EML4450L10

EML4450L10 - S ustainable E nergy S cience and E ngineering...

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Unformatted text preview: S ustainable E nergy S cience and E ngineering C enter Essentials of Quantum Physics References Direct energy conversion by S.W. Angrist, Ch 3. (out of print text book) Essential Quantum Physics by Peter Landshoff, Allen Metherell and Gareth Rees, 1997, Cambridge University Press. S ustainable E nergy S cience and E ngineering C enter Atoms Atoms are believed to be the basic building blocks of matter. An atom consists of a positively charged nucleus together with a number of negatively charged electrons . Inside the nucleus there are protons and neutrons that carry positive charge and no charge respectively. The diameter of the whole atom is about 10-10 m. Since the diameter of the nucleus is of the order of 10-15 m, it may be considered as a point charge. The mass of the proton or neutron is some 2000 times that of the electron (mass = 9.1 x 10-31 kg; charge= -1.6 x 10-19 Coulombs) , hence nearly all the mass of the atom is in the nucleus. Electrons carry a negative charge and circle the nucleus, in special orbits, because of the attractive electrostatic force according to Coulombs law. When they jump between these special orbits, they would cause radiation with a little bursts of light (electromagnetic energy) generally refereed to as photons . This energy is known to change only in little jumps, called quanta . Atoms do not obey classical mechanics and one resorts to Quantum mechanics for their behavior. S ustainable E nergy S cience and E ngineering C enter Photons Photon: a quantum of electromagnetic energy having both particle and wave properties. A photon has no electric charge or mass but posses momentum, energy and spin. In the present context, the focus is on the interaction of electromagnetic waves ( photons ) with electrons to understand the creation and absorption of light. Electromagnetic waves have wavelengths ranging from 1 nm (10-9 m) for x-rays to 1 km for radio waves. All electromagnetic radiation, in vacuum, moves at the speed of light (c = 3 x 10 8 m/s). The speed of the photons changes when they pass through different media, such as water, glass and air. In a metal, the atoms are anchored to fixed sites by the electrostatic forces due to all the other atoms. The outermost orbital electrons of the atoms are almost free, and move through the metal when an electric field is applied. It is known that if one shines a beam of light on a clean surface of a metal, electrons can escape from the metal surface and can be detected as electric current - photoelectric effect . The light has to exceed a certain energy to remove electrons from the metal surface. The number of electrons that escape in a given time rises with the light beam intensity. However, the energy with which they escape does not depend on the beam intensity, rather it depends on the frequency of light, (in Hz). The energy E of each photon is proportional to frequency....
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EML4450L10 - S ustainable E nergy S cience and E ngineering...

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