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Published byLionel Barton Modified over 9 years ago
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Unit 61: Engineering Thermodynamics Lesson 4: Properties of Pure Substances
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Objective The purpose of this lesson is to consider the relationships between pressure, specific volume and temperature for a pure substance.
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The p - V - T Surface A pure substance is homogeneous i.e. it has the same chemical composition throughout in the various combinations of its three phases (solid, liquid and gas) Consider an experiment in which a solid is contained in a piston-cylinder arrangement such that the pressure is maintained at a constant value. Heat is applied to the cylinder causing the substance to pass through all three phases.
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The p - V - T Surface SolidLiquidVapour
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The p - V - T Surface T V Solid Liquid Vapour Saturated Vapour Saturated Liquid Boiling Melting
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The p - V - T Surface If the experiment is repeated a number of times using different pressures a T – v diagram results
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The p - V - T Surface solid S - L Liquid L - V Vapour S - V T v Critical Point P = constant
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The p - V - T Surface A pressures that exceed the pressure of the critical point, there is no longer a distinction between a liquid and a vapour: the substance is referred to as a supercritical fluid. The data obtained in an actual experiment could be presented as a three-dimensional surface with P = P(v,T)
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The p - V - T Surface
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The P- v – T surface may be projected onto the P – v, T – v and the P – T plane
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