2hf
3hf
4hf
0
5hf
Photoelectric effect
• In 1887, Heinrich Hertz– shining ultra-violet light on metal in
vacuum
– If V not large enough, no current
Photoelectric effect
Kinetic energy of liberated electrons is
where is the work function of the metal
Photoelectric effect
• Explained by Einstein in 1905– Based on quantum of light (Planck)
– Nobel Prize in 1914
Photon properties • Recall (for electromagnetic wave) E = pc
• Quantization (Planck): E = hf = hc /
• So= h / p
• Recall from relativity
• Conclusion: m0 = 0 (photons have no mass ! )
Compton effect• In 1920’s, Arthur Compton experiments with X-rays
– Wavelength longer after scattering
– Using quantization he derived
C : Compton wavelength
The waves properties of particles
In 1924, Louis de Broglie postulate: because photons have both wave and particle characteristics, perhaps all forms of matter have both properties
Momentum of the photon
De Broglie wavelength of a particle
Example: An accelerated charged particle
An electron accelerates through the potential difference 50 V. Calculate itsde Broglie wavelength.
Solution:
Energy conservation
Momentum of electron
Wavelength
Birth of quantum mechanics• Erwin Schrödinger
– Wave function & Hamiltonian
• Werner Heisenberg– Uncertainty principle
5 steps methods• Draw and list quantitites
• Concepts and equations needed
• Solve in term of symbols
• Solve with numbers
• Checks values and units