Module 11-Second Law of Thermodynamics

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Second Law of Thermodynamics Created by: Marlon Flores Sacedon Physics section, DMPS June 2010

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The Second Law of Thermodynamics Many thermodynamic processes proceed naturally in one direction but not opposite. Example: Heat always flows from hotter body to cooler body, never reverse. Heat flows from cooler body to hotter body violates the First Law of Thermodynamics, energy would be conserved. But it doesn’t happen in nature . Why not? It is easy to convert mechanical energy complete into heat Introduction Hot Cool Hot

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Converts from mechanical energy to heat energy Yes! possible to happen. By gravity No! impossible to happen by gravity Yes! possible to happen by machine engine Converts from heat energy to mechanical energy Inventors have never succeeded in building a machine that converts heat completely into mechanical energy. Again why or why not? The answer to both of these questions has to do with the directions of thermodynamic processes and is called… The Second Law of Thermodynamics

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The Second Law of Thermodynamics This law places fundamental limitations on the efficiency of an engine or a power plant. It also places limitations on the minimum energy input needed to operate a refrigerator. So the second law is directly relevant for many important practical problems.

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Directions of Thermodynamics Processes Thermodynamic processes that occurs in nature are all Irreversible Process These are processes that proceed spontaneously in one direction but not the other. Example of irreversible process. The flow of heat from a hot body to cooler body is irreversible. Sliding book across a table converts mechanical energy into heat by friction. Reversible Process is an idealization that can never be precisely attained in the real world. Example of reversible process. Heat flow between two bodies whose temperature differ only infinitesimal can be reversed by making only a very small change in one temperature or the other.

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Thermodynamic processes that occur in nature are all irreversible processes . These are processes that proceed spontaneously in one direction but not the other. The flow of heat from a hot body to a cold body is irreversible. However, we can think of a class of idealized processes that would be reversible. An idealized reversible process is always very close to being in thermodynamic equilibrium within itself and with its surroundings. For example, heat flow between two bodies whose temperatures differs only infinitesimally can be reversed by making only a very small change in one temperature or the other.

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While a reversible process is an idealization that can never be precisely attained in the real world, making temperature gradients and the pressure differences in the substance very small, we can keep the system very close to equilibrium states and make the process nearly reversible. For this reason, reversible processes are called equilibrium processes, with the system always in thermodynamic equilibrium.

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Heat Engines Any device that transforms heat partly into work or mechanical energy is called Heat Engine Heat Engine undergoes cyclic process. All Heat Engines absorbs heat from a source at a relatively high temperature, perform some mechanical work, and discard or reject some heat at a lower temperature. so

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Internal-Combustion Engines

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Internal-Combustion Engines

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Gasoline Engine

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Diesel Engine

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