Radaition , Mechanism of heat transfer .

Radiation :

 - The transfer of energy by means of the emission of electromagnetic radiation called radiation heat transfer .

- thermal radiation occurs through the vacuum or any transparent medium . it is the transfer of energy by photons in electromagnetic wave  .

-  thermal radiation is emitted by all matter whose temperature is above absolute zero ( 0 K or -273 C )

 

- emission of energy occurs when  an electron moves from higher energy level to lower energy level and energy released from it has all the properties of electromagnetic waves .

- the radiation that results only due to temperature is called thermal radiation .

- when radiation strikes a surface , part of it is absorbed , part of it is reflected and remaining part if any is transmitted .

- the fraction of radiation absorbed by the surface is called absorptivity , the fraction reflected by the surface is called reflectivuty . the fraction transmitted is called transmitivity .

-  A perfect black body is one that converts all the incident radiation into heat and emits all thermal energy as radiation .

Stefan - Boltzman Law 

the Amount of the energy object radiates depends strongly on temperature . for an object with a temperature T in kelvin and a Surface area A , the energy radiates in a time t is given by the Stefan - Boltzman law of the Radiation .

Q =  e k T^4 A t

e  = emissivity constant and its a measure of the fraction of incident radiation energy is absorbed and radiated by the object .

k = Stefan -  Boltzman constant 

T = temperature 

A = Cross Sectional Area

t = time