Energy Transfer in Closed Systems PDF

Thermodynamics Properties  Any characteristic of a system is called a property. Properties are considered to be either intensive or extensive. Intensive properties are those that are independent of the mass of a system. Extensive properties are those whose values depend on the size—or extent—of...

Thermodynamics Properties  Any characteristic of a system is called a property. Properties are considered to be either intensive or extensive. Intensive properties are those that are independent of the mass of a system. Extensive properties are those whose values depend on the size—or extent—of the system. NOTE: Specific extensive properties are intensive Ex. Specific volume, specific energy, density 15  Thermodynamic Equilibrium A system is said to be in a state of thermodynamics equilibrium if there is no change in any of its macroscopic properties registered when the system is isolated from its surroundings. Three conditions to be satisfied: 1) Mechanical Equilibrium(No unbalanced force) 2) Thermal Equilibrium 3) Chemical Equilibrium(forward and backward reactions are equal) 17 Process  Process Any change that a system undergoes from one equilibrium state to another is Initial thermodynamic state called a process, and the series of states through which a system passes during a process is called the path of the process High Low temperature temperature HEAT TRANSFER Final thermodynamic state Same Same temperature temperature 19  Temperature It is the measure of hotness or coldness of a body. Zeroth law of Thermodynamics When a body C is in thermal equilibrium with body A and also separately with body B, then A and B are said to be in thermal equilibrium with each other. 25  Temperature A thermodynamic property that determines whether or not a system is in thermal equilibrium with another system 26  Quasi-static process  Such a process which is nothing but the locus of all the equilibrium states passed through by the system is called as quasi-static process.  Infinite slowness is the characteristic of the process.  Quasi-static process is a reversible process. NOTE: Criteria for a reversible process- Process should be quasi-static i.e. all the states passed through by the system should be an equilibrium state. Isothermal Isobaric Isochoric 27  Energy transfer in a closed system – Work transfer Energy can cross the boundary in form of heat and work. Work is usually defined as a force F acting through a displacement Work is done by a system if the sole effect on the surroundings (everything external to the system) could be the raising of a weight. The raising of a weight is in effect a force acting through a distance. 28  Energy transfer in a closed system – Work transfer Mechanical forms: Modes of Work transfer i) Pdv (boundary work) ii) Paddle work iii) Shaft iv) Flow work v) Spring work Non-mechanical forms: i) Electrical ii) Magnetic 25 29  Energy transfer in a closed system – Work transfer p-dv work or displacement work 30 Energy transfer in a closed system – Work transfer Path function and point function We cannot evaluate the integral unless we know the “path” 2 2 W12   PdV w12   Pdv 1 1 Different “paths” lead to different “area under the curve”. 1 32 4  Energy transfer in a closed system – Work transfer Path function and point function It is possible to go from state 1 to state 2 through various “paths” Each path will lead to different work done. Hence, Work is “path function”. Differential form for path functions will be denoted by “δ ” State functions like specific volume, pressure, temperature can be integrated and do not depend on the “path”. Their differentials are exact differential and will be denoted by “d” 2 v12   dv  v2  v1 1 33 P-dv work on various quasi-static process Isothermal or Constant temperature process 35 36  γ Adiabatic Work Transfer (pV = C) 37  Energy transfer in a closed system – Heat transfer Heat is defined as the form of energy that is transferred between two systems (or a system and its surroundings) by virtue of a temperature difference. Heat is thermal energy in transition Eg. A hot object has “thermal energy” but it is called “heat” only when it transfers across its boundaries. Once in the surroundings, it is part of “internal energy” of the surroundings. 38  Work Transfer and Heat Transfer Free expansion is said to have zero work transfer 46  Work and heat transfer Consider a system as shown in Fig.; the cylinder is fitted with a piston on which a number of small weights are placed. The initial pressure of gas inside the cylinder is 200 kPa, and the initial volume of the gas is 0.04 m3. Assuming ideal gas behavior. a) Let the cylinder be heated till the volume of the gas increases to 0.1 m3 while the pressure remains constant. Calculate the “work” done. (Ans: 12 kJ) b) In second scenario while the cylinder is heated and the piston is rising, remove weights from the piston in such a manner that the process remains isothermal. Let the final volume of the gas is 0.1 m3. Calculate the “work” done in this case. (Ans: 7.33 kJ) 47 A piston–cylinder device contains 0.05 m3 of a gas initially at 200 kPa. At this state, a linear spring that has a spring constant of 150 kN/m is touching the piston but exerting no force on it. Now heat is transferred to the gas, causing the piston to rise and to compress the spring until the volume inside the cylinder doubles. If the cross- sectional area of the piston is 0.25 m2, determine (a) the final pressure inside the cylinder, (b) the total work done by the gas, and (c) the fraction of this work done against the spring to compress it. Final Pressure = 320kPa Total work done = 13kJ Work done by the spring = 3kJ THANK YOU