Introduction To Radiation

Lecture 33, March 26, 2004 • Project Due Date Last day of classes o Jackson Cluster is available even if McLaughlin is not

• Reminder – Quiz Tuesday (from last quiz to ch12 assignment) Introduction to Radiation (Ch12) All substances continuously emit electromagnetic radiation due to molecular and atomic agitation associated with the internal energy of the material. In the equilibrium state this energy is proportional to the temperature of the material. The emitted radiant energy can range from radio waves which can have wavelengths of 10s of metres to cosmic rays which have wavelengths < 10-14 metres. We are interested in radiation that is detected as heat or light which occupies an intermediate wavelength range. Radiation propagates in a vacuum at the speed of light ( c = λν) = 3 x 108 m/s λ – wavelength [m] ν – frequency [1/s] Thermal radiation 0.1 < λ < 100 µm Visible radiation 0.35 < λ < 0.75 µm Although radiant energy surrounds us constantly, we are not overly aware of it because our bodies are very poor detectors of thermal radiation. Our eyes on the

other hand are extremely sensitive to this radiation in a the very narrow range of visible radiation. If we stop to think a little, however, we realize that our skin is to some degree a detector of thermal radiation, but we really need additional equipment to measure thermal radiation. We are aware of thermal radiation in certain cases, and we notice in several cases that temperatures far exceed the local ambient temperatures when radiation is important. Recall • Walking on a beach in the sun • Getting into a car in the sun • Frost appearing on the ground when the ambient T is above 0 Radiation is very different from conduction and convection in that • It does not need a medium to travel (travels through vacuum) • Depends on the absolute temperature differences to the fourth power (compared to the first power) • Can have significant variations with wavelength • It depends strongly on surface properties

Radiation will therefore be more important at higher temperatures as in furnaces, combustion chambers, nuclear reactors and high T fuel cells. There are significant applications where it must also be

considered at much lower temperatures (greenhouse example). Solar radiation is also the toughest design constraint on both vehicle cooling systems and window systems. Ice forming above 0oC is also a clear example of the importance of radiation even at low temperatures, as is the fact that windows have a chilling effect on us as our bodies radiate energy out to the cooler environment without receiving compensating radiation in return. Simply drawing the blinds will stop this effect immediately as the blinds act as a radiation shield and block the radiative heat transfer from our bodies. Radiation interacts with the other modes of heat transfer by raising of lowering surface temperatures and for example generating free convection flows that would not otherwise exist. Solar energy is also very important and, photovoltaics aside the energy is used to heat domestic hot water and to generate electricity (steam turbines, solar chimneys). A well designed solar collector functions at temperatures of a few hundred degrees above ambient. Radiation incident upon a surface and either be absorbed, reflected or transmitted through the surface, and all of these phenomena can depend on

both direction (directional distribution) and wavelength (spectral distribution). Absorbed radiation is converted to internal energy, raising the temperature, and hence affects the emission of radiation from the surface as well. Emission from a black body It can be demonstrated that (2nd law of thermodynamics) that there is a maximum amount of energy that can be radiated at a given T and lambda. A body emitting this maximum possible amount is termed a black body. Let us define the spectral emissive power of a blackbody in terms of frequency