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Unformatted text preview: Tan 1 3. THE SECOND AND THIRD LAWS OF THERMODYNAMICS The First Law of Thermodynamics defined the role heat plays in the energy conserva- tion/transfer processes. Just as the role of work, heat is not a conserved entity of a physical sys- tem. However, what the First Law does not address is the conditions and extent governing the conversion of one form of energy to another. The Second Law of thermodynamics addresses these aspects, which owes its development to steam engine. Since the invention of the steam en- gine by James Watt in 1769, engineers had been greatly interested in the process of converting heat into work using such engines. Such an engine essentially operates as follows: (i) a source of heat (e.g., a coal or wood fire) is used to heat a working substance (e.g., steam), causing it to ex- pand through a valve into a cylinder fitted with a piston, (ii) the expansion drives the piston for- ward, and mechanical work is done by the engine to a load coupled to the piston; (iii) the work- ing substance is cooled by the expansion and is then withdrawn from the cylinder through a valve; (iv) a flywheel arrangement returns the piston to its original position, in readiness for an- other expansion stroke. Such engines operate in cycles and involve irreversible processes. Through these operations, the engine intakes heat from a heat source or reservoir, converts some of this heat into work, and discards the remainder to a heat sink or reservoir. The Second Law of Thermodynamics is formulated through the studies of these engines. 3.1 Reversible and Irreversible/Spontaneous Processes A reversible process is a process for which, after reaching the final state and then traversed backward along exactly the same path to restore the initial state of the system, neither the system nor its surrounding or reservoir has experienced any changes, i.e., it is as if the process has not been carried out. A reversible process is possible if and only if all the states the process traversed through are equilibrium states. All real processes, including those of all natural phenomena and all laboratory experiments, however, consist of irreversible (or spontaneous) processes. Nonethe- less, reversible processes serve the purpose of providing models for analyzing real processes as well as for setting upper limits of efficiencies of engineering devices that produce work, e.g., automobile engines, and lower limits of performance factors of engineering devices that con- sume work, e.g., air-conditioners. All real processes are irreversible processes because of the presence of irreversibilities . The irreversibilitis are due to existence of a potential gradient of the involved energies of the process, e.g., a temperature gradient or a pressure gradient, either inside Tan 2 the system (internal), or between the system and its surrounding (external), or both. Such gradi- ents lead to irreversiblilities in the process, because when left alone the gradient always tends to smooth out, and once smoothed out will never reverse back to its initial condition naturally. smooth out, and once smoothed out will never reverse back to its initial condition naturally....
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This note was uploaded on 07/13/2011 for the course ME 218 taught by Professor Dr.tan during the Fall '11 term at Duke.
- Fall '11