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Proprties and state of a substance




A thermodynamic property is any measurement or quantity which serves to describe a system. Thermodynamic properties are either intensive or extensive. Intensive properties are independent of mass. Temperature, pressure and density are intensive properties. Extensive properties vary directly with mass. Mass and total volume are extensive properties.

A property of a pure, simple, compressive substance can always be defined in terms of two independent intensive properties. For example, the pressure of a gas can be expressed in terms of its temperature and specific volume: P = p(T,v) (3-5). A pure substance is also homogeneous and of fixed chemical composition. We sometimes speak of air as being pure: however, thermodynamically it is a mixture of several gases and vapors.

A phase is a quantity of matter which is homogeneous throughout. A substance may exist in any one or a combination of three phases - solid, liquid and vapor. Two or more phases may coexist when in a common state, identified by two or more observable properties such as temperature and pressure. Change of phase and phase equilibrium can be understood by considering water. At a pressure of 14.7 psia water is a solid (ice) when below 32F solid, vapor and liquid water can coexist. Further increases in temperature cause the liquid water to vaporize (turn to steam) until it is 100 percent water above 212F. During this transition the quality x, ratio of the mass of vapor to the total mass changes from 0 to 1.00.

Work. Work, one of the basic quantities transferred during a thermodynamic process, is defined from elementary mechanics as a force F acting through a displacement x, where x is positive in the direction of the force; i.e., W = (3-6).

This basic relation enables us to determine the work required to raise weights, propel missiles, etc. But this definition of work is too limited for thermodynamics, where the concern is with the interactions between a system and its surroundings. Therefore, we shall define work compatible with our concepts of systems, properties and processes. Hence, work is done by a system if the sole effect external to the system (on the surroundings) could be the raising of a weight. Work done by a system in assumed to be positive and work done on a system is considered negative. This definition does not state that a weight is raised or that a force actually acts through a distance. This definition is necessary because of the need to distinguish between work and heat in the second law of thermodynamics. The term "sole effect" in the definition of work implies that another effect might be external to the system.

The term "external" in the definition of work suggests that work is defined only with reference to a system boundary.

Heat. The other form of energy of significance in transfer processes, heat, is defined in terms of temperature. Heat is the energy which is transferred across the boundaries of a system interacting with the surroundings by virtue of a temperature difference.

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. , , , F x, x ; .., W = (3-6).

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