State and explain First Law of thermodynamics.

According to first law of thermodynamics, if an amount of heat ∆Q is supplied to a thermodynamics system, a part of it may increase the internal energy of the system by ∆u and the remaining part is used up as the external work done ∆w by the system. thus, we have
∆Q=∆u + ∆w
First Law of thermodynamics follows from the conservation law of energy and establishes an exact relation between heat transferred and mechanical work done. it provides the valuable concept of internal energy.
First Law of thermodynamics is applicable to every process in nature and to all the three states of matter i.e., solid. liquid and gases. Moreover, here ∆u i.e.,changes in internal energy of system may be due to any cause like change in translational or rotational or vibrational kinetic energy or molecular potential energy etc. Again ∆w may be any type of work done by the system.
While applying First Law of thermodynamics, ∆Q is taken +ve if heat is supplied to the system and –ve if heat is withdrawn from the system. work done by the system is taken as +ve and work done on the system is considered negative, Finally increase in internal energy of a system is considered +ve but decrease in internal energy of a system is considered –ve.