講者： 許永昌 老師 1

Contents 2

Turning Heat into Work ( 請預讀 P567) Example: The gas do positive work on the piston. W<0 (work done by the environment on the system) W s =  W (work done by the system) W+Q=  E th. Q=W s +  E th. Based on the 2 nd Law T flame > T f > T i. Q W 3

Energy Transfer diagram ( 請預讀 P568) P V 4

Energy Transfer diagram (continue) 5

Energy Transfer Diagram (continue) 6 ThTh TcTc ThTh TcTc No perfect engine

Heat engine: Thermal Efficiency ( 請預讀 P570~P572) 7

A Heat-Engine Example Description: PV diagram: Energy Transfer diagram: 323 3131 1212

Refrigerators 9 ThTh TcTc Adiabatic process Tc’Tc’ Th’Th’

No perfect Heat Engine Proof: Suppose we had a perfect engine We could use its output to provide the work input to the refrigerator. The combined system becomes a perfect refrigerator. It conflicts with the 2 nd Law of thermodynamics. In fact, we can get the upper limit of the efficiency. 10

Homework Student Workbook

Ideal-Gas Heat Engines ( 請預讀 P575~P579) 12

Ideal-gas summary 13 Table 19.1 Table 19.2  ProcessGas LawWork WHeat Q  E th Isochoric  V=0 0 Q=nC V  T  E th =Q Isobaric  P=0 PVPVQ=nC p  T  E th =Q+W Isothermal  T=0 nRTln(V f /V i ) Q=WQ=W  E th =0 Adiabatic nC V  T Q=0  E th =W PV=nRT Area under curve  E th =Q+W MonatomicDiatomic E th 3/2*nRT5/2*nRT CVCV 3/2*R5/2*R CPCP 7/2*R  5/3~1.677/5=1.4

Work done during one full cycle 14

Stop to Think ( 請預讀 P577) Stop to think 19.3 What is the thermal efficiency of this heat engine? a b c d. 4 e. Can’t tell without knowing Q C. 15 V(m 3 ) P(Pa) Q H =4000J 40,000 20,

Some Examples of Heat Engines Otto Cycle: Diesel Cycle: Brayton Cycle: 16 Engine.gif

The Brayton Cycle ( 請預讀 P578~P579 ) 17

The conditions of T H & T C ( 請預讀 P580) 18

Homework Student Workbook

Ideal-Gas Refrigerators ( 請預讀 P579~P581) 20

Homework Student Workbook

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What is a perfectly reversible engine? 23

A perfectly reversible engine must use only two types of processes ( 請預 讀 P584) A perfectly reversible engine must use only two types of processes: Adiabatic Process: Frictionless mechanical interactions with no heat transfer (Q=0). Isothermal Process: During this process, the heat should be transferred infinitely slowly.  reversible. Any engine that uses only these two types of processes is called a Carnot engine. 24

The Carnot Cycle ( 請預讀 P585~P588) QHQH QCQC

The maximum efficiency 26

Entropy 27

For an isolated process,  S  0 28 a b 用虛線是因為不可逆 過程在過程中，一般 我們無法決定它所有 的巨觀量如 P&V 。

Homework Student Workbook: Student Textbook: 請自行製作 terms and notation 的卡片。 29