Basics of Thermodynamics, Study notes of Mechanical Engineering

Download Basics of Thermodynamics and more Study notes Mechanical Engineering in PDF only on Docsity! Professor K.Srinivasan Department of Mechanical Engineering Indian Institute of Science Bangalore Fundamental Concepts and Definitions THERMODYNAMICS: It is the science of the relations between heat, Work and the properties of the systems. How to adopt these interactions to our benefit? Thermodynamics enables us to answer this question. Examples (Contd…) On the other hand we burn, Some coal/gas in a power plant to generate electricity. Petrol in a car engine. What is the largest energy we can get out of these efforts? Thermodynamics allows us to answer some of these questions Definitions In our study of thermodynamics, we will choose a small part of the universe to which we will apply the laws of thermodynamics. We call this subset a SYSTEM. The thermodynamic system is analogous to the free body diagram to which we apply the laws of mechanics, (i.e. Newton’s Laws of Motion). The system is a macroscopically identifiable collection of matter on which we focus our attention (eg: the water kettle or the aircraft engine). The rest of the universe outside the system close enough to the system to have some perceptible effect on the system is called the surroundings. The surfaces which separates the system from the surroundings are called the boundaries as shown in fig below (eg: walls of the kettle, the housing of the engine). Boundary Surroundings System Isolated System - in which there is no interaction between system and the surroundings. It is of fixed mass and energy, and hence there is no mass and energy transfer across the system boundary. System Surroundings Choice of the System and Boundaries Are at Our Convenience We must choose the system for each and every problem we work on, so as to obtain best possible information on how it behaves. In some cases the choice of the system will be obvious and in some cases not so obvious. Important: you must be clear in defining what constitutes your system and make that choice explicit to anyone else who may be reviewing your work. (eg: In the exam paper or to your supervisor in the work place later) The boundaries may be real physical surfaces or they may be imaginary for the convenience of analysis. eg: If the air in this room is the system,the floor,ceiling and walls constitutes real boundaries.the plane at the open doorway constitutes an imaginary boundary. Choice of the System and Boundaries Are at Our Convenience (contd…) Microscopic Approach In microscopic approach, the effect of molecular motion is Considered. eg: At microscopic level the pressure of a gas is not constant, the temperature of a gas is a function of the velocity of molecules. Most microscopic properties cannot be measured with common instruments nor can be perceived by human senses Property It is some characteristic of the system to which some physically meaningful numbers can be assigned without knowing the history behind it. These are macroscopic in nature. Invariably the properties must enable us to identify the system. eg: Anand weighs 72 kg and is 1.75 m tall. We are not concerned how he got to that stage. We are not interested what he ate!!. Examples (contd...) We must choose the most appropriate set of properties. For example: Anand weighing 72 kg and being 1.75 m tall may be a useful way of identification for police purposes. If he has to work in a company you would say Anand graduated from IIT, Chennai in 1985 in mechanical engineering. Anand hails from Mangalore. He has a sister and his father is a poet. He is singer. ---If you are looking at him as a bridegroom!! Specific property: It is the value of an extensive property per unit mass of system. (lower case letters as symbols) eg: specific volume, density (v, ρ). It is a special case of an intensive property. Most widely referred properties in thermodynamics: Pressure; Volume; Temperature; Entropy; Enthalpy; Internal energy (Italicised ones to be defined later) Property (contd..) State: It is the condition of a system as defined by the values of all its properties. It gives a complete description of the system. Any operation in which one or more properties of a system change is called a change of state. Phase: It is a quantity of mass that is homogeneous throughout in chemical composition and physical structure. e.g. solid, liquid, vapour, gas. Phase consisting of more than one phase is known as heterogenous system . Path And Process The succession of states passed through during a change of state is called the path of the system. A system is said to go through a process if it goes through a series of changes in state. Consequently: A system may undergo changes in some or all of its properties. A process can be construed to be the locus of changes of state Processes in thermodynamics are like streets in a city eg: we have north to south; east to west; roundabouts; crescents If we remove the weights slowly one by one the pressure of the gas will displace the piston gradually. It is quasistatic. On the other hand if we remove all the weights at once the piston will be kicked up by the gas pressure.(This is unrestrained expansion) but we don’t consider that the work is done - because it is not in a sustained manner In both cases the systems have undergone a change of state. Another eg: if a person climbs down a ladder from roof to ground, it is a quasistatic process. On the other hand if he jumps then it is not a quasistatic process. Quasi-static Processes (contd…) Equilibrium State A system is said to be in an equilibrium state if its properties will not change without some perceivable effect in the surroundings. Equilibrium generally requires all properties to be uniform throughout the system. There are mechanical, thermal, phase, and chemical equilibria Equilibrium State (contd) Nature has a preferred way of directing changes. eg: water flows from a higher to a lower level Electricity flows from a higher potential to a lower one Heat flows from a body at higher temperature to the one at a lower temperature Momentum transfer occurs from a point of higher pressure to a lower one. Mass transfer occurs from higher concentration to a lower one Zeroth Law Of Thermodynamics (Contd…) Two systems are said to be equal in temperature, when there is no change in their respective observable properties when they are brought together. In other words, “when two systems are at the same temperature they are in thermal equilibrium” (They will not exchange heat). Note:They need not be in thermodynamic equilibrium. Zeroth Law If two systems (say A and B) are in thermal equilibrium with a third system (say C) separately (that is A and C are in thermal equilibrium; B and C are in thermal equilibrium) then they are in thermal equilibrium themselves (that is A and B will be in thermal equilibrium TA TB TC Explanation of Zeroth Law Let us say TA,TB and TC are the temperatures of A,B and C respectively. A and c are in thermal equilibrium. Ta= tc B and C are in thermal equilibrium. Tb= tc Consequence of of ‘0’th law A and B will also be in thermal equilibrium TA= TB Looks very logical All temperature measurements are based on this LAW.