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Identify the unique vocabulary associated with thermodynamics through the precise definition of basic concepts to form a sound foundation for the development of the principles of thermodynamics. • Review the metric SI and the English unit systems. • Explain the basic concepts of thermodynamics such as system, state, state postulate, equilibrium, process, and cycle. • Review concepts of temperature, temperature scales, pressure, and absolute and gage pressure.
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Muhammad Ahmad KFUEIT R.Y.K Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Thermodynamics: An Engineering Approach Seventh Edition in SI Units Yunus A. Cengel, Michael A. Boles McGraw-Hill, 20 11
The molecules in a solid are kept at their positions by the large springlike inter-molecular forces. In a solid, the attractive and repulsive forces between the molecules tend to maintain them at relatively constant distances from each other.
It probably came as no surprise to you that water started to boil at 100 °C. Strictly speaking, the statement “water boils at 100 °C” is incorrect. The correct statement is “water boils at 100 °C at 1 atm pressure.” The only reason water started boiling at 100 °C was because we held the pressure constant at 1 atm ( 101. 325 kPa). If the pressure inside the cylinder were raised to 500 kPa by adding weights on top of the piston, water would start boiling at 151. 8 °C. That is, the temperature at which water starts boiling depends on the pressure; therefore, if the pressure is fixed, so is the boiling temperature. At a given pressure, the temperature at which a pure substance changes phase is called the saturation temperature T sat. Likewise, at a given temperature, the pressure at which a pure substance changes phase is called the saturation pressure P sat. At a pressure of 101. 325 kPa, T sat is 99. 97 °C. Conversely, at a temperature of 99. 97 °C, P sat is 101. 325 kPa. (At 100. 00 °C, P sat is 101. 42 kPa).
Saturation Temperature and Saturation Pressure
The general shape of the P - v diagram of a pure substance is very much like the T - v diagram, but the T constant lines on this diagram have a downward trend. THE PV DIAGRAMS FOR PHASE-CHANGE
Consider again a piston–cylinder device that contains liquid water at 1 MPa and 150 °C. Water at this state exists as a compressed liquid. Now the weights on top of the piston are removed one by one so that the pressure inside the cylinder decreases gradually (Fig. 3 – 20 ). The water is allowed to exchange heat with the surroundings, so its temperature remains constant. As the pressure decreases, the volume of the water increases slightly. When the pressure reaches the saturation-pressure value at the specified temperature ( 0. 4762 MPa), the water starts to boil. During this vaporization process, both the temperature and the pressure remain constant, but the specific volum e increases. Once the last drop of liquid is vaporized, further reduction in pressure results in a further increase in specific volume. Notice that during the phase-change process, we did not remove any weights. Doing so would cause the pressure and therefore the temperature to drop [since T sat f ( P sat)], and the process would no longer be isothermal When the process is repeated for other temperatures, similar paths are obtained for the phase-change processes. Connecting the saturated liquid and the saturated vapor states by a curve, we obtain the P - v diagram of a pure substance. THE PV DIAGRAMS FOR PHASE-CHANGE
Saturated Liquid and Saturated Vapor States
Saturated liquid and saturated vapor states of water on T-v and P-v diagrams.
y v , u , or h.
mixture states on T-v and P-v diagrams.
