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1、Heat and Work,熱與功,2.1 Introduction,Energy cannot be created or destroyed; it can only change forms (the first law) Energy has quality as well as quantity, and actual processes occur in the direction of decreasing quality of energy. (the second law),2.2 Forms of Energy,Ability to work 功 Capacity to m
2、ove a force through a distance The total Energy全能: E , kJ/kg-Kinetic Energy 動能-Potential Energy 位能-Internal energy 內能,2.2 Forms of Energy (1),The macroscopic 巨觀的energy;A system possesses as a whole with respect to some outside reference frame,i.e. kinetic energy, potential energy. The microscopic 微觀
3、的energy;Related to the molecular structure of a system and the degree of the molecular activity, and independent of outside reference frames,2.2 Forms of Energy (2),Sensible energy顯能: Associated with the kinetic energy of the molecules Latent energy潛能: Associated with the phase of a system Chemical
4、energy化學能: Associated with the atomic bonds in a molecule Nuclear energy核能: Associated with the strong bonds within the nucleus of the atom itself Thermal energy熱能: The forms of energy associated with temperature,FIGURE 25 The various forms of microscopic energies that make up sensible energy.,Copyr
5、ight The McGraw-Hill Companies, Inc. Permission required for reproduction or display.,1-8,2-3 Energy transfer by Heat,Energy can cross the boundaries of a closed system in the form of heat and work. Heat : The form of energy that is transferred between two systems(or a system and its surroundings) b
6、y virtue of a temperature difference.-Heat is transferred from hot bodies to colder ones) by virtue of a temperature difference.-Energy is recognized as heat transfer only as it crosses the system boundary.,2-3 Energy transfer by Heat,Adiabatic Process: A process during which there is no heat transf
7、er. -During an adiabatic process, a system exchanges no heat with its surroundings.-Energy is recognized as heat transfer only as it crosses the system boundary.,Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.,3-1,FIGURE 2-11 Specifying the directions of he
8、at and work.,2-4 Energy Transfer by Work,Energy can cross the boundaries of a closed system in the form of heat and work. - If the energy cross the boundaries of a closed is not heat, it must be work.-Work is the energy transfer associated with a force acting through a distance.,WORK,Processes 過程,In
9、teractions,Mechanical work flow,Mechanical Work 機械功,Work at a system boundary.,Deformation變形 of the system boundary,Closed system with boundary deformation,Work transfer at a boundary,Evaluating work at a boundary.,Total work done,Component of work due to expansion of the gas,Work to raise the pisto
10、n,Electrical Work,Electrical work電功-the generalized force is the voltage, the generalized displacement is the electrical charge We = VI- V the potential difference- I the current (number of electrical charges flowing per unit time),2-5 Mechanical forms of Work,Shaft work軸功 Spring work彈簧功 Other mecha
11、nical forms of work,2-6 Moving Boundary Work,The work associated with a moving boundary is called moving boundary work. The expansion膨脹 and compression 壓縮works,Work of Expansion: p-dV work,Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.,3-2,FIGURE 4-2 A gas
12、 does a differential amount of work dWb as it forces the piston to move by a differential amount ds.,Evaluating an equilibrium expansion process,Evaluating the work Integral for a quasi-static process,Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.,3-3,FIGU
13、RE 4-3 The area under the process curve on a P-V diagram represents the boundary work.,Moving boundary Work,The area under the process curve on a P-V diagram represents the boundary work. -The boundary work done during a process depends on the path followed as well as the end states. -The net work d
14、one during a cycle is the difference between the work done by the system and the work done on the system.,Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.,3-4,FIGURE 4-5 The net work done during a cycle is the difference between the work done by the system a
15、nd the work done on the system.,Key concepts and terms,Equilibrium process Kinetic energy Path-dependent work Quasistatic process Work at a system boundary Work transfer Work of expansion,PROCESSES INVOLVING IDEAL GASES,Polytropic processes.多變過程,The polytropic process: PVn=Const.,Work under the curv
16、e,The ideal gas PVn=Const,Increment of work done,Assumptions,Changes in KE and PE are zero Quasi-static process Polytropic process, PVn=Const Ideal gas,Increment of work done,Initial and final states,Calculation of total work done,From the system perspective, work is done by the system:,Expression for work:,Process equation:,Evaluating the integral:,Note that n cannot equal one, which is the general case.,For the special case when n = 1: Isothermal Expansion,
