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Transcript
19 Second Law Thermo • Heat Engines and 2nd Law Thermodynamics • Hk: 27, 35. Second Law of Thermodynamics • Heat flows spontaneously from a substance at a higher temperature to a substance at a lower temperature. • The reverse situation does not occur spontaneously. • Ex. Hot drink cools but does not reheat Heat Engines • • • • Device that uses heat to perform work Hot Reservoir (e.g. steam) Cool Reservoir (e.g. pool of water) Efficiency is work done per unit of input heat (e = W/QH) • Ex. A heat engine does 100J of work when given 300J from the hot reservoir. The efficiency is 100J/300J = 0.33 = 33%. Heat Engine Flow Chart Summary • Heat engines use part of heat and reject rest, efficiency = work out/heat in • 2nd Law of Thermodynamics prescribes that efficiency cannot be 100% • / 19-2 Refrigerators and the Second Law of Thermodynamics Refrigerators and the Second Law of Therm. Omit 19-3 Equivalence of the HeatEngine and Refrigerator Statements 19-4 The Carnot Engine Carnot Theorem The Carnot Cycle Heat Engine Omit 19-5 Heat Pumps 19-6 Irreversibility and Disorder Irreversible Processes Introduce Disorder. Example: Container of gas collides inelastically with wall. Translational kinetic energy is added to general thermal energy of gas 19-7 Entropy Entropy • Entropy is a measure of the disorder of a system • Low entropy (ordered) • High entropy (disordered) • Reversible Process: Entropy is constant • Irreversible Process: Entropy increases Second Law in terms of Entropy • the total entropy of the universe does not change when a reversible process occurs. • the total entropy does increase when an irreversible process occurs. Omit 19-8 Entropy and the Availability of Energy Omit 19-9 Entropy and the Probability Summary: • Heat engines accept heat and output work and heat. Refrigerators reverse this process. Carnot engine is most efficient engine. • 2nd Law of Therm. dictates the direction of natural processes. • Irreversible processes increase disorder. • Entropy is a measure of disorder and increases for any irreversible process. Heat engines typically have efficiencies in the range 1. 3% to 5% 2. 20% to 60% 3. 80% to 90% 0% 1 0% 2 0% 3