First Law of Thermodynamics

The first law of thermodynamics states that energy cannot be created or destroyed, although it may be changed from one form to another. Stated in equation form, it is written as follows:


Qh = heat supplied to a system

Wt = work done by the system

AE = change in energy of the system

If the change in energy to the system is expanded, then


AU = change in internal energy APE = change in potential energy AKE = change in kinetic energy

If the work term Wt is expanded to breakdown shaft work done to or from the system and the work done by the system, then


W = shaft work in or out of the system p = fluid pressure in the system v = specific volume of the fluid in the system Am = mass of fluid working in the system

If Equation 2.36 is rewritten in a general form using specific energy notation, yZ

where u = specific form of internal energy V = velocity of the gas z ~ height above some arbitrary reference

By substituting Equations 2.37 and 2.38 into Equation 2.35, maintaining the specific energy form, and regrouping, the following equation can be written:

By defining enthalpy as h = u + Pv

and substituting into Equation 2.39, and substituting into Equation 2.39,

Equation 2.41 is the general energy equation for a steady flow process.

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