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Quantum Physics
Black Body Radiation
Intensity of blackbody radiationClassical Rayleigh-Jeans law forradiation emission
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
Quantum energy levels
Energy
E
0
1
3
4
5
2
n
hf
2hf
3hf
4hf
0
5hf
Photoelectric effect
Kinetic energy of liberated electrons is
where is the work function of the metal
Atomic Spectra
a) Emission line spectra for hydrogen, mercury, and neon;b) Absorption spectrum for hydrogen.
Bohr’s quantum model of atom
+e
e
r
F v
1. Electron moves in circular orbits.2. Only certain electron orbits are stable.3. Radiation is emitted by atom when electron jumps from a more energetic orbit to a low energy orbit.
4. The size of the allowed electron orbits is determined by quantization of electron angular momentum
Bohr’s quantum model of atom
+e
e
r
F v
Newton’s second law
Kinetic energy of the electron
Total energy of the electron
Radius of allowed orbits
Bohr’s radius (n=1)
Quantization of the energy levels
Bohr’s quantum model of atom
Orbits of electron in Bohr’s model of hydrogen atom.
An energy level diagram for hydrogen atom
Frequency of the emitted photon
Dependence of the wave length
The waves properties of particles
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