| Radiation Pressure |
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| SHOPPER'S DELIGHT | |
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For example, the radiation of the Sun at the Earth has an energy flux density of 1370 W/m2, so the radiation pressure is 4.6 µPa (absorbed) (see also Climate Model ). DISCOVERY The fact that Electromagnetic Radiation exerts a pressure upon any surface exposed to it was deduced theoretically by James Clerk Maxwell in 1871 , and proven experimentally by Lebedev in 1900 and by Nichols and Hull in 1901 . The pressure is very feeble, but can be detected by allowing the radiation to fall upon a delicately poised vane of reflective metal ( Nichols Radiometer ). THEORY It may be shown by electromagnetic theory, by Quantum Theory , or by Thermodynamics , making no assumptions as to the nature of the radiation, that the pressure against a surface exposed in a space traversed by radiation uniformly in all directions is equal to 1/3 the total radiant energy per unit volume within that space. For Black Body radiation, in Equilibrium with the exposed surface, the energy density is, in accordance with the Stefan-Boltzmann Law , equal to ''σT''4/3''c''; in which ''σ'' is the Stefan-Boltzmann Constant , ''c'' is the Speed Of Light , and ''T'' is the absolute Temperature of the space. One third of this energy is equal to 6.305×10−17''T''4 J/(m3K4), which is therefore equal to the pressure in Pascal s. In interplanetary space For example, at the , YORP Effect ). In stellar interiors In Stellar interiors the temperatures are very high. Stellar models predict a temperature of 15 MK in the center of the Sun and at the cores of Supergiant stars the temperature may exceed 1 GK. As the radiation pressure scales as the fourth power of the temperature, it becomes important at these high temperatures. In the Sun, radiation pressure is still quite small when compared to the gas pressure. In the heaviest stars, radiation pressure is the dominant pressure component. SOLAR SAILS Solar Sail s, a proposed method of Spacecraft Propulsion , would use radiation pressure from the Sun as a motive force. Private spacecraft Cosmos 1 was to have used this form of propulsion. RADIATION PRESSURE IN ACOUSTICS In Acoustics , radiation pressure is the unidirectional pressure force exerted at an interface between two media due to the passage of a sound wave. REFERENCES
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