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pee, 8C.1. Thermodynamics, 8Ct, , , , ee, , Thermodynamics is mainly, the heat energy into mechanical en, , Then eee of a body is due to the random motion of its molecules. Although molecules ofa, body have random motion yet they may have some degree of ordered motion also. Thus total energy of, a body is the sum of thermal as well as mechanical energies. As such thermodynamics is the theory of, heat as well as theory of mechanics supported by certain experimental facts., , Steam, diesel and petrol engi ', gines convert heat energy i 0 ical energy. Thermodyanics helps, gh thesdbaipn sorineteacéenteae eal energy into mechanic Tey ¥, , . cration of such devices to obtain maximum efficiency., , i Thermodynamics does not deal with temperature (or heat exchange) alone rather it links many, physical quantities like volume, Pressure, internal energy and mechanical energy also., 8C.2. Thermodynamic System, , the study of exchange of heat energy between bodies and conversion of, ergy and vice versa., , A collection of an extremely, such that it has a certain value, system., , Anything outside the thermod:, surroundings., , large number of atoms or molecules confined within certain boundaries, Of pressure (P), volume (V) and temperature (T) is called a thermodynamic, , lynamic system to which energy or matter is exchanged is called its, , Taking into consideration the interaction between a system and its surroundings, a system may be, divided into three classes :, , (2) Open system. A system is said to be an open system if it can exchange both energy and matter, with its surroundings., , (6) Closed system. A system is said to be closed system if it can exchange only energy (not matter), with its surroundings., , (c) Isolated system. A system is said to be isolated if it can neither exchange energy nor matter with, its surroundings., , 8C.3. Zeroth Law of Thermodynamics* (Thermal equilibrium and Temperature), , It states that two thermodynamic systems, which are separately in thermal equilibrium with @ third, thermodynamic system, are also in thermal equilibrium with each other., , Suppose systems I and II are separated by a wall which does not allow any transfer of heat, whereas system I and II can separately exchange heat with system III and thus attain thermal, equilibrium with III (Figure 1). On the removal of wall between I and II, no transfer of heat takes place, between them which shows that system I and II are also in thermal equilibrium with each other., , , , “Although Zeroth law of thermodynamics was found much after the second law of thermodynamics yet it has to be, Studied before the first law of thermodynamics because it defines temperature. It is therefore placed before the first law and is, , hence named as Zeroth law. roe, , Scanned with CamScanner

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Si ThermodyNAmive au~, , , , , , , , Wn aie petmometer will record same readings for systems I, II and, tm ich are now in thermal equilibrium. This physical quantity, Casured by thermometer is nothing but temperature,, , Temperature of | = Temperature of ITI and, Temperature of II = Temperature of III, Then, Temperature of I = Temperature of II. Figure 1., Thus modern definition of temperature can be given as below :, It is the physical quantity, equality of which is the only condition, for the thermal equilibrium of thermodynamic systems., , 8C.4. Thermodynamic Variables, , , , , , , , , , , , , , . A thermodynamic system can be described by specifying its pressure, ee, internal energy and the number of moles («). These parameters or variables are callc odynamic, variables. Thus, the variables which are required to specify the state of thermodynamic system are called, , thermodynamic variables., 8C.5. Equation of State, , The relation between the thermodynamic variables (P, V, T) of the system is called equation of, state., The equation of state for an ideal gas having « moles is given by. PV = #RT, where R is gas, constant. Had, For one mole of gas, 4 = 1, therefore, equation of state becomes PV = RT., , ‘ a, For one mole of real gas, the equation of state given by Vander Waal’s equation : (ra (V—b)=RT, , where a and b are Vander Waal’s constants, (V — xb) = #RT, , , , For # moles of real gas the above equation is given by P+ a, , aC,6 t External Work, , Work is said to be done if the body moves through a certain distance in the direction of applied force., The amount of work done is equal to the product of the magnitude of the force and the distance, through which the point of application of force moves. Thus,, , Work done = Force x displacement, , if a force is exerted by'a system on its surroundings and the displacement takes place, then the, work done is called external work. For example, a gas contained in a cylinder at a uniform pressure, while expanding pushes out the piston of the cylinder and thus. external work is done on its, surroundings., , The work done by one part of the system on another part of the same system is called the intemal, work, For example, in a gas, there are inter-molecular interactions and when a gas expands, some work, has to be done against these inter-molecular interactions among the, molecules of the gas. This work done is called internal work., , 8C.7. Work done during Expansion, , Let us consider a known mass of an ideal gas in a perfectly, insulated cylinder fitted with a non-conducting and. frictionless piston, (Figure 2). Assume that the external pressure is slightly less than the, internal pressure of the gas. Under this condition, the gas will expand., Let P and V be the pressure and volume of the gas respectively., , , , , , , , , , , , , , Scanned with CamScanner

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IS" ENGiNnes 787, , , , If A be the area of the p;, Let the piston move oa then force exerted by gas on the piston is F = P x A., «. Work done by the gas ae ee dx during the expansion of the gas., , é i = PAdr, , Since A dt = dV, increase in volume of the ga:, iS, , sc.8. Internal Energy, SY, , The internal energy of a ., system. It is denoted byU, of @ system 18 the sum of kinetic and potential energies of the molecules of the, Thus, U, . = KE. + PE,, , According to kineti 7 RE, ‘, of continuous oe Matter is made up of large number of molecules which are in a state, arises due to inter-mableculag 2 and hence possess kinetic energy. The potential energy of the system, psolute temperature (Tr ‘u n interactions, Since kinetic energy of the molecules is proportional to the, he system, therefore, wae system and potential energy of the system depends upon the volume of, , 4 : : *, , and its volume (V). ° ‘al energy (U) of the system is the function of its absolute temperature (T), That is,, , U=, In case of an ideal gas, inter fv), , . : molecular interaction (i.e. force) is zero. Hence its potential energy, is also zero. In this case, the internal energy is only due to kinetic energy of the molecules, which, , depends on the absolute temperature (T) of the gas. Hence internal energy (U) of an ideal gas is the, function of absolute temerpature (T) only., , That is, U =f(T), 8C.9. Isothermal Process, , A process in which pressure and volume of the system change at constant temperature is called, , isothermal process. In this case, P and V change but T = constant. i.e. AT (change in temperature) = 0, Consider a gas contained in a cylinder with conducting walls, , fitted with a conducting piston as shown in figure 3. When the gas is, , compressed suddenly, some heat is produced and its temperature, , increases. But if the compression is slow and the heat produced is, , lost to the surroundings immediately, then the temperature of the Conducting, , gas remains constant. walls, Similarly when the gas is allowed to expand suddenly, work is, , done by the gas and some heat is absorbed and hence its, , temperature falls. In order to keep the temperature of the gas, constant, it must gain heat from outside. Thus, for isothermal, change there should be free exchange of heat between the surroundings and the system., , Therefore, for an isothermal process to take place, the following conditions must be fulfilled :, , () There should be free exchange of heat between the system and its surroundings. All walls of the, container and the piston must be perfectly conducting., , (ii) The system should be compressed or allowed to expand very slowly so that there is a sufficient, time for the exchange of heat between the system and its surroundings., , Since the two conditions are not fully realised in practice, therefore, no process is perfectly, , Figure 3., , isothermal., , $c eT PY, , Scanned with CamScanner