The following facts are worth noting regarding the first law of thermodynamics.

(a) This law, which is basically the law of conservation of energy, applies to every process in nature.

(b) The law applies equally to all the three phases of matter, i.e., solid liquid and gas.

(c) Out of the three quantities, dQ, dW and dU, the first two are dependent on the path the system follows in going from initial to final state. However, dU does not depend on path. It depends only on initial and final states of the system.

(d) While applying the first law of thermodynamics, care must be taken that all the three quantities dQ, dU and dW must be expressed either in joule or in calorie.

(e) Just as Zeroth law of thermodynamics introduces the concept of temperature, the first law of thermodynamics introduces the concept of internal energy.

(f) From the first law of thermodynamics, we learn that it is impossible to get work from any machine, without giving an equivalent amount of energy to the machine.

(g) The first law of thermodynamics establishes the essential equivalence between work and heat, as according to this law, internal energy (and hence temperature) of a system can be increased either by supplying heat to it or by doing work on the system or both.