Convection it is the process in which heat is transferred from one place to the other by the actual movement of heated substance (usually fluid). Convection requires medium.
When a liquid is heated the particle of the liquid which gets heated moves upwards, delivers heat to the other particles by bodily movement and gets cooled. It comes down at the heating point gets heated and goes back to its journey to distribute heat. This type of convection which results from difference in densities, is called natural convection, however, if a heated fluid is forced to move by a blower fan or pump, it is called forced convection.
It is found that heat convection from an object is proportional to the temperature difference ?θ between the object and convective fluid and the contact area A.
[dQ/dt] convection = h A?θ where h is a constant.
Lapse rate (the temperature falls by 6 C/Km as we move up) is an example of natural convection. Blood circulation, which helps in maintaining the temperature of the body, is an example of forced convection.
Radiation the process by which heat is transferred directly from one body to another without requiring a medium is called radiation. It is an electromagnetic radiation of wavelength 1 mm to 10-7 m. velocity is equal to the speed of light. If can be detected by thermocouple, thermopile or radiometer, bolometer, and so on.
Black body a black body is capable of emitting or absorbing radiation of all possible wavelengths. Initially it was thought that. Only a body which absorbs all possible radiation is a black body. Later on emission characteristics were also included.
The blackest body on the earth is lamp black. It is 98% black.
Absorptive power the absorptive power of a substance is defined as the ratio of the radiant energy absorbed by it in a given time to the total radiant energy incident nit in the same time.
Spectral absorptive power
A = ∫ ∞0 aλ dλ unit (Wm -2)
For a perfectly black body absorptive power is 1.
Emissive power (e) radiant energy emitted per unit area of the surface. However, if we consider emissive power of a surface for a particular wavelength it is called spectral emissive power. Spectral emissive power.
E = ∫∞0 eλ dλ unit = Wm-2
It is maximum for a black body and minimum for a white shining and smooth body.
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