Conservation of Energy - First Law of Thermodynamics

First Law of Thermodynamics

For a closed thermodynamic system, the first law of thermodynamics may be stated as:

where is the amount of energy added to the system by a heating process, is the amount of energy lost by the system due to work done by the system on its surroundings and is the change in the internal energy of the system.

The δ's before the heat and work terms are used to indicate that they describe an increment of energy which is to be interpreted somewhat differently than the increment of internal energy (see Inexact differential). Work and heat are processes which add or subtract energy, while the internal energy is a particular form of energy associated with the system. Thus the term "heat energy" for means "that amount of energy added as the result of heating" rather than referring to a particular form of energy. Likewise, the term "work energy" for means "that amount of energy lost as the result of work". The most significant result of this distinction is the fact that one can clearly state the amount of internal energy possessed by a thermodynamic system, but one cannot tell how much energy has flowed into or out of the system as a result of its being heated or cooled, nor as the result of work being performed on or by the system. In simple terms, this means that energy cannot be created or destroyed, only converted from one form to another.

Entropy is a function of the state of a system which tells of the possibility of conversion of heat into work.

For a simple compressible system, the work performed by the system may be written

where is the pressure and is a small change in the volume of the system, each of which are system variables. The heat energy may be written

where is the temperature and is a small change in the entropy of the system. Temperature and entropy are variables of state of a system.