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Chapter 1
INTRODUCTION AND BASIC
CONCEPTS
Muhammad Ahmad Jamil Khwaja Fareed University of Engineering and Information Technology Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Thermodynamics: An Engineering Approach
Seventh Edition in SI Units Yunus A. Cengel, Michael A. Boles McGraw-Hill, 20 11
Objectives
- Identify the unique vocabulary associated with thermodynamics through the precise definition of basic concepts to form a sound foundation for the development of the principles of thermodynamics.
- Review the metric SI and the English unit systems.
- Explain the basic concepts of thermodynamics such as system, state, state postulate, equilibrium, process, and cycle.
- Review concepts of temperature, temperature scales, pressure, and absolute and gage pressure.
THERMODYNAMICS AND ENERGY
- Conservation of energy principle : Energy cannot be created or destroyed. During an interaction, energy can change from one form to another but the total amount of energy remains constant.
- The first law of thermodynamics : An expression of the conservation of energy principle.
- The first law asserts that energy is a thermodynamic property.
- The second law of thermodynamics: It asserts that energy has quality as well as quantity, and actual processes occur in the direction of decreasing quality of energy.
- Classical thermodynamics : A macroscopic approach to the study of thermodynamics that does not require a knowledge of the behavior of individual particles. It deals with the macrostate variable like pressure, volume instead of dealing thermodynamic system as a combination of moving molecules.
- It provides a direct and easy way to the solution of engineering problems and it is used in this text.
- Statistical thermodynamics : A microscopic approach, based on the average behavior of large groups of individual particles.
- It is used in this text only in the supporting role.
7 IMPORTANCE OF DIMENSIONS AND UNITS
- Any physical quantity can be characterized by dimensions.
- The magnitudes assigned to the dimensions are called units.
- Some basic dimensions such as mass m , length L , time t , and temperature T are selected as primary or fundamental dimensions , while others such as velocity V , energy E , and volume V are expressed in terms of the primary dimensions and are called secondary dimensions , or derived dimensions.
- Metric SI system : A simple and logical system based on a decimal relationship between the various units.
- English system : It has no apparent systematic numerical base, and various units in this system are related to each other rather arbitrarily.
Some SI and English Units Work = Force Distance 1 J = 1 N∙m 1 cal = 4.1868 J 1 Btu = 1.0551 kJ
Specific weight : The weight of a unit volume of a substance.
SYSTEMS AND CONTROL VOLUMES
- System : A quantity of matter or a region in space chosen for study.
- Surroundings : The mass or region outside the system
- Boundary : The real or imaginary surface that separates the system from its surroundings. It can be fixed or movable and has zero thickness thus can neither contain any mass nor occupy any volume.
- Systems may be considered to be closed or open.
- Closed system (Control mass): A fixed amount of mass, and no mass can cross its boundary. Energy in the form of heat or work can cross the boundary and the volume of a closed system does not have to be fixed.
- Isolated System is a special case of control mass system in this case even energy is not allowed to cross the boundary.
- Thermos is an excellent example of Isolated system.
- Boundaries of a control volume are called a control surface, and they can be real or imaginary. In the case of a nozzle, the inner surface of the nozzle forms the real part of the boundary, and the entrance and exit areas form the imaginary part, since there are no physical surfaces there.
- A control volume can be fixed in size and shape, as in the case of a nozzle, or it may involve a moving boundary, as shown in previous slide. Most control volumes, however, have fixed boundaries and thus do not involve any moving boundaries. A control volume can also involve heat and work interactions just as a closed system, in addition to mass interaction
PROPERTIES OF A SYSTEM
- Property: Any characteristic of a system.
- Some familiar properties are pressure P , temperature T , volume V , and mass m.
- Properties are considered to be either intensive or extensive.
- Intensive properties: Those that are independent of the mass of a system.
- If we divide the system into two equal parts, each part will have a value of intensive properties exactly equal to the original system such as temperature, pressure, and density.
Continuum
- Matter is made up of atoms that are widely spaced in the gas phase. Yet it is very convenient to disregard the atomic nature of a substance and view it as a continuous, homogeneous matter with no holes, that is, a continuum.
- The continuum idealization allows us to treat properties as point functions and to assume the properties vary continually in space with no jump discontinuities.
- This idealization is valid as long as the size of the system we deal with is large relative to the space between the molecules.
- This is the case in practically all problems.
- In this text we will limit our consideration to substances that can be modeled as a continuum. For instance we assume density of water in a glass is the same at all points
DENSITY AND SPECIFIC GRAVITY Density is mass per unit volume; specific volume is volume per unit mass. Specific gravity : The ratio of the density of a substance to the density of some standard substance at a specified temperature (usually water at 4°C). Density Specific weight : The weight of a unit volume of a substance. Specific volume
The State Postulate
- The state of a system is described by its properties. We do not need to specify all the properties in order to fix a state.
- The number of properties required to fix the state of a system is given by the state postulate : ✓ The state of a simple compressible system is completely specified by two independent, intensive properties.
The state of nitrogen is
fixed by two
independent, intensive
properties.
The State Postulate
- Simple compressible system: If a system involves no electrical, magnetic, gravitational, motion, and surface tension effects. These are general due to external force fields and are negligible for most engineering problems.
The state of nitrogen is
fixed by two
independent, intensive
properties.
