Conduction, Convection and Radiation

Modes of heat transfer

The heat transfer from one medium to another medium by any of the following methods

( 1 ) Conduction

( 2 ) Convection

( 3 )  Radiation

( 1 ) Conduction

• The heat transfers from one part of substance to another part without movement in the molecule of substance. 
• The heat transfer takes places between object and direct contact. 
• The rate of conduction of heat along the substance depends upon the temperature difference.

Example : e.g. heating of insulating materials, 

• The amount of heat transfer through cubic body with two parallel plates during T hours is given by

Q = [ kA ( T₁ – T₂ ) / t ] T Mega Joules

Where

k = Co – efficient of thermal conductivity of the material ( MJ / m³ / ^oC / hour )

t = Thickness of cubic body

A = Cross section area of plate

T₁ = Temperature of first plate in centigrade

T₂ = Temperature of second plate in centigrade

( 2 ) Convection

• The heat transfers from one part to another part of substance due to actual motion of molecules. 
• The rate of conduction of heat mainly depends upon difference in fluid density at different temperature. 

Example : Boiling water, immersion heater, ice melting, hot air balloon, radiator

• The heat dissipation is given by H = a ( T₁ – T₂ )^b Watt / meter²

( 3 ) Radiation

• The radiation does not require any heat transfer medium. 
• The heat transfer takes place through electromagnetic waves. 
• The object is not necessary to be in direct contact with another object to transmit heat. 
• The energy transmission through electromagnetic waves is called as radiant energy.

Exmaple : Sun energy , solar heaters

• The rate of heat dissipation through radiation is given by

        H = 5.72 × 10⁴ ke [ ( T₁ / 1000 )⁴ - ( T₂ / 1000 )⁴ ] watt / meter²

Where

T₁ = Temperature of source in Kelvin

T₂ = Temperature of substance to be heater in Kelvin

k = Radiant efficiency

   = 1 Single element

   = 0.5 to 0.8 for several elements

e = Emissivity

   = 1 for Black body

   = 0.9 for resistance heating element

• The radiant heat is directly proportional to difference of the fourth power of the temperature.

Final Conclusion points

• The speed of conduction and convection is slower than radiation. 
• The radiation obeys the law of reflection and refraction whereas the conduction and convection do not follow it. 
• The conduction mainly occurs in solids through collision of molecules whereas the convection occurs in fluids by mass motion of molecules.
• The radiation takes places through the vaccum of space. 
• The conduction takes place due to temperature difference whereas the convection takes place due to density difference.
  

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