Glossary of Thermodynamic Terms

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P
 Primary Word Secondary Word Definition Tutorial Page Link
 partial pressure For each species in a gas phase, it is the fraction of the total pressure that is caused by the presence of that species. If all of the other type of molecules were removed from the vessel, the final pressure would be the partial pressure of the remaining species. 2D1  ,  2  ,  4  ,  5  ,  6  ,  8  ,  9 partially miscible 2A6 Pascal 1D2 path The actual series of states that the system passes through as it moves from the initial state to the final state during a process. 3D3, 4A17, path equation 4A17 - 18 path variable 4A8  ,  12  ,  13, 4B3, 4C5  ,  6 perpetual motion machine 6C10 perpetual motion machine first kind 6C10 perpetual motion machine second kind 6C10 perpetual motion machine third kind 6C10 phase 2A3 - 10 phase change 2A4  ,  8  ,  10 phase diagram A plot of one intensive variable against another intensive variable.  The intensive variables are often, but not always, P, V or T.  A Phase Diagram is useful because it shows what phase will exist for given values of the two intensive variables plotted on the axes.  A Phase Diagram also presents curves along which phase transitions occur.  PV Diagram, PT Diagrams and TV Diagrams are discussed in this program. 2B1, 7B1, phase equilibrium In a system at Phase Equilibrium, the rate at which molecules are making the transition OUT of each and every phase is exactly equal to the rate at which molecules are making the transition INTO the same phase. 2A9 pipe 5C6  ,  18 piston-and-cylinder device 1C2, 2A10, 4A1  ,  9 Pitzer accentric factor A property of the molecule of a given species that reflects the symmetry of the geometry of a molecule as well as its electrical polarity. 2F3 polytropic process A process in which the volume and pressure of a system are related by an equation of the form: PV^n = C , where C and n are constants. 4A17  ,  19, 7A1, 7E7 polytropic process ideal gas 7E8 polytropic process internally reversible boundary work 7E14  ,  15 polytropic process internally reversible boundary work, ideal gas 7E16 - 19 polytropic process logarithmic TS diagram 7E9 - 13 polytropic process PV diagram 8B11 polytropic process relationship to isentropic process 7E7 polytropic process semi-logarithmic PV diagram 7E9 - 13 polytropic process shaft work 8B12 - 20 polytropic process shaft work on a PV diagram 8B19 polytropic process shaft work, ideal gas 8B14 polytropic process shaft work, ideal gas, constant heat capacities 8B15 polytropic process shaft work, ideal gas, exponent = 1 8B16 polytropic process shaft work, ideal gas, isochoric 8B18 polytropic process shaft work, ideal gas, isothermal process 8B17 potential energy The energy of a piece of matter associated with the position of the matter within a potential field.  The field is usually gravitational. 1A4, 5B3  ,  4, 5E5 pound force 1B5 pound mass 1B4  ,  5 pound mole 1B4 power The rate at which the energy of a system changes or the rate at which energy is teansferred.  Power has units of energy per time,  such as J/s (Watts) or Btu/h 4A6 power cycle A cycle in which energy, in the form of heat, is transferred from a hot reservoir into a system in order to do work on the surroundings. 4F1  ,  3 - 6 power cycle gas 4F3, 9E1 - 13, power cycle gas, closed 9E2  ,  3  ,  5 - 13 power cycle gas, open 9E2 - 4 power cycle thermal efficiency A measure of the performance of a power cycle. Thermal Efficiency is the ratio of the work produced by the cycle to the heat transferred into the heat engine from the hot reservoir. 4F6, 6B4, 6C4  ,  5 power cycle vapor 9B1 pressure Force per unit area.  In the context of thermodynamics, the pressure force is generally exerted by a fluid (gas or liquid) on a solid surface.  The pressure force is due to the sum of all the random collisions of fluid molecules with the solid surface.  [ atm, kPa, bar, psi, etc. ] 1D2  ,  3 pressure absolute The pressure measured relative to a vacuum (absolute zero pressure).  [ atm, kPa, bar, psia, etc. ] 1D3 pressure atmospheric 1D2 pressure gauge Also known as Gage Pressure.  The difference between the absolute pressure and atmospheric pressure.  Gauge pressure is what is normally read from a pressure gauge.  You must ADD atmospheric pressure to the gauge pressure to obtain the absolute pressure.  Gauge pressures CAN be negative !  [ atm, kPa, bar, psig, etc. ] 1D3 pressure partial For each species in a gas phase, it is the fraction of the total pressure that is caused by the presence of that species. If all of the other type of molecules were removed from the vessel, the final pressure would be the partial pressure of the remaining species. 2D1  ,  2  ,  4  ,  5  ,  6  ,  8  ,  9 pressure reduced A dimensionless pressure used with the generalized compressibility charts and other EOS's. The Reduced Pressure, Pr, is defined as the ratio of the absolute pressure of the system to the critical pressure of the species in the system. As the reduce pressure approaches one, the pressure of the system approaches the critical pressure. 2E9 pressure vacuum Pressures below atmospheric pressures. Vacuum Pressure equals atmospheric pressure minus the absolute pressure in the system. Vacuum pressures are only used when the system pressure is below atmospheric, so they generally only take positive values.  For example, when the vacuum pressure is 10 kPa, the gauge pressure is - 10 kPa. 1D3 pressure vapor The pressure that a pure liquid COULD overcome in order to boil.  Vapor pressure depends on temperature only.  If we keep the temperature constant and begin reducing the pressure starting from a very high pressure, then the vapor pressure at our fixed temperature is the pressure at which the liquid begins to boil (first bubble of vapor forms).  [atm, Pa, bar,...] 2B2  ,  6, 2C2, 2D2  ,  6  ,  8  ,  9 pressure gage 1D7 pressure ratio 9F5  ,  6 pressure transducer A sensor device that converts the pressure applied to its surface into an electrical signal.  The pressure can be determined from the electrical signal. 1D7 principle of corresponding states For all gases at the same reduced temperature and reduced pressure, many physical properties, including the compressibility factor, Z, are the same. 2E8 principle of increasing entropy 7C6  ,  7, 8A2, principle phases The three principle phases are gas, liquid, and solid. 2A4 problem solving procedure 4D1 - 10 process A Process occurs in thermodynamics when any property of a system changes. 1D1  ,  3  ,  4 process adiabatic A process in which no heat transfer across the system boundary occurs. 4B8  ,  9, 4C3 - 5, 6F11 process cyclic process irreversible 6D2 process isentropic 7E1 - 7 process isobaric A process during which the pressure remains constant. 1D3, 4E1 - 3, process isochoric A process during which the volume of the system remains constant. 1D3, 4E1 - 2  ,  5, process isothermal A process during which the temperature remains constant. 1D3, 4A17 - 18, 6F9  ,  12 process internally reversible process path 1D2 process polytropic A process in which the volume and pressure of a system are related by an equation of the form: PV^n = C , where C and n are constants. 4A17  ,  19, 7A1, 7E7 process reversible 6D2, 7B2, process steady-state A process in which properties may vary with posistion within the system, but none of the properties of the system change with time. 5C5 process transient A process during which at least one property of the system changes as a function of time.  This type of process is also known as an unsteady processes. 5D1 process path The actual series of states that the system passes through as it moves from the initial state to the final state during a process. 3D1 - 4, 4A17, 4E1 process path hypothetical 3D4 - 22, 3E1  ,  4 - 9, property Also known as thermodynamic properties.  Any characteristic of a system is a property of the system.  Thermodynamic properties generally relate to the pressure, volume, temperature or energy of a system. 1C4 property extensive 1C4  ,  5, 3C4, property intensive 1C4 - 6, 3C4, 3D1 - 4 property molar 1C4 property specific 1C4 pseudo-component 2A2 psia 1D2 psig 1D2 PT diagram 2B8 pump 6B5, 6E17, pure substance A pure substance has uniform chemical composition.  This usually means that only one type of molecule is present.  But a mixture such as air can be considered to be a pure substance as long as the composition of the air is the same throughout the system and none of the molecules that make up air (principally O2 and N2) participate in a chemical reaction. 2A1 - 3, 2D1, PV diagram 2B6, 4A9 - 13, 7B11 PV diagram area under a process curve 7D3 PV diagram Carnot cycle 6E3 - 23 PV diagram enclosed area 7B14 PV diagram power cycle 7B14 PV work 4A9, 5B6,