Heat And Thermodynamics
Thermal Radiation
For thermal radiations/infrared region of electromagnetic spectrum, 
is confined from 8000 Å
to about 4 mm. Roughly speaking 
is between 
to 
m.
Thermal radiations can be detected by Crook's radiation micrometer, Boy's radiomicrometer, Bolometer, Pyrometer, differential air thermometer and thermopile.
Thermal radiations are emitted by every body whose temperature is greater than zero kelvin.
Higher the temperature of body, greater will be the amount of radiations emitted and vice versa.
When a body ansorbs heat radiations, its temperature rises. The temperature falls when a body emits heat radiations.
Thermal radiations exhibit all properties of electromagnetic radiations emitted by tSpectrum of infrared region is studied with the prism of Quartz or rock salt. Glass absorbs heat radiations and so prisms of glass are not used.
Energy flux:
- It is the energy flowing per second unit area normal to any surface.
- Its units are
= 
Energy density:
- It is the total energy per unit volume.
- Its unit are
.
Due to point source of radiation,
- Intensity (I)
- Amplitude A
A 
Due to linear source of radiation, I 
and 
For normal radiations, radiation pressure exerted
- on perfectly absorbing surface is P = energy density
- on perfectly reflecting surface is P =
energy density - Energy flux = velocity (c) energy density
For inclined radiations,
- P = energy density/3, on perfect absorber
- P =
energy density on perfect reflector - Energy flux = velocity (c)
Reflectance, absorptance ans transmittance:
(i) Let in a given time t, amount of thermal radiation
incident = Q; reflected = 
; absorbed = 
; transmitted = 
then Q = 
+
+
.
(ii) Let r = reflectance = reflecting power = 
a = absorptance = absorbing power = 
t = transmittance = transmitting power = 
r + a + t = 
= 
= 1.
(iii) If t = 0, r + a = 1 
a = 1 _ r.
This shows that good reflectors are bad absorbers and good absorbers are bad reflectors.
(iv) Similarly, good transmitters are bad absorbers and bad reflectors. Vice-versa is also true.
Prevost's theory of heat exchange:
- Every body emits heat radiations at all temperatures, except zero kelvin , as well as absorbs radiations incident upon it.
- Temperature of a body rises if the energy absorbed is more than the energy emitted.
- Temperature of a body falls if the energy emitted is more than the energy absorbed.
- Temperature of a body remains constant when the energies absorbed and emitted are equal. Thermal equilibrium is reached in such a case.
- At absolute zero temperature (0 K or 273°C) there is no heat exchange among various bodies.