Chapter 10-3 The Second Law of Thermodynamics St. Augustine Preparatory School January 26, 2017

So What is the Second Law of Thermodynamics? “No cyclic process that converts heat entirely into work is possible.” “Heat will not flow spontaneously from a cold object to a hot object.” “Any system which is free of external influences becomes more disordered with time. This disorder can be expressed in terms of the quantity called entropy. “

Cyclic Processes Cyclic processes are thermodynamic processes in which a system returns to the same conditions under which it started. Heat engines use heat to do work (convert thermal energy into mechanical energy) and are cyclical processes. A cyclic process resembles an isothermal process in that all energy is transferred as work and heat. But now the process is repeated with no net change in the system’s internal energy

Entropy Definition: A measure of the randomness or disorganization of a system.

Heat Engines (also called Carnot Engine) A basic heat engine model consists of the following: Heat is transferred into the engine The engine uses that energy to do work Leftover heat is expelled from the engine. Point number 3 is the basis for the Second Law of Thermodynamics: No cyclic process that converts heat entirely to work is possible.

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Calculating the Efficiency of a Heat Engine Formula: Efficiency is unitless and will be: 0 ≤ eff < 1

Practice Problem Calculate the efficiency of a heat engine that receives 204 J of energy from combustion and loses 153 J as heat to the exhaust.

Practice Problems Page 351 #1 – 4 Answers: 0.1504 0.59 a. 0.247 b. 4.9 x 104 J 210 J