Physics

where u is the energy emitted by a body per unit time,

e is the emissivity

$σ$ is the Stefan-boltzmann constant

A is the surface area and

T is the temperature

$Δu_{1}=eσA(T_{4}−T_{0})$

$=4eσAT_{0}(T−T_{0})$ if the temperature difference is small

$I=I_{o}/r_{2}$

where

$I:$ Intensity at a distance $r$ from the reference point

$I_{o}:$ Intensity at the reference point

Example:

Find the ratio of amplitudes of radiation emitted by a cylindrical source at distances $2r$ and $18r$ from its axis.

Solution:

Let the intensity of the cylindrical source be $I_{o}$

The intensity at a distance $2r$ is given by: $I_{2r}=4r_{2}I_{o} $

The intensity at a distance $18r$ is given by: $I_{18r}=18_{2}r_{2}I_{o} $

$I_{18r}I_{2r} =2_{2}18_{2} =81$

$I∝A_{2}$ where $A:$ Amplitude

$∴A_{18r}A_{2r} =9$

ExampleDefinitionsFormulaes

ExampleDefinitionsFormulaes