Work can always be converted to heat directly and completely, but the reverse is not true. Heat engines and other cyclic devices usually involve a fluid to and from which heat is transferred while undergoing a cycle. They convert part of this heat to work (usually in the form of a rotating shaft.) They reject the remaining waste heat to a low-temperature sink (the atmosphere, rivers, etc.). They receive heat from a high-temperature source (solar energy, oil furnace, nuclear reactor, etc.). In practice, large bodies of water such as oceans, lakes, and rivers as well as the atmospheric air can be modeled accurately as thermal energy reservoirs because of their large thermal energy storage capabilities or thermal masses.Ħ HEAT ENGINES The devices that convert heat to work. A hypothetical body with a relatively large thermal energy capacity (mass x specific heat) that can supply or absorb finite amounts of heat without undergoing any change in temperature is called a thermal energy reservoir, or just a reservoir. Bodies with relatively large thermal masses can be modeled as thermal energy reservoirs. The second law of thermodynamics is also used in determining the theoretical limits for the performance of commonly used engineering systems, such as heat engines and refrigerators, as well as predicting the degree of completion of chemical reactions.Ī source supplies energy in the form of heat, and a sink absorbs it. The second law provides the necessary means to determine the quality as well as the degree of degradation of energy during a process. The first law is concerned with the quantity of energy and the transformations of energy from one form to another with no regard to its quality. The second law also asserts that energy has quality as well as quantity. MAJOR USES OF THE SECOND LAW The second law may be used to identify the direction of processes. Transferring heat to a wire will not generate electricity.Ĥ Processes occur in a certain direction, and not in the reverse direction.Ī process must satisfy both the first and second laws of thermodynamics to proceed. These processes cannot occur even though they are not in violation of the first law. Transferring heat to a paddle wheel will not cause it to rotate. Determine the expressions for the thermal efficiencies and coefficients of performance for reversible heat engines, heat pumps, and refrigerators.Ī cup of hot coffee does not get hotter in a cooler room. Examine the Carnot principles, idealized Carnot heat engines, refrigerators, and heat pumps. Apply the second law to develop the absolute thermodynamic temperature scale. Apply the second law of thermodynamics to cycles and cyclic devices. Discuss the concepts of perpetual-motion machines. Describe the Kelvin–Planck and Clausius statements of the second law of thermodynamics. Discuss thermal energy reservoirs, reversible and irreversible processes, heat engines, refrigerators, and heat pumps. Identify valid processes as those that satisfy both the first and second laws of thermodynamics. Permission required for reproduction or display.Ģ Objectives Introduce the second law of thermodynamics. Boles McGraw-Hill, 2011© Chapter 6 THE SECOND LAW OF THERMODYNAMICS Mehmet Kanoglu Copyright © The McGraw-Hill Companies, Inc. Thermodynamics: An Engineering Approach, 7th Edition Yunus A. Second Law of Thermodynamics: All systems tend to go from a state of greater organization to a state of lesser organization with a concommitant loss of usable energy.1 Chapter 6 THE SECOND LAW OF THERMODYNAMICS How do the first and second laws of thermodynamics apply to photosynthesis?įirst Law of Thermodynamics: Energy cannot be created or destroyed, but it can be changed in form. Living systems can not be closed systems or they are not living. No The Second Law of thermodynamics applies in the truest sense to closed systems. Human organisms are not a closed system and thus the energy input and output of an the organism is not relevant to the second law of thermodynamics directly. Does the formation of living organisms violate the second law of thermodynamics? Some energy is reflected and some is lost as heat. In photosynthesis, for example, not all of the light energy is absorbed by the plant. Two statements Is photosynthesis An example of the second law of thermodynamics?Īs with other biological processes, the transfer of energy is not 100 percent efficient.