• The temperature difference between system and surrounding.
  • This exchange of heat takes place through thermally conducting walls (boundary).
  • If the temperature of a system is higher than the surrounding, heat is lost from the surrounding and heat is gained when the system has a lower temperature than the surrounding.
  • So, the amount of heat that is gained or lost by the system is symbolically represented by q.
  • ‘q > 0’ is positive (+ve) when system gains or absorbs heat.
  • ‘q < 0’ is negative (-ve) when system loses heat.


  • The energy between system and surrounding can take place in the form of work which may be a mechanical, pressure-volume or electrical.
  • For example, Take a thermos flask (thermally insulated) with some water in it and note down its temp., after doing some work (mechanical) by inserting a paddle and rotating it we will notice an increase in temp. This indicates the increase in internal energy, i.e., gaining of energy by the system by doing work on it.
  • The work done by the system or on the system is denoted by ‘w’.
  • Work is positive (w>0) when done by the system.
  • Work is negative (w<0) when done on the system.
  • The pressure-volume work is also called expansion work.


  • When heat is added, molecules start moving in different directions and thus, system attains randomness whereas when work is done molecules attain an order.
  • Hence, work can be regarded as an organised form of energy and heat as a random form of energy.
  • Work, a high-grade energy whereas Heat, is a low-grade energy.

Note- In few texts convention for work is opposite as mentioned above.

Also Read Modes of Heat Transfer

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