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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