- Radiation is the energy emitted by matter in the form of electromagnetic waves.
- Here we will look into thermal radiation (10-7 m to 10-4 m) since we are dealing with heat transfer.
- In radiation the internal energy of an object decreases.
Rate of emission of radiation by a body depends on-
- Surface temperature
- Surface Nature
- Wavelength or frequency of radiation
Total Emissive Power (E)
- Total amount of radiation (all wavelength range) emitted by a body per unit area and time.
- A/C to Stefan-Boltzman, for a black body emissive power is proportional to absolute temperature to the fourth power.
Eb = σAT4 W/m2
σ = Stefan-Boltzman Constant (5.67*10-8 W/m2K4)
Monochromatic Emissive Power (Eλ)
- Rate of energy radiated per unit area of the surface per unit wavelength.
- At any given temperature the amount of radiation emitted per unit wavelength varies at different wavelengths. So, Monochromatic Emissive Power (Eλ) of the surface is used.
- Ability of the body surface to radiate heat.
- It is also defined as the ratio of emissive power of any body to emissive power of a black body of equal temperature.
- Its value varies for different substances ranging from 0 (white body) to 1 (black body).
- Emissivity may vary with temperature and wavelength.
- Total incident radiation on a surface from all directions per unit time per unit area of the surface.
- It refers to all of the radiant energy leaving a surface per unit area of the surface.
J = ε + ρG
ρ = reflectivity
- Fraction of incident radiation reflected.
- For white body :- ρ = 1, α = 0, τ = 0
- Fraction of incident radiation absorbed.
- For Black Body α = 1, ρ = 1, τ = 0
- τ = 0 for black, white and opaque body (α + ρ = 1).
- Fraction of incident radiation transmitted.