First Law of Thermodynamics:The first law of thermodynamics states that the amount of heat given to a system is equal to the sum of the increase in the internal energy of the system and the external work done.
When the whole of the work done is used in producing heat or vice versa then,
W = JH [J is the Joule’s mechanical equivalent of heat]
δH = the quantity of heat
δW = amount of external work done
dU = the increase in internal energy.
Then, mathematically we can write,
δH = dU + δW
The above equation represents the first law of thermodynamics.