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First Law of Thermodynamics

The first law of thermodynamics is a statement of conservation of energy in thermodynamical process.
According to it, heat given to a system (ΔQ) is equal to the sum of increase in its internal energy (ΔU) and the work done (ΔW) by the system against the surroundings, i.e.,


Important Points
  • It makes no distinction between work and heat as according to it the internal energy (and hence temperature) of a system may be increased either by adding heat to it or doing work on it or both.
  • ΔQ and ΔW are path functions, but ΔU is point function.
  • In Eq. (1) all three quantities ΔQ, ΔU, and ΔW must be expressed either in Joule or in calorie.
  • Just as zeroth law of thermodynamics introduces the concept of temperature, the first law introduces the concept of internal energy.

Sign conventions



When heat is supplied to a system.


When heat is drawn from the system.



When work done by the gas (expansion).


When work done on the gas (compression).



When temperature increases, internal energy increases.


When temperature decreases, internal energy decreases.


The first law of thermodynamics does not indicate the direction of heat transfer. It does not tell anything about the conditions under which heat can be transformed into work and also it does not indicate as to why the whole of heat energy cannot be converted into mechanical work continuously.

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