Radiation is the way of heat transfer that doesn't need the material medium. This is in contrast to conduction or convection that requires the material medium to transmit heat energy. Radiant energy comprises of electromagnetic waves that travel with speed of light. Radiation from sun comprises of light and ultra violet and infrared heat waves. All these travel with speed of light. 3.0 x 108 m/s.
Properties of Radiation:
Similar to light radiation, study of which comes under waves and wave phenomena, heat radiation has properties which are given below:
Like light energy, it is in form of electromagnetic waves. Though, its wavelength is longer than wavelength of red light. Heat radiation that is invisible is hence known as infrared radiation. Wave length of light waves range from 4 x 10-7m (400nm) in violet to 7.5 x 10-7m (750nm) in red, while infrared radiation ranges from 750nm to about 100.00nm. Though, most objects emit heat radiation in invisible part of spectrum or range of wavelengths, some very hot objects like the sun emit heat radiation in visible part of the spectrum.
Detecting Heat Radiation:
Dull or black surfaces are the best radiators and absorbers of heat radiation respectively. Though, bright, shiny, polished or silvery surfaces are the bad radiators and absorbers of heat radiation. Naturally skins are able to detect heat radiation but more sensitive and consistent instrument which is more usually utilized to detect heat radiation is thermopile.
The thermopile is the series of arrangements of thermocouples composed of two dissimilar metals like Bismuth (Bi), Antimony (Ag). Such arrangement can be utilized to detect and to provide the rough measurement of intensity of heat radiation.
Black Body Radiation:
Black body is the best radiator or absorber of heat radiation which falls on it. The perfectly black body is hence stated as one which emits every wavelength with maximum energy for each wavelength for particular temperature of body. This black body is also called as ideal radiator. The good example of black body is the ceramic-lined closed container with the hole in it. It may also be vacant tin with the hole punched on lid.
Any radiation that enters hole is reflected numerous times round inside surface and tends to be trapped inside it.
Absorber inside the black body may be silvery so that reflection is high. With numerous reflections and absorption, hole looks black. As good absorber of radiation is good radiator, a hole in a closed container is also the black radiator.
Inside is ceramic-lined and blackened to decrease quickly any reflected radiation and it is heated to the high temperature in the furnace or heat chamber. It must be noted that radiation from the perfectly black body relies only on temperature. It doesn't depend on nature of the surface inside. Black body radiation is therefore also known as temperature radiation. Non-black body radiators like hot filament of the n electric lamp may have some wavelengths of lower intensity compared with those emitted by the perfectly black body at same temperature.
Provost's Theory of Heat Exchange:
It defines that when the object is at constant temperature or is in thermal equilibrium, it is losing and gaining heat at equal rates.
Let us consider the enclosure P at constant temperature T. Inside the enclosure are two objects A and B. A is cold while object B is hot.
Temperature T is though greater than temperature of cold body TA and less than the temperature of the hot body TB.
TB > T > TA
At first, body A receives more radiation falling on it from B that it emits. As a result, temperature of A rises that is it warms up. Also, body B emits more radiation than it receives from A, therefore temperature of B decreases that is it cools down. Ultimately, equilibrium temperature T is obtained. At this temperature both bodies are emitting and absorbing radiation at same rate. Above procedure shows exchange of heat through radiation of bodies at different temperatures to reach the thermal equilibrium as opposed to what occurs if they were to be in contact. Though, if body A is the black body, then at temperature T, it radiates a considerable amount of that heat at same rate. If B is though a silvery surface, then, at temperature T, it radiates and absorbs less heat than A.
Stefan-Boltzmann Law of Radiation:
Stefan found by experiment, while Boltzmann illustrated theoretically, that total rate of radiation emitted per unit area by the perfectly black body was proportional to fourth power of its absolute temperature (T) in Kelvin.
E/tA ∝ T4
E/tA σ T4
E = total heat energy emitted
t = time of emission
A = total surface area
T = absolute temperature
σ = constant known as Stefan constant
Value of σ = 5.7 x 10-8Wm-2k-4
For any other body or surface different from the black body
Where, e = emissivity of the surface (e is a number characterizing the emitting properties of a particular surface). But, rate of emission is power, that can be represented as:
E/t = power P
Therefore P = eAσ T4
If the black body X is placed inside the enclosure at the constant temperature To, then X will ultimately reach temperature To. From Provost's theory of heat exchanges, X receives from enclosure as much radiation as it emits, which is σATo4
If X is initially at temperature T inside the enclosure, net heat per second radiated by X is
P = eσA(T4 - To4)
Practical Application of Transfer of Heat:
Some of the practical applications comprise: screening action of clouds, green house and thermos flask. Thermos flask is described to illustrate how knowledge of conduction, convection and radiation has been utilized to keep materials at constant temperatures without loss of heat.
The Thermos Flask:
The thermos flask was initially designed for objective of storing liquefied gases. But now it is utilized for maintaining temperature of hot and cold liquids for long periods.
Basically, flask is composed of double-walled thin glass. Space in between flask is evacuated to make the vacuum between walls of the glass. If you observe the broken flask you will see a small protruding notch at the bottom. This is where glass was sealed off after generating vacuum with glass walls. Open end of flask is covered with the cork stopper or plastic cover that is non-conductors of heat. Double walled vacuum flask is sealed again in the insulator in entire case containing the flask. Evacuated space of flask is to prevent loss of heat by conduction and convection that need material medium. To minimize loss or gain of heat by radiation, the surfaces facing evacuated space are covered with silver. Therefore, radiant heat from hot liquid is reflected back at the outer wall. If cold liquid is put inside the flask, heat radiation from the surrounding is reflected back from silvered face of the inner wall. As a result, any radiant heat entering vacuum space is therefore arrested.
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