704. Problem 49.7 (RHK) We have to show (a) that a human body of area , emissivity , and temperature emits radiation at the rate of 910 W. (b) We have to answer why people do not glow in the dark. |
Solution: Click For PDF Version (a) We will use the Stefan-Boltzmann law for answering the first part of the problem. It states that energy emitted by a body, which is at temperature T, per unit area per unit time of emissivity is given by As the surface area of a human body is assumed to be and the temperature of the skin is assumed to be , and emissivity , the rate at which emission of radiation is taking place will be (b) The wavelength at which Planck’s spectral radiation has its maximum for will be The radiation emitted by a human body is therefore in infrared part of the electromagnetic spectrum, and so the people do not ‘glow’. |