describe emission of heat raddiation and coefficient of emistivity
Answers
Answer:
The emissivity of the surface of a material is its effectiveness in emitting energy as thermal radiation. ... Quantitatively, emissivity is the ratio of the thermal radiation from a surface to the radiation from an ideal black surface at the same temperature as given by the Stefan–Boltzmann law.
Answer:
The emissivity of the surface of a material is its effectiveness in emitting energy as thermal radiation. ... Quantitatively, emissivity is the ratio of the thermal radiation from a surface to the radiation from an ideal black surface at the same temperature as given by the Stefan–Boltzmann law.
Explanation:
Thermal radiation is electromagnetic radiation emitted from a material that is due to the heat of the material, the characteristics of which depend on its temperature. An example of thermal radiation is the infrared radiation emitted by a common household radiator or electric heater.A person near a raging bonfire will feel the radiated heat of the fire, even if the surrounding air is very cold. Thermal radiation is generated when heat from the movement of charges in the material (electrons and protons in common forms of matter) is converted to electromagnetic radiation. Sunshine, or solar radiation, is thermal radiation from the extremely hot gasses of the sun, and this radiation heats the earth. The earth also emits thermal radiation but at a much lower intensity because it is cooler. The balance between heating by incoming solar thermal radiation and cooling by the earth's outgoing thermal radiation is the primary process that determines the earth's overall temperature. As such, radiation is the only form of heat transfer that does not require a material to transmit the heat. Radiative heat is transferred from surface to surface, with little heat absorbed between surfaces. However, the surfaces, once heated, can release the heat via conduction or convection to the surroundings.
Thermal radiation is conducted via electromagnetic waves. As such, this form of heat transfer is not only a function of the temperature difference between the two surfaces but also the frequency range of the emitted and received energy. As an example, sunlight is composed of the visible light spectrum as well as infrared energy and ultraviolet energy.
As can be seen, thermal radiation is affected by the frequency of the emitted energy. This is why sunscreen ointments have ultraviolet protection, since this type of energy can be damaging to skin. In addition, it is common to feel warmer on the sunny side of the street as opposed to the shady side, given the radiative heat transfer. Radiation can be a significant source of heat as compared to the other forms (conduction and convection) because radiation is composed primarily of sunlight and the respective heating of the earth.
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