. Applied thermodynamics for engineers. Repre-sentation of InternalEnergy. = i pdv = -^â where v is the initial volume, p the initial pressure, and y the adiabaticexponent. This is a finite and commensurable quantity. 120. Representation by Isodynamic Lines. A defect of the precedingrepresentation is that the areas cannot be included on a finite diagram. GRAPHICAL REPRESENTATIONS 63 In rig. S6j consider the path AB. Let -BC be an adiabatic and AC an-isodynamie. It is required to find the change of internal energy betweenA and B. The external work done during adi-abatic expansion from Bio C is
. Applied thermodynamics for engineers. Repre-sentation of InternalEnergy. = i pdv = -^â where v is the initial volume, p the initial pressure, and y the adiabaticexponent. This is a finite and commensurable quantity. 120. Representation by Isodynamic Lines. A defect of the precedingrepresentation is that the areas cannot be included on a finite diagram. GRAPHICAL REPRESENTATIONS 63 In rig. S6j consider the path AB. Let -BC be an adiabatic and AC an-isodynamie. It is required to find the change of internal energy betweenA and B. The external work done during adi-abatic expansion from Bio C is equal to BCcb;and this is equal to the change of internal en-ergy between B and C. But the internal energyis the same at C as at A, because AC is anisodynamic. Consequently, the change of in-ternal energy between ^ and B is representedby the area BCcb; or, generally, by the areaincluded between the adiabatic through the finalstate, extended to its intersection with the iso-dynamic through the initial state, and the hori-zontal Fig. 36. Arts. 120, 121. âIn-ternal Energy, Second Dia-gram. 121. Source of External Work. If in Fig. 36 the path is such as to increasethe temperature of the substance, or even to keep itstemperature from decreasing as much as it wouldalong an adiabatic, then heat must be , comparing the paths ad and ac, Fig. 37, aNand cm being adiabatics, the external work donealong ad is adef, no heat is absorbed, and the internalenergy decreases by adef. Along ac, the externalwork-done is acef, of which ar/e/was done at the ex-pense of the internal energy, and acd by reason ofthe heat absorbed. The total heat absorbed was Nacm, of which acd was expended in doing external work, while Ndcm went to increase the stock of internal energy.
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