Chapter 2 Control Volumes and Units.pdf

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Mechanical Engineering
Chris Ford

Chapter 2: Control Volumes and Units Monday, 11 November 2013 7:57 AM Thermodynamics: the science of energy and entropy; science that deals with heat and work and properties relating to heat and work A Thermodynamic System and Control Volume • Thermodynamic system: control volume that contains the matter and devices inside a control surface • Isolated system: no mass, heat or work cross the system boundary • Closed system: fixed mass, fixed volume • Open system: flow of mass, fixed volume Macroscopic versus Microscopic Points of View • Only investigate from macroscopic point of view; concerns overall or average effects of many molecules rather than x, y, z coordinates of each atom at all points in time Properties and State of a Substance • Phase: the quantity of mass that is homogenous throughout the system; eg. Liquid, vapour, solid • State: identified by values of each macroscopic property at that point and position in time; eg. Temperature, pressure, density, etc. • Intensive (absolute) property: independent of mass; pressure, temperature, density, specific volume, etc. • Extensive property: dependent on mass; can become an intensive property by dividingby the mass; mass, total volume, etc. Processes and Cycles • Process: path from one state to another • Quasi-static process: occurs so slowly that if stopped at any point in time, the system would be in equilibrium • Isothermal process: constant temperature • Isobaric (isopiestic) process: constant pressure • Isochoric process: constant volume • Isotropic: conntant entropy; reversible and adiabatic • Polytropic: PV =constant • Thermodynamic Cycle: where a system undergoes a number of changes of states and finally returns to the original state Units for Mass, length, time and force Force (F) F = ma Newtons (N) Time (t) Seconds (s) Length (l)
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