Definition Of Energy

The rise of energy of a system during a change of state is numerically equivalent to the total heat minus the total work during the procedure.
In symbols,

E₂ – E₁ = Q - W

Here E₂ and E₁ are the energies of the system in the final state 2 and the preliminary state 1, correspondingly.

Remarks on the Definition of Energy:

• It might be seen that only changes of energy have been stated. This means that energy has to be measured with respect of some random base or datum.  

• The energy of a system of unit mass is termed as the specific energy, and is specified by the lower-case symbol e.

• Till now, it has been supposed that the path of the whole process among states 1 & 2 is identified. It will be shown currently that only the end states 1 & 2 require to be specified, and hence E is a property.