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1 Chapter 2 Control Volumes and Units 2.1 A thermodynamics system and the control volume Thermodynamic system is a device or combination of devices containing a quantity of matter (working fluids; 工作流體 )that is being studied. Fluids:Liquid and Gas (Ex:Water, Air) 機械工程系陳俊勳老師 1

2 All the laws of mechanics are written for a system, which is defined as an arbitrary quantity of mass of fixed identity. Boundaries Surrounding System( 特定對象 ) 力學定律用來描述系統 (system) 和其周遭環境 (surrounding) 之間的交互作用機械工程系陳俊勳老師 2

3 Basic Laws for a System Conservation of Mass 機械工程系陳俊勳老師 3

4 Basic Laws for a System Momentum Equation P MV 機械工程系陳俊勳老師 4

5 Basic Laws for a System The First Law of Thermodynamics 機械工程系陳俊勳老師 5

6 Basic Laws for a System The Second Law of Thermodynamics 機械工程系陳俊勳老師 6

7 機械工程系陳俊勳老師 7

8 Control Volume (C.V.) : An imagination is chosen so that it contains the matter and devices inside a control surface (C.S.;boundary of control volume). Surrounding:Outside the control volume Closed System C.V. Open System 機械工程系陳俊勳老師 8

9 Relation of System Derivatives to the Control Volume Formulation Intensive Properties:Values are Independent of Mass;Ex. P, T, ρ Extensive Properties: Values vary with mass; Ex. Mass, volume. 機械工程系陳俊勳老師 9

10 Relation of System Derivatives to the Control Volume Formulation Reynolds Transport Theorem 機械工程系陳俊勳老師 10

11 Mass, momentum and energy can flow across the control surface. (Fig. 2.2, P.14) Control mass: no mass allows to flow across the control surface, which may not be fixed. (Fig. 2.1, P.14) Isolated system;a system that is not influenced in any way by the surrounding. no mass, momentum and energy cross the boundary. Ex. Universe 機械工程系陳俊勳老師 11

12 Closed system: fixed mass Open system:mass can flow across the boundaries 機械工程系陳俊勳老師 12

13 2.2 Macroscopic point of view ( 巨觀 ) versus Microscopic point of view ( 微觀 ) Describe system by microscopic method too many moleculars to be handled Ex. 1 mole of air 6 x moleculars Consider velocity (3 components) only 1.8 x components impossible 機械工程系陳俊勳老師 13

14 Solutions: Statistical approach: Average probability Kinetic theory Statistical mechanics Macroscopic viewpoint; P, K.E., P.E. Classical thermodynamics Variables can be measured by instruments directly or indirectly Time-averaged and gross effects of many molecules 機械工程系陳俊勳老師 14

15 Continuum:the basic assumption for classical thermodynamics and fluid mechanics Physical length >> mean free path of molecules Examples: P lim A A* F A lim A A* M V 機械工程系陳俊勳老師 15

16 2.3 Properties and state of substance Phase ( 相 ): various form of substance Solid, Liquid and Gas Fluid A phase is defined as a quantity of matter that is homogeneous throughout. State ( 狀態 ): Can be described by certain observable, macroscopic properties, such as temperature, pressure, density, etc. 機械工程系陳俊勳老師 16

17 Each of the properties of a substance in a given state has only one definite value, and is independent of the path. (or process) 不和如何達到此狀態的過程有任何關聯 Minimum number of properties (in equilibrium) must be specified to fix the state of the substance. 機械工程系陳俊勳老師 17

18 Thermodynamic properties (substance) Intensive properties:independent of mass Ex. T, ρ, P Extensive properties Ex. M, V, H(enthalpy; 焓 ), U(internal energy) Ex. Specific volume ν = V/M = 1/ρ, h = H/M, u = U/M 機械工程系陳俊勳老師 18

19 Equilibrium: for a system (not a substance) Thermal equilibrium T (property) will not change with time. (Fig. 2.1, p.14) Note:Allow heat to flow in and out. Mechanical equilibrium P will not changes with time (Variation in height is negligible) Chemical equilibrium (Chapter 16) Thermodynamic equilibrium: when a system is in equilibrium regarding all possible changes of state. 機械工程系陳俊勳老師 19

20 2.4 Processes and Cycles In a System T 1 T 2 P 1 process P 2 ρ 1 (succession of states) ρ 2 M 1 (path) M 2 State 1 state 2 (equilibrium) (equilibrium) Quasi-equilibrium slow process (deviation from thermodynamic equilibrium is infinitesimal) See Fig. 2.3 for Ex. (p. 16) Non equilibrium process:during the change of states, the system would not be inequilibrium at any time. (actual situation) 機械工程系陳俊勳老師 20

21 Isothermal process T 1 = T 2 = T (constant temperature) Isobaric process P 1 = P 2 = P (constant pressure) Isochoric process V 1 = V 2 = V (constant volume) 機械工程系陳俊勳老師 21

22 Cycle: the system undergoes a cycle when process 1-2 state 1 state 2 process 2-3 process 2-3 state 3 Thermodynamic cycle: the substance throughout the cycle is the same. Ex. Steam power plant (see next page) 機械工程系陳俊勳老師 22

23 Mechanical cycle:4-stroke I-C engine;otto cycle;the working fluid changes its own compositions in different processes during the cycle. ( 見第一章模型 ) 機械工程系陳俊勳老師 23

24 2.5 Units for mass, Length, time and force (Kg) (m) (sec) (Newton) Basic quantities: M, L, t and T (Temperature) F =ma; 1 N = 1 Kg.m/s 2 Force mass; F, L, t and T SI system ( 公制 ) English system ( 英制 ) Reading assignment 機械工程系陳俊勳老師 24

25 2.6 Energy Generate Force Energy can be stored within a system (microscopic), 內能 and can be transferred from one system to another in forms of work and/or heat (macroscopic). 機械工程系陳俊勳老師 25

26 2.7 Specific volume and density Specific volume:see powerpoint # 15 Satisfied with the requirement of continuum lim V V V m (see Fig. 2.7, p.22) ' ; unit: m 3 /Kg Density:ρ = 1 / ν Discussion of Fig. 2.8 (p. 22) Example 2.2 (p.23), note the remark 機械工程系陳俊勳老師 26

27 2.8 Pressure P lim A A ' F A F n A Remind that both force and area are vector, but pressure is scalar Unit:Pascal (Pa) = N/m 2 Standard Atmosphere ( 標準大氣壓 ) 1 atm = 101,325Pa = lb f /in 2 1 Bar = 0.1 Mpa 機械工程系陳俊勳老師 27

28 Some Comments for Fig and Ex. 2.3 Absolute pressure = Atmosphere pressure + Gauge pressure (Fig. 2.12, p. 26) Atmosphere pressure: known value Gauge pressure: frequently measured by manometer (filled with Hg, H 2 O etc.). Principles of hydrostatics (1) 在同一流體中, 在相同高度處所受之壓力相等 (2) 不同高度處之壓力差 (ΔP) 為 ΔP = ρlg 機械工程系陳俊勳老師 28

29 ρ: 所在流體的密度 (Ex. ρ water = 1000 kg / m 3, ρ Hg = 13.6ρ water ) L : 兩者在重力方向的高度差 g: 當地的重力加速度 (9.8 m/s 2 ) Discussion on Figs. 2.3, 2.5, and Exs. 2.4 to 2.6 ΔP =P fluid -P P fluid = P +ΔP (Gauge pressure) 機械工程系陳俊勳老師 29

30 2.9 Equality of temperature 因很直接難定義 temperature 為何? After Δt, we can say that two bodies have equality of temperature. If, when they in thermal communication, no change in any observable property (ex: ΔV,R, T) occurs. 機械工程系陳俊勳老師 30

31 The zero law of thermodynamics (logical) When two bodies have equality of temperature with a third body (T A = T C & T B = T C ), they in turn have equality of temperature with each other (T A = T B ). 機械工程系陳俊勳老師 31

32 2.11 Temperature scales At 1 atmosphere F Fahrenheit, ice point = 32F, steam point = 212F Celsius, ice point = 0, steam point = 100 Absolute temperature K = (Kelvin) R = F (Rankine) 機械工程系陳俊勳老師 32

Microsoft PowerPoint - 热学002.pptx

Microsoft PowerPoint - 热学002.pptx 热学 ( 求是科学班 ) 第二节物态和物态方程吴建澜浙江大学物理系 2014 年 1) 物体的基本宏观参数 : 2) 理想气体物态方程理想气体微观模型 3) 非理想气体物态方程 van der Waals 方程 ; Viral (Onnes) 方程 4) 相与相变的初步概