Heat can be defined as the energy transferred from one body to another body due to the difference in temperature. Heat can flow spontaneously from an object at higher temperature to another object at lower temperature.

It is denoted by Q. Heat transfer to the system is taken +ve and heat transfer from the system is taken as –ve`.

The heat is measured in joules.


There are three methods of transfer of heat from one object to other.

Conduction: It is the most significant means of heat transfer in a medium (solid, liquid and gases). In this method the hot and vibrating atoms come in contact with the neighboring atoms and molecules, transferring a part of heat to these neighboring atoms. It can be shown as

Q = kA (t1-t2)/l

Where Q is the heat transfer rate, t1-t2 is the temperature difference, A is the area of the heat transfer surface and k is the thermal conductivity of the material.

Convection: In conduction the heat transfer takes place by fluid flow. As we give heat to the fluid, it will transfer the heat to its neighboring molecule and so on. There are two types of convection.

  1. Free convection: In force convection the fluid moves under gravity.
  2. Forced convection: In this type of convection we take help of fan or blower to move the fluid 

Radiation: It is the heat transfer process in which there is no medium present for the heat transfer from one body to another body. It is the transmission of heat in the form of radiant energy. It occurs in the vacuum because the medium will affect the rate of heat transfer. The equation for radiation heat transfer given by the Stefan-Boltzman is

E = σAt4

Where E is the energy radiated, σ is the Stefan-Boltzman’s constant, t is the temperature of the surface and A is the area of the surface.

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