ch05_slides - Chapter 5 The 2nd Law of Thermodynamics This...

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Unformatted text preview: Chapter 5: The 2nd Law of Thermodynamics This 1000 hp engine photo is courtesy of Bugatti automobiles. Motivations • 1st Law: Energy is a conserved property. • However, satisfying the 1st Law does not ensure the process will actually take place. • How to determine the direction of a process? • How to determine the theoretical limits of energy systems, such as engines and refrigerators? Spontaneous Processes Objects spontaneously tend to cool Fluids move from higher to lower pressure environments spontaneously Objects spontaneously fall from elevated positions Spontaneous processes allows occur in a predictable direction, and have the potential to produce work The Many Uses of the 2nd Law • • • • • Predict process direction Establish equilibrium conditions Determine theoretical best performance Evaluate factors limiting best performance Define a temperature scale independent of properties • Develop means for property evaluation for derived properties, such as h and u Thermal Reservoirs • Thermal Energy Reservoir: 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. • What can be modeled as thermal reservoirs? Heat Engines TH th = Wcycle Qin Qout QC =1 =1 Qin QH TH Can we save Qout? See notes. TC Statements of the 2nd Law • Kelvin-Planck Statement It is impossible for any system to operate in a thermodynamic cycle and deliver a net amount of energy by work to its surroundings while receiving energy from a single thermal reservoir Notes: • Negative statement, impossible to prove; • Alternative statement: “no heat engine can have a thermal efficiency of 100 percent”; • The impossibility to achieve 100% efficiency is NOT due to friction or other dissipative effects; * Wcycle 0 Single reservoir Refrigerators / Heat Pumps COP for a refrigerator: Qc QC = = Wcycle QH QC COP for a heat pump: QH QH = = Wcycle QH QC Statements of the 2nd Law • Clausius Statement • It is impossible for any system to operate in such a way that the sole result would be an energy transfer by heat from a cooler to a hotter body Equivalence of the Two Statements Violation of the Kelvin-Plank statement 3 Violation of the Clausius statement 1 2 Thus a violation of the Clausius statement implies a violation of the Kelvin-Plank statement. See notes for another example. Reversible and Irreversible Processes • Reversible Process: Both the system and surroundings can be returned to their initial states. – Internally reversible process: there are no irreversibility within the system – Externally reversible process: no irreversibility occur outside the system boundaries during the process – Reversible process = Internally reversible + Externally reversible • Irreversible Process: not reversible. • The second Law can be used to determined whether a given process is reversible or irreversible. • , and reach their maximum when the cycles are reversible. Irreversibilities • Friction (sliding and flow) • Heat transfer through a finite temperature difference – Since heat transfer occur only when there is a temperature difference between a system and its surroundings, it is physically impossible to have a reversible heat transfer process. – However, heat transfer through a differential temperature difference dT can be considered to be reversible. • • • • • • • Unrestrained expansion of a gas or liquid Spontaneous chemical reaction Spontaneous mixing Electric current flow through a resistance Magnetization or polarization with hysteresis Inelastic deformation And many more … Carnot corollaries of the 2nd Law . th ,irrev < th , rev • All reversible power cycles operating between the same two thermal reservoirs have the same thermal efficiency (regardless the nature of the substance or the series of processes). -> the efficiency must be related only to the nature of the reservoirs. W cylce < 0 single reservoir WI WR < 0 The Kelvin Temperature Scale In general, th QC =1 QH th , rev For a reversible process, and QC ( ) rev = f (TH , TC ) QH QC = 1 ( ) rev QH The Kelvin temperature scale is define in the way so that: QC TC ( ) rev = QH TH The Triple Point temperature of water is assigned to be 273.16 K. The Gas Thermometer Maximum Performance Heat Engines Refrigerators & Heat Pumps max max TC =1 TH = TC TH TC max = TH TH TC Tc=298K Carnot Cycle Reversible power cycle: Two adiabatic processes alternated with two isothermal processes Carnot power cycles operated in reverse may be regarded as a reversible refrigeration or heat pump cycle, with maximum coefficient of performance ...
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This note was uploaded on 03/15/2010 for the course ENGRD 2210 at Cornell.

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