Question

Define emissive power.

Answer 1

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All body will emit heat when the body temperature is above absolute zero , and this is called emission and when this emitted heat travels in the form of electro magnetic wave it is called thermal radiation. Emissive power is the energy of thermal radiation emitted in all directions per unit time per unit area of a surface at any given temperature.

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Answer 2

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The emissivity of the surface of a material is its effectiveness in emitting energy as thermal radiation. Thermal radiation is electromagnetic radiation and it may include both visible radiation (light) and infraredradiation, which is not visible to human eyes. The thermal radiation from very hot objects (see photograph) is easily visible to the eye. 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. The ratio varies from 0 to 1. The surface of a perfect black body (with an emissivity of 1) emits thermal radiation at the rate of approximately 448 watts per square metre at room temperature (25 °C, 298.15 K); all real objects have emissivities less than 1.0, and emit radiation at correspondingly lower rates..

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