The work done in stretching an elastic wire per unit volume is on strain energy in a stretched string is
K is the force constant of a spring. The work done in increasing its extension from l1 to l2 will be
If 
and 
represent the heat supplied to the system and the work done on the system respectively, then the first law of thermodynamics can be written as
(where 
is the internal energy)
In an isothermal reversible expansion, if the volume of 96 gm of oxygen at 27°C is increased from 70 litres to 140 litres, then the work done by the gas will be
A gas expands under constant pressure P from volume V1 to V2. The work done by the gas is
