| Basic Thermodynamics ~ J. Pohl © | www.THERMOspokenhere.com (183-E010) |
The systems of engineering analysis (mechanical systems, electrical, and all others) consist of matter. Events of matter (as an event in action, an event anticipated or some event observed) are marked by the change (or non-change) of some (or none) of the system physical properties. There are many physical properties. Basic thermodynamics addresses the three basic properties: mass, momentum and energy.
Equation Development Finalization Of the pages prior to this as many were devoted to introduction and explanation of equations as were to their application. Perhaps it seemed that equations would be changing "evermore." Fortunately the will not.
Our progress has brought us to SINGLE property equations that apply to all (nearly all) systems and models of system. In our studies we developed equations for the BODY, then for the SUBSTANCE, then for the FLOWING SUBSTANCE. It was necessary to take the simpler path through models, their masses momentums and energies. At last we are HERE! Hereafter we will use three equations to apply to any circumstance. These general or accounting-type equation forms are rate-forms representations: Mass Equation, Momentum Equation and Energy Equation (angular momentum and entropy are not addressed here). These are thermodynamic system equations but it is logical to call the group, simply, "Property Equations."
Mass Equation: We have been using the final form of the mass equation which has the term, system mass, left-of-equality and terms right-of-equality for possible change of system mass by mass entering and/or leaving the system. Of the system equations, the mass equation is always applicable and the easiest to apply. We have used it and will use it with some diversity.
Momentum Equation and Energy Equation: Neither the momentum equation nor energy equation we previously used is complete. Logically, if by the mass equation, mass enters or leaves the system during an event, then momentum, the product of the mass times its velocity, crosses the system boundary also. The same idea applies to the potential, kinetic and internal energies of entering or leaving mass flows. Our momentum equation has lacked "right-of-equality" terms to account for what some call a "momentum-flux." That deficit is corrected below. The energy equation also will be rewritten, upgraded to have "energy-flux" or flow terms. And with our consideration of matter entering (or leaving) a system, we will discover that work is involved.
Property Equations The foundation ideas of these equations were introduced in high school physics usually as "conservation principles." For the simple body (or extended body) as system, the perspective "conservation of mass, of momentum or of energy" is adequate. But with systems that are fluid, shape-changing with possibly growing or shrinking boundaries, the system approach in which mass is permitted to flow across the system boundaries is superior because in such cases the where and how conservation occurs is blurred.