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Chapter 15 | Thermodynamics 669
Exercise 15.3
In an air conditioner, 12.65 MJ of heat transfer occurs from a cold environment in 1.00 h. (a) What mass of ice melting would involve the same heat transfer? (b) How many hours of operation would be equivalent to melting 900 kg of ice? (c) If ice costs 20 cents per kg, do you think the air conditioner could be operated more cheaply than by simply using ice? Describe in detail how you evaluate the relative costs.
Glossary
adiabatic process: a process in which no heat transfer takes place
Carnot cycle: a cyclical process that uses only reversible processes, the adiabatic and isothermal processes Carnot efficiency: the maximum theoretical efficiency for a heat engine
Carnot engine: a heat engine that uses a Carnot cycle
change in entropy: the ratio of heat transfer to temperature
coefficient of performance: for a heat pump, it is the ratio of heat transfer at the output (the hot reservoir) to the work supplied; for a refrigerator or air conditioner, it is the ratio of heat transfer from the cold reservoir to the work supplied
cyclical process: a process in which the path returns to its original state at the end of every cycle
entropy: a measurement of a system's disorder and its inability to do work in a system
first law of thermodynamics: states that the change in internal energy of a system equals the net heat transfer into the system minus the net work done by the system
heat engine: a machine that uses heat transfer to do work
heat pump: a machine that generates heat transfer from cold to hot
human metabolism: conversion of food into heat transfer, work, and stored fat
internal energy: the sum of the kinetic and potential energies of a system's atoms and molecules
irreversible process: any process that depends on path direction
isobaric process: constant-pressure process in which a gas does work
isochoric process: a constant-volume process
isothermal process: a constant-temperature process
macrostate: an overall property of a system
microstate: each sequence within a larger macrostate
Otto cycle: a thermodynamic cycle, consisting of a pair of adiabatic processes and a pair of isochoric processes, that converts heat into work, e.g., the four-stroke engine cycle of intake, compression, ignition, and exhaust
reversible process: a process in which both the heat engine system and the external environment theoretically can be returned to their original states
second law of thermodynamics: heat transfer flows from a hotter to a cooler object, never the reverse, and some heat energy in any process is lost to available work in a cyclical process
second law of thermodynamics stated in terms of entropy: the total entropy of a system either increases or remains constant; it never decreases
statistical analysis: using statistics to examine data, such as counting microstates and macrostates Section Summary
15.1 The First Law of Thermodynamics
• The first law of thermodynamics is given as , where is the change in internal energy of a system, is the net heat transfer (the sum of all heat transfer into and out of the system), and is the net work done (the sum of all