Emission PDF

Emission   Emission is the process by which some of the internal energy of a material is converted into radiant energy  All materials above absolute zero (0 Kelvin, K) in temperature emits radiation.  1) Our own bodies lose heat energ...

Emission   Emission is the process by which some of the internal energy of a material is converted into radiant energy  All materials above absolute zero (0 Kelvin, K) in temperature emits radiation.  1) Our own bodies lose heat energy through emission of EMISSION radiation. We do not notice because of a near-balance between heat we lose via emission and that we absorb from our surroundings.  2) A burning wood stove radiates heat that you can feel from far.  3) Glowing embers in a fireplace —— visible emission   Emissivity is the ratio of what is emitted by a given surface to what would be emitted if it were a blackbody.  Two cases: Emission  1) The emissivity at a single wavelength: Monochromatic Emissivity  2) Emissivity over a broad range of wavelengths: Graybody Emissivity   Blackbody: is the perfect emitter, which emits the maximum amount of radiation at each wavelength.  A blackbody is a hypothetical body comprising a sufficient number of molecules absorbing and emitting EM radiation in all parts of the EM spectrum so that:  1) All incident radiation is completely absorbed.  2) In all wavelength bands and in all directions, the maximum possible emission is realized. EMISSION  Properties of blackbody radiation  1) Blackbody radiation is uniquely determined by the temperature of the emitter.  2) For a given temperature, the radiant energy emitted is the maximum possible at all wave lengths.  3) The radiation is isotropic.   The Planck’s Radiation Law describes the spectral density of electromagnetic radiation emitted by a black body in thermal equilibrium at a given temperature T, when there is no net flow of matter or energy between the body and its environment. PLANCK’S  An object having temperature 𝑇 will generally emit radiation at all FUNCTION possible wavelengths. However, for any particular wavelength 𝜆, there is a hard upper bound on the amount of that radiation. The function of 𝑇 and 𝜆 that gives that upper bound is called Planck’s function   Where 𝐵𝜆 is radiance (intensity) in 𝑊𝑚−2𝑆𝑟−1𝜇𝑚−1  𝑘𝐵 is Boltzmann’s constant, PLANCK’S 𝑘𝐵 = 1.381 𝑥 10−23𝐽𝐾−1 FUNCTION  𝑐 is speed of light, 𝑐 = 2.998 𝑥 108𝑚𝑠−1  ℎ is Planck’s constant, ℎ = 6.626 𝑥 10−34𝐽𝑠  𝑇 is absolute temperature (in Kelvin, K),  𝜆 is wavelength in 𝜇𝑚   For any given absolute temperature, Planck’s Wein’s  T is the absolute temperature function has its peak at a wavelength that is inversely Displacement  b is a constant of proportional to that Law proportionality called Wien's displacement constant temperature.  Thus, peak emission from a 2.897771955...×10−3 m⋅K or b cool object, like the earth, ≈ 2898 μm⋅K occurs at much longer wavelengths that that from a very hot object, like the sun. Wein’s  A piece of metal heated by a Displacement blow torch Law  E = σT4  E is the radiant heat energy  According to Stefan emitted from a unit area in Boltzmann law, the amount of radiation emitted per unit Stefan one second (that is, the power from a unit area) and time from area A of a black Boltzmann  T is the absolute temperature body at absolute temperature T is directly (in kelvins) Law proportional to the fourth power of the temperature.  σ = Stefan-Boltzmann constant 5.670374419 × 10−8 watt per metre2 per K4   Where 𝐵𝜆 is radiance (intensity) in 𝑊𝑚−2𝑆𝑟−1𝜇𝑚−1  The Rayleigh–Jeans Law is an approximation of the Planck’s Rayleigh-Jeans  𝑘𝐵 is Boltzmann’s constant, 𝑘𝐵 = 1.381 𝑥 10−23𝐽𝐾−1 law for a blackbody that states that emitted radiance Approximation  𝑐 is speed of light, 𝑐 = 2.998 𝑥 is directly proportional to the 108𝑚𝑠−1 blackbody temperature.  𝑇 is absolute temperature (in Kelvin, K),  𝜆 is wavelength in 𝜇𝑚