|THERMO Spoken Here! ~ J. Pohl © ~ 2019||www.ThermoSpokenHere.com (E0900-219)|
The first step of any engineering analysis is to select a system. All systems have in common that they consist of matter and that matter is part of the event. The system event might be modeled after something "on-going" or "in action." The event of interest might be "anticipated or imagined," or having "happened, was observed" with the analysis done to provide reason or insight.
Events of matter (actual observed or imagined) have the commonality of time and properties. Events are marked by "change" or "constancy (non-change)" of some (or none) of the system physical properties.
Equation Development Finalization: At this beginning level, thermodynamics addresses three basic system properties: mass, momentum and energy. Our task in this section is to develope final equations for these properties and demonstrare their applicability.
In our studies we developed equations for the BODY, then for the thermodynamic SUBSTANCE, then for the flowing, then phase-changing substances. It was necessary to take the simpler path through models. The equations for mass, momentum and energy changed as did the complexity of the system matter. Here we introduce and use equations for all models. The three equations apply to any circumstance, in a retro-active way (by omission of selcct terms) to the simplest system, the BODY. These (general, accounting-type) equations are rate-form representations: Mass Equation, Momentum Equation and Energy Equation (angular momentum and entropy are not addressed here). It is logical to call these thermodynamic system equations simply, "Property Equations."
Mass Equation: We have been using the mass equation form which has 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.
Momentum Equation: The momentum equation previously used is incomplete. Logically, if mass enters or leaves the system then momentum also crosses the system boundary. Our momentum equation has lacked "right-of-equality" terms to account for what some call a "momentum-flux."
Energy Equation: The same idea applies to the potential, kinetic and internal energies of entering or leaving mass flows. 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, from high school physics, are called "conservation principles." For the simple BODY (or extended body) as system, the perspectives "conservation of mass, of momentum or of energy" is adequate. However for flowing, shape-changing fluid systems with possibly growing or shrinking boundaries, the conservation becomes unclear. Where and what does the conservation happen. The new need is realized with the examples.