Heat Transfer – Radiation and Conduction, the Different Modes of Heat Transfer Include (For CBSE, ICSE, IAS, NET, NRA 2022)

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Heat Transfer – Radiation and Conduction

Heat Transfer

Heat Transfer
  • The movement of heat across the border of the system due to a difference in temperature between the system and its surroundings.
  • So, when there is a temperature difference between two bodies, heat is transferred from the hot body to the colder body.
  • There are three common modes of heat transfer – conduction, convection, and radiation.
  • Convection is either the heat transfer due to bulk movement.
  • The process of heat transfer between a surface and a fluid flowing in contact with it is called convective heat transfer.

The Different Modes of Heat Transfer Include

  • Conduction
  • Convection
  • Radiation


Conduction, Radiation
  • The process in which heat flows from objects with higher temperature to objects with lower temperature
  • Heat transfers by conduction through some solid objects (metallic spoon) from one side to the other, Cooking pans are made up of copper & aluminium because they are good conductors of heat, When you put a metallic spoon in a cup of hot tea for a time, we feel the hotness of the spoon because heat is transferred through solids by conduction.
  • Transfer of heat by conduction is the transfer of heat through some solid objects from the part with higher temperature to that with lower temperature, You feel hot when you touch a hot metallic spoon because heat is transferred from the hot object (spoon) to the cold object by conduction

Conduction Equation

  • The coefficient of thermal conductivity shows that a metal body conducts heat better when it comes to conduction.
  • The rate of conduction can be calculated by the following equation:


  • Q is the transfer of heat per unit time
  • K is the thermal conductivity of the body
  • A is the area of heat transfer
  • is the temperature of the hot region
  • is the temperature of the cold region
  • d is the thickness of the body

Conduction Examples

  • Ironing of clothes where the heat is conducted from the iron to clothes.
  • Heat is transferred from hands to ice cube resulting in melting of ice cube when held in hands.
  • Heat conduction through sand at the beaches. This can be experienced during summers. Sand is a good conductor of heat.

What is Radiation?

What is Radiation
  • Thermal radiation is generated by the emission of electromagnetic waves.
  • These waves carry away the energy from the emitting body.
  • It takes place through a vacuum or transparent medium which can be either solid or liquid.
  • It is the result of the random motion of molecules in the matter.
  • Movement of charged electrons and protons is responsible for the emission of electromagnetic radiation.

Radiation Equation

  • The wavelengths in the spectra of the radiation emitted decreases and shorter wavelengths radiations are emitted as temperature rises.
  • Thermal radiation can be calculated by Stefan-Boltzmann law:


  • P is the net power of radiation
  • A is the area of radiation
  • is the radiator temperature
  • is the surrounding temperature
  • e is emissivity and is Stefan՚s constant

Radiation Example

  • Release of alpha particles during the decaying of Uranium-238 into Thorium-234.
  • Microwave radiation emitted in oven is an example of radiation.
  • UV rays coming from the sun is an example of radiation.

Unit of Heat Transfer

Unit of Heat Transfer
SI systemJoule
MKS systemcal
Rate of transfer of heatKW

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