The first law of thermodynamics leads directly to the non-flow energy equation and embodies four important concepts, as follows:
1. Heat and work are mutually convertible one into the other as they are both modes of energy transfer.
2. The existence of a type of energy (internal energy) that depends on the thermodynamic state of a system.
3. The possibility of measuring a difference in internal energy between thermodynamic states by making measurements of heat transfer and work.
4. The fact that energy is conserved whenever the thermodynamic state of a closed system changes.
