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Quantum Physics

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Black Body Radiation Intensity of blackbody radiation Classical Rayleigh-Jeans law for radiation emission Planck’s expression h = 6.626 10 -34 J · s : Planck’s constant Assumptions: 1. Molecules can have only discrete values of energy E n; 2. The molecules emit or absorb energy by discrete packets - photons

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Quantum energy levels Energy E 0 1 3 4 5 2 n hf 2hf 3hf 4hf 0 5hf

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Photoelectric effect Kinetic energy of liberated electrons is where is the work function of the metal

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Atomic Spectra a)Emission line spectra for hydrogen, mercury, and neon; b)Absorption spectrum for hydrogen.

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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

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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

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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

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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

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Example: An accelerated charged particle An electron accelerates through the potential difference 50 V. Calculate its de Broglie wavelength. Solution: Energy conservation Momentum of electron Wavelength

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