CHAPTER 4 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 move 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 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

Copyright © The McGraw-Hill Companies, Inc Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. FIGURE 2–5 The various forms of microscopic energies that make up sensible energy. 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) by 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 transfer. -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.

FIGURE 2-11 Specifying the directions of heat and work. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. FIGURE 2-11 Specifying the directions of heat and work. 3-1

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 Process line, or path路徑 State 1 State 2 P1 P3 P2

Interactions System Surroundings Ma f(Pk, k =1...N)=0 Mass Flow Heat Work Heat

Mechanical work flow Work Flow The turning fan represents the Motor Electrical Power System Boundary Work Flow The turning fan represents the result of a mechanical work transfer.

Mechanical Work　機械功 m

Work at a system boundary...

Deformation變形 of the system boundary Note: Pgas > Pambient Direction of Motion Piston Wall System: A gas at p,V, and T. x

Closed system with boundary deformation Initial System State 1 Deformed System Boundary State 2 x y z P1 1 2 P3 P2 Process Path

Work transfer at a boundary System Surroundings W > 0 W< 0 System Boundary

Evaluating work at a boundary...

pambient pgas Force balance at the boundary on the x Note: Pgas > Pambient Direction of Motion x X pambient The gas is the system for analysis. Force balance at the boundary on the piston, where the boundary deforms. pgas

X Pambient Pgas dx The net force on the piston.

Total work done

pambient pgas > pambient Component of work due X Component of work due to expansion of the gas Work to raise the piston

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 mechanical forms of work

The expansion膨脹 and compression 壓縮works 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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. FIGURE 4-2 A gas does a differential amount of work dWb as it forces the piston to move by a differential amount ds. 3-2

Evaluating an equilibrium expansion process V = Ax V1 V2 p1 p2

Evaluating the work Integral for a quasi-static process V = Ax V1 V2 p1 p2

Copyright © The McGraw-Hill Companies, Inc Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. FIGURE 4-3 The area under the process curve on a P-V diagram represents the boundary work. 3-3

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 done 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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. FIGURE 4-5 The net work done during a cycle is the difference between the work done by the system and the work done on the system. 3-4

Work at a system boundary 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. State 1 State 2

Work under the curve p Work done is proportional State 1 State 2 Work done is proportional to the area under the curve for a quasi-static process. The. process can go either forward (1-2) or the reverse (2-1)

The ideal gas PVn=Const

Increment of work done Area = A x

Assumptions Changes in KE and PE are zero Quasi-static process Polytropic process, PVn=Const Ideal gas

Increment of work done x A

Initial and final states Area = A x x1 x2

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

Polytropic processes p n > 1 V1 V2 V T1 T2 Isothermal Process (n = 1) n > 1 p1 p2

Alternative expressions for W1-2

Constant pressure processes...

Constant pressure process Consider as a limiting case of the general polytropic process. P = Constant Evaluation of the work integral

Constant pressure, constant temperature and polytropic processes: 1 2 P V P = Constant (n = 0) Isobaric process Constant pressure, constant temperature and polytropic processes:

Isothermal process Polytropic process Isobaric process Key concepts and terms Isothermal process Polytropic process Isobaric process

Definition of heat...

Heat is defined as the form of energy that is transferred between two systems (or a system and its surroundings) by virtue of a temperature difference.

Several phrases which are in common use today such as: heat flow, heat addition, heat rejection, heat removal , heat gain, heat loss, heat storage, heat generation, electrical heating, resistance heating, heat of reaction, specific heat, sensible heat, latent heat, waste heat, body heat, are not consistent with the strict thermodynamic meaning of the term heat, which limits its use to the transfer of thermal energy during a process. In thermodynamics the term heat simply means heat transfer.

A process during which there is no heat transfer is called an adiabatic process.

Heat has energy units, kJ or Btu. The amount of heat transferred during the process between two states is denoted by Q12 or just Q. Heat transfer per unit mass of a system is denoted q and is determined from

The sign for heat is as follows: heat transfer to a system is positive, and heat transfer from a system is negative. Modes of heat transfer Heat can be transferred in three different ways: conduction (傳導), convection (對流), and radiation (輻射).

Heat: The result of non-adiabatic processes path, i.e., Q1-2 > 0. Adiabatic path, i.e., Q1-2 = 0. P1 P2