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1、Chapter 17 otherwise negative p V O Experimentally it is found that both work W and heat Q are dependent on the nature of the process However a surprising thing is that the quantity Q W is the same for all processes between two states It depends only on the initial and final states The first law of
2、thermodynamics P407 One can infer that the quantity Q W must represent a change in some intrinsic property of the system This intrinsic property is called the internal energy U which is a function of state or a state function 内能是状态的单值函数 状态量 So The First Law of Thermodynamics For a differential proce
3、ss it can be written as It is hold for any system and any process 系统从外界吸收的热量 一部分使系统的内能增加 另一部分用于对外做功 About the sign of W Q and U work done by the system W 0 system absorbs heat Q 0 system s energy increasing U 0 包括热现象在内的能量守恒定律 The internal energy U of a system tends to increase if energy is added as
4、heat Q and tends to decrease if energy is lost as work done by the system 第一类永动机 不需能量 输入而能继续做功的机器 不存在 Two ways to change system s internal energy Energy may be transferred into or out of a system as either work W or heat Q Relationship between W and Q 1 cal 4 18 J The transformation between work and
5、 heat is not direct but on the system The direct results is that the internal energy of the system changes The change of the internal energy is by the amount of work E 0 if the work is done on the system 17 2 Some Special Cases of the First Law of Thermodynamics P409 1 Adiabatic process 绝热过程 It is o
6、ne that occurs so rapidly or occurs in a system well isolated that no transfer of energy as heat between the system and its environment Q 0 特征 Q 0 U 0 system absorbs heat from its environment 0 from the high temperature reservoir and releases heat QL 0 to the low temperature reservoir and does work
7、W to the environment at the same time Using Ther I law to the whole cyclic process indicates the principle of engine In each cycle substance absorbs heat QH from TH partly does work outside partly transfers QL to TL For a cyclic process E 0 So the efficiency of an engine can be written as The effici
8、ency h of any engine is defined by 意义 在一次循环中工作物质对外所做的净功占 它从高温热源吸收的热量的比例 Eq 18 1 of P432 2 A Carnot engine and its efficiency P433 One of important reversible cycles is Carnot cycle This ideal engine turns out to be the best in principle at using energy as heat to do useful work In an ideal engine su
9、ch as Carnot engine all processes are reversible and no wasteful energy transfers occur due to friction and turbulence O p V V1 a p1 Adiabatic Isotherm p2 p3 p4 V2V3V4 b c d QL QH Carnot cycle consists of two isothermal processes and two adiabatic processes as the Fig State parameters a TH p1 V1 b T
10、H p2 V2 c TL p3 V3 d TL p4 V4 a b isothermal process TH feature is E 0 TL 高温热源TH QH QL W Operation of Carnot engine c d isothermal process TL E 0 From the adiabatic process equations in b c d a O p V V1 a p1 Adiabatic Isotherm p2 p3 p4 V2V3V4 b c d QL QH So the efficiency of the Carnot engine is giv
11、en by 1 The efficiency of the cycle of Carnot engine depends only on TL and TH 高温 低温两热源的温度 Summary 2 The efficiency of Carnot engine is always less than 1 总是小于1 3 The definition of thermodynamics scale of temperature 热力学温标 卡诺循环的效率 Eq 18 3 One mole of a monatomic ideal gas is taken through the revers
12、ible cycle shown in Fig Process bc is an adiabatic expansion with pb 10 0atm and Vb 1 00 10 3 m3 Find a the energy added to the gas as heat b the energy leaving the gas as heat c net work done by the gas and d the efficiency of the cycle Solution a a b V const W 0 Vb8Vb V p idiabatic o c b a Example
13、 3 Refrigerators 致冷机 and its efficiency P438 A refrigerator is a device that uses work to transfer energy from a low temperature reservoir to a high temperature reservoir as it continuously repeated a set of series of thermodynamic processes The efficiency of any refrigerator 致冷系数coefficient of perf
14、ormance can be defined as The Ther I law in refrigerator W QH QL Carnot Refrigerator the basic elements of an ideal refrigerator that operates in the reverse of the Carnot engine Adiabatic Isotherm O p V V1 a p1 p2 p3 p4 V2V3V4 b c d QL QH Since Carnot refrigerator is a Carnot engine operating in re
15、verse so 卡诺致冷机致冷系数 or Question Can we make an engine Suit with the first Law Precise statements of second law were proposed by Lord Kelvin and by Rudolph Clausius in the early 1850s Although worded differently their statements are equivalent 17 7 Engine Efficiency and The Second Law I Kelvin Planck
16、1824 1907 statement No device is possible whose sole effects is to transform a given amount of heat completely into work 不可能从单一个热源吸取热量 使之全部变为 功 而不产生其他影响 P432 第二类永动机不可能制成 效率 永远小于1 You should remember II Clausius 1822 1888 statement P438 No device is possible whose sole effects is to transfer heat from one system at one temperature into a second system at a higher temperature 热量不可能自动地从低温物体传向高温物体 1 Irreversible processes 不可逆 All real thermodynamic process in natural is in a certain direction 自然界一切实
