Basic Concepts of Thermodynamics Notes, Study notes of Thermodynamics

Download Basic Concepts of Thermodynamics Notes and more Study notes Thermodynamics in PDF only on Docsity! UNIT 2 Basic Concepts of Thermodynamics After going through this unit, the reader would be able to understand the following: (1) Thermodynamic system and its types. (2) Thermodynamic properties. (3) Quasi-static process. (4) Zeroth, I, II and III laws of thermodynamics. (5) Concept of entropy. 2.1 BASIC CONCEPTS — Thermodynamics is the science of energy and entropy. — Thermodynamics is the science that deals with heat and work and the properties of substances that bear a relation to heat and work. — The basis of thermodynamics is experimental evidence and observation. These observations have been formalized into certain basic laws, which are known as zeroth, first, second and third laws of thermodynamics. Definitions Thermodynamic System: It is a three-dimensional region of space or an amount of matter which is under consideration. It is enclosed by an imaginary surface or real surface which may be at rest or in motion, and can change its size or shape. Everything outside the arbitrarily selected boundaries of the system is called surrounding. Universe: Combination of system and surrounding is called universe. Open System: The system which can exchange both mass and energy with its surroundings is called an open system. The region of space for observation is described as control volume. The boundary of control volume is called control surface. In such a system, flow type of processes occur. For example, turbine, boiler pump, compressor, condenser, nozzle, diffuser, etc. Basic Concepts of Thermodynamics 25 Extensive Property: Properties which depends on the mass of the system or in other words, value for the whole system is the sum of its values for the various subsystems or parts. Examples: Volume (V), Energy (E). Intensive Property: These properties have values that are independent of the size or amount of mass of the system. These have fixed value. If a given phase system in equilibrium is divided into n parts, then the value of given intensive property will be the same for each of the subsystems. Examples: Temperature, pressure, density velocity and chemical concentration, etc. If extensive property like Energy (E) of overall system is divided by the mass (m) of overall system, the resulting property is called specific property. e = E/m. A specific property is an intensive property. State: When all the properties of a system have definite values, then the system is said to exist at a definite state. Properties are the coordinates to describe the state of a system. Any operation in which one or more of the properties of a system changes is called a change of state. Path: The succession of states passed through during a change of state is called the path of the change of state. Process: When the path is completely specified, the change of state is called a process, e.g. constant temperture process (isothermal process), constant pressure process (isobaric process). Cycle: A thermodynamic cycle is defined as a series of state change such that the final state is identical with the initial state. Phase: A quantity of matter homogeneous throughout in chemical composition and physical stucture is called a phase. Every substance can exist in any one of the three phases: (1) Solid (2) Liquid (3) Gas Homogeneous System: A system consisting of a single phase is called a homogeneous system. Heterogeneous System: A system consisting of more than one phase is called a heterogeneous system. Thermodynamic Equilibrium: If the system is isolated from its surroundings and no change in its macroscopic properties occur, then it is called in thermodynamic equilibrium. A system will be in state of thermodynamic equilibrium if the condition for the following three types of equilibrium are satisfied: (a) Mechanical Equilibrium (b) Chemical Equilibrium (c) Thermal Equilibrium                                                                                                                                                                                                                                                                  Pulley Thermometer Pulley Z W Water Adiabatic vessel Paddle 1 2 W (In)1-2 Q (out)2-1 V P                             Σ = Σ                                      32 Mechanical Engineering: Fundamentals £ = Sum of microscopic and macroscopic modes of energy. E=U+ PE+ KE PE is bulk potential energy, KE is bulk kinetic energy and U is the total intemal energy of the system. In an ideal gas, there are no intermolecular forces of attraction and repulsion and the internal energy depends only on temperature. Thus, U=f (Z) only for an ideal gas. A system can possess other forms of energies also, like magnetic energy, electrical energy and surface (tension) energy. In the absence of these forms; the total energy £ of a system is given by: E=U+ PE+ KE (5) In the absence of motion and gravity, KE = 0 and PE = 0, E=U Therefore, equation 5 becomes 8Q- 8W=dE 8Q — SW is independent of path and depends upon end states only, internal energy U which is equal to f@o — 87) is also independent of path and hence a property of the system. Internal Energy: Internal energy can be defined as a form of stored energy in a system in the absence of magnetism, electricity, capillarity, surface tension, motion and gravity. 2.5 LIMITATIONS OF THE FIRST LAW AND INTRODUCTION TO THE SECOND LAW It is a common experience that while work is very easily converted into heat (by rubbing both hands we generate heat) but heat cannot be easily converted into work. The first law, however, places no restriction on the direction of flow of heat and work. For example, in the case of disc brakes, when it is stopped by friction pad, the pad as well as disc gets heated up, because the KE lost by the disc is converted into heat energy. The first law of thermodynamics would be equally satisfied if the brakes were to cool off and give back its internal energy to disc causing it to resume its rotation again. This may, however, never occur. The action of the brake in stopping the disc by friction is irreversible process. Thus, we can conclude that there exists a directional law which imposes limitation on energy transformation. Statement of second law also called Kelvin-Planck statement. “It is impossible to construct an engine which will work in a complete cycle and produce no other                                                                                                Engine E Q Heat source Q = W W Heat source TH QH Engine E Heat sink T2 Q2 A Possible Engine W = Q – QH 2 An Impossible Engine                                                                                                                                                                       High temperature sink Heat Pump or Refrigerator W=0 System boundary Low temperature source Impossible                                                                                                                                                                            Hot reservoir T1 Reversible engine =60% 100 kJ 80 kJ Irreversible energy =15% assumed  Cold reservior 40 kJ (assumed) 20 kJ Reversible engine Irreversible energy Cold reservoir T2 40 kJ 25 kJ 100kJ H =60 KJ w = 15 kJ                                                                                                                                                          = =                          = =∫ T1 T T2 0 2 S2S1 1 S                                                                          −= ×         =                                                                                                Basic Concepts of Thermodynamics 39 becomes less predictible and the entropy increases. Thus, it is not surprising that the entropy ofa substance is lowest in the solid phase and highest in the gas phase. In the solid phase, the molecules of a substance continually oscillate about their equilibrium positions, but they cannot move relative to each other and their positions at any instant can be predicted with good certainty. In the gas phase, however, the molecules move about at random, collide with each other, and change direction, making it extremely difficult to predict accurately, the microscopic state of a system at any instant. Associated with this molecular chaos is a high value of entropy. Heat is, in essence, a form of disorganized energy, and some disorganization (entropy) will flow with heat. As a result, the entropy and the level of molecular disorder or randomness of the hot body will decrease with the entropy and the level of molecular disorder of the cold body will increase. The second law requires that the increase in entropy of the cold body be greater than the decrease in entropy of the hot body and thus, the net entropy of the combined system (the cold body and hot body) increases. That is, the combined system is at a state of greater disorder at the final state. Clausius Inequality When any closed system undergoes a cyclic process, the sum of all (8q/7) terms at the system boundary for each differential element of the process will always be equal to or less than zero. Thus, pa < for any cyclic process (possible) (Clausius’ inequality) If p2- 0, the cycle is reversible ds = 0 p2 <0, the cycle is irreversible and possible ds > 0 p2 > 0, the cycle is impossible 2.8 THIRD LAW OF THERMODYNAMICS The third law of thermodynamics often referred to as the Nernst law, provides the basic for the calculation of absolute entropy of the substances. Third Law Statement The entropy of all perfect crystalline solid is zero at absolute zero temperature.      =∫      =∫      >∫                   = +∫ ∫ ∫   ∴          = − =∫                 = − =∫ ∴             = − =∫    ∴                              =                                                                     44 Mechanical Engineering: Fundamentals Intensive property of a system is one whose value. (a) depends on the mass of the system like volume (b) does not depend on the mass of the system, like temperature pressure etc. (c) isnot dependent on the path followed but on the state (d) is dependent on the path followed and not on the state Heat and work are (a) point functions (b) system properties (c) path functions (d) intensive properties Which of the following is the property of a system (a) pressure and temperature (b) intemal energy (c) volume and density (d) all of the above Work done is zero for the following process (a) constant volume (b) free expansion (c) throttling (d) all of the above Entropy change depends upon (a) heat transfer (b) mass transfer (c) change of temperature (d) thermodynamic state First law of thermodynamics (a) enables to determine change in internal energy of the system (b) does not help to predict whether the system will or will not undergo a change (c) does not enable to determine change in entropy (d) all of the above If a heat engine attains 100% thermal efficiency it violates (a) zeroth law of thermodynamics (b) first law of thermodynamics (c) second law of thermodynamics (d) all of the above laws According to Clausius’ statement (a) heat flow from hot substance to cold substance. (b) heat cannot flow from cold substance to hot substance. (c) heat can flow from cold substance to hot substance with the aid of extemal work. (d) none of the above 46 17. 18. 19. 20. 21. 22. 23. 24. Mechanical Engineering: Fundamentals (c) OK (d) equal to inlet temperature Calorie is a measure of (a) specific heat (b) quantity of heat (c) thermal capacity (d) entropy The value of 1 bar in S.I. units is equal to (a) 1 N/m? (b) 1 kN/m? (c) 1 x 104 Nim? (d) 1 x 10° Nim? Compressed air coming out from a punctured football (a) becomes hotter (b) becomes cooler (c) remains at the same temperature (d) attains atmospheric temperature Which of the following cycles has maximum efficiency (a) Rankine (b) Stirling (c) Carmot (d) Brayton Which of the following is extensive property? (a) entropy (b) intemal energy (c) kinetic energy (d) all of the above A process occurs spontaneously if its entropy (a) increases (b) decreases (c) remains same (d) becomes zero Entropy is called the property of the system because (a) Its derivative is zero for any process. (b) It has some value at any two equilibrium states (c) Ithasa single value at each equilibrium state (d) It has a constant value at each equilibrium state A reversible engine working between the temperatures limits of 600°K and 12°K receives 50 kJ of heat. The work done by the engine will be (a) 50kJ (b) 100 kJ (c) 25 kJ (4) -25 kJ