Spectral Energy Density, Total Energy Density, Emmisive Power & Absorptive Power

Spectral Energy Density: 

Spectral Energy Density for a particular wave length is the energy per unit volume per unit range of wavelength.

Total Energy Density:

Total Energy Density of Thermal Radiation at any point is the total radiant energy per unit volume around that point due to all wavelengths.

Emmisive Power:

The emmisive power of a body at a given temperature and for a given wavelength, is defined as the ratio of the radiant energy absorbed for second by unit surface area of the body per unit wavelength range.

Absorptive Power

The absorptive power of a body at a given temperature and wavelength is defined as the ratio of radiant energy absorbed per second by unit surface area of the body to the total energy falling per second on the same area.   

Concept of Thermodynamics - Zeroeth Law of Thermodynamics

For a system to be in thermodynamic equilibrium the following conditions must be full filled:-

i) Mechanical equilibrium

ii) Thermal equilibrium

iii) Chemical equilibrium

Mechanical equilibrium:

When there is no unbalanced force between system and its surroundings, the system is said to be in mechanical equilibrium.

Thermal equilibrium:

When the temperature in all parts of system is same as that of surroundings, the system is said to be in thermal equilibrium.

Chemical equilibrium:

If the chemical composition is same throughout the system and surroundings it is said to be in chemical equilibrium.

Zeroeth Law of Thermodynamics:

This law was first enunciated by RH Flower in 1831. According to this law when two systems ‘A’ and ‘B’ are in thermal equilibrium with another system ‘C’ then ‘A’ and ‘B’ will also be in thermal equilibrium.