Thermodynamics and Energy Chapters 1 PDF

THERMODYNAMICS AND ENERGY Thermodynamics: The science of energy. Energy: The ability to cause changes. The name thermodynamics stems from the Greek words therme (heat) and dynamis (power). Conservation of energy principle: During an interaction, energy can change from one for...

THERMODYNAMICS AND ENERGY Thermodynamics: The science of energy. Energy: The ability to cause changes. The name thermodynamics stems from the Greek words therme (heat) and dynamis (power). Conservation of energy principle: During an interaction, energy can change from one form to another but the total amount of energy remains constant. Energy cannot be created or destroyed. 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 Energy cannot be created or quality as well as quantity, and actual processes occur in the destroyed; it can only direction of decreasing quality of energy. change forms (the first law). Classical thermodynamics: A macroscopic approach to the study of thermodynamics that does not require a knowledge of the behavior of individual particles. 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. Heat flows in the direction of decreasing temperature. 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 ## The SI unit prefixes are used in all branches of engineering. ## The weight of a unit mass at sea level. 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. The boundary of a system can be fixed or movable. Systems may be considered to be closed or open. ## A schematic shows Closed system (Control mass): A fixed amount of mass, and System, Surroundings, no mass can cross its boundary. and Boundary. Open system (control volume): A properly selected region in space. m It usually encloses a device that involves mass flow such as a V T compressor, turbine, or nozzle. P Both mass and energy can cross the boundary of a control ρ volume. Control surface: The boundaries of a control volume. It can After be real or imaginary. dividing by two PROPERTIES OF A SYSTEM 0.5 m 0.5 m 0.5 V 0.5 V Property: Any characteristic of a system. T T Some familiar properties are pressure P, temperature T, volume V, and mass m. P P ρ ρ Properties are considered to be either intensive or extensive. Intensive properties: Those that are independent of the mass of a system, such as temperature, pressure, and density. Extensive properties: Those whose values depend on the size—or extent—of the system. Specific properties: Extensive properties per unit mass. Density ## Density is mass per unit volume; specific volume is volume per unit mass. Specific volume 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). Specific weight: The weight of a unit volume of a substance. STATE AND EQUILIBRIUM Thermodynamics deals with equilibrium states. Equilibrium: A state of balance. In an equilibrium state there are no unbalanced potentials (or driving forces) within the system. Thermal equilibrium: If the temperature is the same throughout the entire system. Mechanical equilibrium: If there is no change in pressure at any point of the system with time. Phase equilibrium: If a system involves two phases and when the mass of each phase reaches an equilibrium level and stays there. Chemical equilibrium: If the chemical composition of a system does not change with time, that is, no chemical reactions occur. PROCESSES AND CYCLES Process: Any change that a system undergoes from one equilibrium state to another. Path: The series of states through which a system passes during a process. To describe a process completely, one should specify the initial and final states, as well as the path it follows, and the interactions with the surroundings. Quasistatic or quasi-equilibrium process: When a process proceeds in such a manner that the system remains infinitesimally close to an equilibrium state at all times. Process diagrams plotted by employing thermodynamic properties as coordinates are very useful in visualizing the processes. Some common properties that are used as coordinates are temperature T, pressure P, and volume V (or specific volume v). The prefix iso- is often used to designate a process for which a particularproperty remains constant. Isothermal process: A process during which the temperature T remains constant. Isobaric process: A process during which the pressure P remains constant. Isochoric (or isometric) process: A process during which the specific volume v remains constant. Cycle: A process during which the initial and final states are identical. The Steady-Flow Process The term steady implies no change with time. The opposite of steady is unsteady, or transient. A large number of engineering devices operate for long periods of time under the same conditions, and they are classified as steady-flow devices. Steady-flow process: A process during which a fluid flows through a control volume steadily. Steady-flow conditions can be closely approximated by devices that are intended for continuous operation such as turbines, pumps, boilers, condensers, and heat exchangers or power plants or refrigeration systems. TEMPERATURE AND THE ZEROTH LAW OF THERMODYNAMICS The zeroth law of thermodynamics: If two bodies are in thermal equilibrium with a third body, they are also in thermal equilibrium with each other. By replacing the third body with a thermometer, the zeroth law can be restated as two bodies are in thermal equilibrium if both have the same temperature reading even if they are not in contact. Temperature Scales All temperature scales are based on some easily reproducible states such as the freezing and boiling points of water: the ice point and the steam point. Ice point: A mixture of ice and water that is in equilibrium with air saturated with vapor at 1 atm pressure (0°C or 32°F). Steam point: A mixture of liquid water and water vapor (with no air) in equilibrium at 1 atm pressure (100°C or 212°F). Celsius scale: in SI unit system Fahrenheit scale: in English unit system Thermodynamic temperature scale: A temperature scale that is independent of the properties of any substance. Kelvin scale (SI) Rankine scale (E) A temperature scale nearly identical to the Kelvin scale is the ideal-gas temperature scale. The temperatures on this scale are measured using a constant-volume gas thermometer. PRESSURE Pressure: A normal force exerted by a fluid per unit area Absolute pressure: The actual pressure at a given position. It is measured relative to absolute vacuum (i.e., absolute zero pressure). Gage pressure: The difference between the absolute pressure and the local atmospheric pressure. Most pressure-measuring devices are calibrated to read zero ## The normal in the atmosphere, and so they indicate gage pressure. stress (or Vacuum pressures: Pressures below atmospheric “pressure”) on pressure. the feet of a chubby person Pascal’s law: The pressure applied to a confined fluid is much increases the pressure throughout by the same amount. greater than on the feet of a slim person.  l ATMOSPHERIC PRESSURE Atmospheric pressure is measured by a device called a barometer; thus, the atmospheric pressure is often referred to as the barometric pressure. ## Throughout this text, the pressure P will denote A frequently used pressure unit is the standard absolute pressure unless specified otherwise. atmosphere, which is defined as the pressure produced by a column of mercury 760 mm in height at 0 °C (UHg = 13,595 kg/m3) under standard gravitational acceleration (g = 9.807 m/s2). ## The length or the cross-sectional area of the tube has no effect on the height of the fluid column of a barometer, provided that the tube diameter is large enough to avoid surface tension (capillary) ## The basic barometer. effects.