# Stefan Boltzmann Law

Physics

## formula

### Energy emitted per unit time by a body

where u is the energy emitted by a body per unit time,
e is the emissivity
is the Stefan-boltzmann constant
A is the surface area and
T is the temperature

## definition

### Thermal energy lost per unit time due to radiation

Suppose a body of surface area A at an absolute temperature is kept in a surrounding having a lower temperature  . The net rate of loss of thermal energy from the body due to radiation is

if the temperature difference is small

## definition

### Intensity at a finite distance from the source

Intensity of thermal radiation decays as:

where
Intensity at a distance from the reference point
Intensity at the reference point

Example:
Find the ratio of amplitudes of radiation emitted by a cylindrical source at distances and from its axis.
Solution:
Let the intensity of the cylindrical source be
The intensity at a distance is given by:
The intensity at a distance is given by:

where Amplitude