IB Basic Concepts and Definitions PDF

Kwame Nkrumah University of Science & Technology, Kumasi, Ghana ME 266 Thermodynamics 1 Introductory Concepts and Definitions D. A. Quansah, PhD, MGhIE Department of Mechanical Engineering Introductory Co...

Kwame Nkrumah University of Science & Technology, Kumasi, Ghana ME 266 Thermodynamics 1 Introductory Concepts and Definitions D. A. Quansah, PhD, MGhIE Department of Mechanical Engineering Introductory Concepts and Definitions System – Closed – Open – Isolated surroundings system boundary working fluid www.knust.edu.gh www.knust.edu.gh Introductory Concepts and Definitions System boundaries may be drawn to reflect the type of analysis one wishes to perform. nptel.ac.in www.knust.edu.gh www.knust.edu.gh Introductory Concepts and Definitions Control Volume is defined by a surface (real or imaginary) enclosing a volume of interest. www.knust.edu.gh www.knust.edu.gh  Introductory Concepts and Definitions Property State A property is a quantifiable Process macroscopic characteristic of Path a system. Flow process e.g. mass, volume, density, pressure, Cycle temperature, etc Equilibrium Quasi-equilibrium www.knust.edu.gh www.knust.edu.gh  Introductory Concepts and Definitions Property State Process The thermodynamic state of path a system is defined by the Flow process values of all of the system Cycle thermodynamic properties. Equilibrium Quasi-equilibrium www.knust.edu.gh www.knust.edu.gh  Introductory Concepts and Definitions Property A process occurs whenever a State system changes from one state Process to another state. Path Flow process E.g. when the temperature or Cycle pressure of a system changes. Equilibrium Quasi-equilibrium www.knust.edu.gh www.knust.edu.gh Introductory Concepts and Definitions Property State Types of Processes Process Constant-temperature process– Isothermal Constant-Pressure process- Isobaric Path Constant-Enthropy process- Isentropic Flow process Constant-Volume – Isochoric Cycle More will be encountered in this course! Equilibrium Quasi-equilibrium www.knust.edu.gh www.knust.edu.gh  Introductory Concepts and Definitions Pressure Property State Process Path Flow process Cycle A PATH is the series of states Equilibrium through which a system passes Quasi-equilibrium during a process. www.knust.edu.gh www.knust.edu.gh  Introductory Concepts and Definitions Property State Process Path A flow process occurs whenever the Flow process state of a fluid entering a control Cycle volume is different from the Equilibrium state of the fluid exiting the Quasi-equilibrium control volume. www.knust.edu.gh www.knust.edu.gh  Introductory Concepts and Definitions Property State Process Path Flow process Cycle Equilibrium Quasi-equilibrium A thermodynamic cycle consists of a sequence of processes in which the working fluid returns to its original thermodynamic state. www.knust.edu.gh www.knust.edu.gh  Introductory Concepts and Definitions Property State When the properties of a thermodynamic Process system are constant from point to point Path and when there is no tendency for change with time, a condition of thermodynamic Flow process equilibrium is said to exist. Cycle Equilibrium Quasi-equilibrium www.knust.edu.gh www.knust.edu.gh  Introductory Concepts and Definitions Conditions for Complete Equilibrium Property ✓ Thermal equilibrium State Uniform system temperature and is same temperature as its surroundings. Process ✓ Mechanical equilibrium Path achieved when the pressure throughout the system is uniform and there are no unbalanced forces at the Flow process system boundaries. Cycle ✓ Phase equilibrium Phase equilibrium requires that the amount of a Equilibrium substance in any one phase not change with time. Quasi-equilibrium ✓ Chemical equilibrium system is in chemical equilibrium if its chemical composition does not change with time, that is, no chemical reactions occur. www.knust.edu.gh www.knust.edu.gh  Introductory Concepts and Definitions Property Illustration of Thermal equilibrium State Process Path Flow process Cycle Equilibrium Quasi-equilibrium www.knust.edu.gh www.knust.edu.gh  Introductory Concepts and Definitions Property A quasi-static or quasi-equilibrium State process is a process that happens Process sufficiently slow such that departures Path from thermodynamic equilibrium are Flow process always so small that they can be neglected. Cycle NB: These are idealized processes. Equilibrium Quasi-equilibrium www.knust.edu.gh www.knust.edu.gh Introductory Concepts and Definitions On quasi-static processes ✓ Although they are idealized, engineers are interested in quasi-static processes because they are easy to analyze. ✓ work-producing devices deliver maximum work when they operate on quasi-static processes. ✓ Work-consuming devices consume minimum work when they operate on quasi-static processes. ✓ Quasi-static processes serve as standards to which actual processes can be compared. 16 www.knust.edu.gh www.knust.edu.gh  Introductory Concepts and Definitions Path and Point Functions ❑The value of a property of a system is independent of the path of the process undergone by the system and is therefore referred to as a point function (also called state function). ❑Quantities such as heat and work which are not properties of the system and dependent on the path of the process undergone by the system are classified as path functions. 17 www.knust.edu.gh www.knust.edu.gh  Introductory Concepts and Definitions Path and Point Functions ❑If X is a property, then the change in its value is independent of path, hence: 2 න 𝑑𝑋 = 𝑋2 − 𝑋1 1 ❑ This requires that dX be an exact differential, where X2-X1 represents the change in the property as the system changes from state 1 to state 2. 18 www.knust.edu.gh www.knust.edu.gh  Introductory Concepts and Definitions Path and Point Functions ❑ Quantities such as heat and work which are not properties of the system and are dependent on the path of the process undergone by the system are classified as path functions. 2 2 න 𝑑𝑄 = 𝑄12 and න 𝑑𝑊 = 𝑊12 1 1 ❑NB: a quantity is a property only if, its change in value between states is independent of the path of the process. 19 www.knust.edu.gh www.knust.edu.gh  Introductory Concepts and Definitions Extensive and Intensive Properties ❑An intensive property is one that does not depend on the mass of the system. ❑Temperature, pressure, density, and velocity are examples. ❑If two (or more) systems are brought together, intensive properties are not summed. www.knust.edu.gh www.knust.edu.gh  Introductory Concepts and Definitions Extensive and Intensive Properties ❑ An extensive property is one that does depend on the mass of the system; mass, volume, momentum, and kinetic energy are examples. ❑ If two systems are brought together the extensive property of the new system is the sum of the extensive properties of the original two systems. ❑ If an extensive property is divided by the mass, a specific property is obtained. 21 www.knust.edu.gh www.knust.edu.gh Introductory Concepts and Definitions ❑ Mass density (or simply density)- mass per unit volume. ❑ Specific volume - volume per unit mass. ❑ Weight density (or specific weight) – Weight per unit volume. 22 www.knust.edu.gh www.knust.edu.gh  Introductory Concepts and Definitions Thermodynamic Pressure The pressure, P, of a system is the total normal force, per unit area, exerted by the system within and at the boundary. Collision with other molecules and with the walls of the containing vessel. The SI unit of pressure and stress is the Pascal (Pa). 23 www.knust.edu.gh www.knust.edu.gh  Introductory Concepts and Definitions Thermodynamic Pressure The SI unit of pressure and stress is the Pascal (Pa) – 1 Pa = 1 N/m2 It is convenient sometimes to work in multiples of Pa. 24 www.knust.edu.gh www.knust.edu.gh Introductory Concepts and Definitions Thermodynamic Pressure The pressure of a system is usually measured relative to the atmosphere, and it is called “gauge pressure”. Absolute pressure = Gauge pressure + Atmospheric pressure 25 www.knust.edu.gh www.knust.edu.gh  Pressure Variation with Elevation F1 = P. A A Force Balance on the fluid element yields: F1 + W − F2 = 0 ∆𝒁 𝜕𝑃 W PA + W − P + ∆𝑍 A = 0 𝜕𝑍 Recall: 𝜕𝑃 𝑊 = 𝑚𝑔 𝑊 = 𝜌𝑉𝑔 𝑊 = 𝜌𝐴∆𝑍𝑔 F2 = P + ∆𝑍. A 𝜕𝑍 𝜕𝑃 PA + 𝜌𝐴∆𝑍𝑔 − P + ∆𝑍 A = 0 𝜕𝑍 𝜕𝑃 = −𝜌𝑔 න 𝑑𝑃 = න 𝜌𝑔 𝑑𝑧 𝑃 = 𝜌𝑔𝑍 𝜕𝑍 KEY LESSON: Pressure varies with altitude or depth. 26 www.knust.edu.gh www.knust.edu.gh MANOMETRY - BASICS P=Patm+PHg www.knust.edu.gh www.knust.edu.gh  Temperature Scale Temperature is a measure of molecular activity. Equality of Temperatures If two systems are equal in temperature to a third, they are equal in temperature to each other: Zeroth law of thermodynamics. www.knust.edu.gh www.knust.edu.gh Temperature Scale To establish a temperature scale, the ice point and the steam point are chosen and a number of subdivisions, are created between them. ice point exists when ice and water are in equilibrium at a pressure of 101 kPa steam point exists when liquid water and its vapor are in a state of equilibrium at a pressure of 101 kPa. www.knust.edu.gh www.knust.edu.gh  Temperature Scale Reference Point oF oC K Water boils 212 100 373 Water Freezes 32 0 273 Absolute Zero -460 -273 0 Kelvin Scale There is a limit to how cold something can be. The Kelvin scale is designed to go to zero at this minimum temperature. At a temperature of Absolute Zero there is no motion and no heat. Absolute zero is where all atomic and molecular motion stops and is the lowest temperature possible.www.knust.edu.gh Absolute Zero occurs at 0 K or -273.15 oC or at www.knust.edu.gh -460 oF.