State the first law of thermodynamics and write its limitations.

(N/A) The first law of thermodynamics is a statement of the law of conservation of energy. It states that if a quantity of heat $dQ$ is supplied to a system,it is used to increase the internal energy $dU$ of the system and to do work $dW$ by the system against the surroundings. Mathematically,it is expressed as: $dQ = dU + dW$.
Limitations of the first law of thermodynamics:
$1$. It does not indicate the direction of the process. For example,it does not explain why heat cannot spontaneously flow from a cold body to a hot body.
$2$. It does not specify the conditions under which the conversion of heat into work is possible.
$3$. It does not tell us about the extent of conversion of heat into work,i.e.,it does not explain why heat cannot be completely converted into work in a cyclic process.