State the first law of thermodynamics.

Explanation

First law of thermodynamics : The heat energy (dQ) supplied to a system is equal to the sum of the increase in the internal energy (dU) of the system and external wrork done (dW) by it
i.e., `dQ = dU+ dW " " …..(1)`
If dV is the increase in the volume under constant pressure (P) then dw = PdV `therefore dQ=dU+PdV" " ....(2)`
The improtance of this law is that it defines , the theromynamic quantity , internal energy which has a fixed value in a state .
Increase in internal energy `dU=nC_(v)dT`
Where n is the number of moles of the gas . This equation helps to calculate change of internal energy of the system when the temperature change by `DeltaT`.
Limitations of 1st law of thermody namics :
(1) It does not tell about the direction of heat flow . That is it does not specify the conditions under which a body can use the heat energy to produce the work .
It does not give any information about the efficiency with which heat can be converted into work .