FTFS Chap21 P001 - Chapter 21 Fundamentals of Thermal...

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Chapter 21 Fundamentals of Thermal Radiation Chapter 21 FUNDAMENTALS OF THERMAL RADIATION Electromagnetic and Thermal Radiation 21-1C Electromagnetic waves are caused by accelerated charges or changing electric currents giving rise to electric and magnetic fields. Sound waves are caused by disturbances. Electromagnetic waves can travel in vacuum, sound waves cannot. 21-2C Electromagnetic waves are characterized by their frequency v and wavelength . These two properties in a medium are related by   c v / where c is the speed of light in that medium. 21-3C Visible light is a kind of electromagnetic wave whose wavelength is between 0.40 and 0.76 m . It differs from the other forms of electromagnetic radiation in that it triggers the sensation of seeing in the human eye. 21-4C Infrared radiation lies between 0.76 and 100 m whereas ultraviolet radiation lies between the wavelengths 0.01 and 0.40 m The human body does not emit any radiation in the ultraviolet region since bodies at room temperature emit radiation in the infrared region only. 21-5C Thermal radiation is the radiation emitted as a result of vibrational and rotational motions of molecules, atoms and electrons of a substance, and it extends from about 0.1 to 100 m in wavelength. Unlike the other forms of electromagnetic radiation, thermal radiation is emitted by bodies because of their temperature. 21-6C Light (or visible) radiation consists of narrow bands of colors from violet to red. The color of a surface depends on its ability to reflect certain wavelength. For example, a surface that reflects radiation in the wavelength range 0.63-0.76 m while absorbing the rest appears red to the eye. A surface that reflects all the light appears white while a surface that absorbs the entire light incident on it appears black. The color of a surface at room temperature is not related to the radiation it emits. 21-7C Radiation in opaque solids is considered surface phenomena since only radiation emitted by the molecules in a very thin layer of a body at the surface can escape the solid. 21-8C Because the snow reflects almost all of the visible and ultraviolet radiation, and the skin is exposed to radiation both from the sun and from the snow. 21-1
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Chapter 21 Fundamentals of Thermal Radiation 21-9C Microwaves in the range of 10 2 to 10 m 5 are very suitable for use in cooking as they are reflected by metals, transmitted by glass and plastics and absorbed by food (especially water) molecules. Thus the electric energy converted to radiation in a microwave oven eventually becomes part of the internal energy of the food with no conduction and convection thermal resistances involved. In conventional cooking, on the other hand, conduction and convection thermal resistances slow down the heat transfer, and thus the heating process.
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FTFS Chap21 P001 - Chapter 21 Fundamentals of Thermal...

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