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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