Thermodynamics

Chapter 1 | 39SUMMARYIn this chapter, the basic concepts of thermodynamics areintroduced and discussed. Thermodynamics is the science thatprimarily deals with energy. The first law of thermodynamicsis simply an expression of the conservation of energy principle,and it asserts that energy is a thermodynamic property.The second law of thermodynamics asserts that energy hasquality as well as quantity, and actual processes occur in thedirection of decreasing quality of energy.A system of fixed mass is called a closed system, or controlmass, and a system that involves mass transfer across itsboundaries is called an open system, or control volume. Themass-dependent properties of a system are called extensiveproperties and the others intensive properties. Density is massper unit volume, and specific volume is volume per unit mass.A system is said to be in thermodynamic equilibrium if itmaintains thermal, mechanical, phase, and chemical equilibrium.Any change from one state to another is called aprocess. A process with identical end states is called a cycle.During a quasi-static or quasi-equilibrium process, the systemremains practically in equilibrium at all times. The stateof a simple, compressible system is completely specified bytwo independent, intensive properties.The zeroth law of thermodynamics states that two bodiesare in thermal equilibrium if both have the same temperaturereading even if they are not in contact.The temperature scales used in the SI and the English systemtoday are the Celsius scale and the Fahrenheit scale,respectively. They are related to absolute temperature scales byT 1K2 T 1°C2 273.15T 1R2 T 1°F2 459.67The magnitudes of each division of 1 K and 1°C are identical,and so are the magnitudes of each division of 1 R and1°F. Therefore,¢T 1K2 ¢T 1°C2and¢T 1R2 ¢T 1°F2The normal force exerted by a fluid per unit area iscalled pressure, and its unit is the pascal, 1 Pa 1 N/m 2 .The pressure relative to absolute vacuum is called theabsolute pressure, and the difference between the absolutepressure and the local atmospheric pressure is called thegage pressure. Pressures below atmospheric pressure arecalled vacuum pressures. The absolute, gage, and vacuumpressures are related byP gage P abs P atm 1for pressures above P atm 2P vac P atm P abs 1for pressures below P atm 2The pressure at a point in a fluid has the same magnitude inall directions. The variation of pressure with elevation isgiven bydPdz rgwhere the positive z direction is taken to be upward. Whenthe density of the fluid is constant, the pressure differenceacross a fluid layer of thickness z is¢P P 2 P 1 rg ¢zThe absolute and gage pressures in a liquid open to the atmosphereat a depth h from the free surface areP P atm rghorP gage rghSmall to moderate pressure differences are measured by amanometer. The pressure in a stationary fluid remains constantin the horizontal direction. Pascal’s principle states thatthe pressure applied to a confined fluid increases the pressurethroughout by the same amount. The atmospheric pressure ismeasured by a barometer and is given byP atm rghwhere h is the height of the liquid column.REFERENCES AND SUGGESTED READINGS1. American Society for Testing and Materials. Standardsfor Metric Practice. ASTM E 380-79, January 1980.2. A. Bejan. Advanced Engineering Thermodynamics. 2nded. New York: Wiley, 1997.3. J. A. Schooley. Thermometry. Boca Raton, FL: CRCPress, 1986.