Page 1 : Detween Same values O} LVALLAL ANd Jindal VOLUMES, Wadia a Wiso, , Comparison between isothermal and adiabatic changes, , . Isothermal change Adiabatic change, , 1. Temperature remains, constant so AT or dT = 0, , , , , , , , , , , , 1. Heat content remain, constant dQ or AQ = 0, , 2. Change occur suddenly., , , , , , , , , , , , , , 2. Change occur slowly., , 3. System is thermally, insulated from |, surroundings., , 4. Specific heat becomes, Zero., , 5, Equation of adiabatic, change PV’ = constant, , 3... System. 1s, thetmially, conducting to, Surroundings., , , , , , , , 4. Specific heat becomes, infinite,, , , , , , 5. Equation of isothermal, change PV = constant, , , , , , 6. Slope of adiabatic curve, , ae (= |, dN V, . Coefficient of. adiabatic, , 6. Slope of isothermal curve,, , IBLE PRC ES, , 22, REVERSIBLE AND IRE, , (i) Reversible Process : A thermodynamic process 1S Salc, to be reversible if the various stages of an operation to which, it is subjected can be traversed back in the opposite direction, such that the substance passes through exactly the same, conditions at every step in the reverse process as in the direct, , \process./S0, 1 ih a particatar-step In iTeCT Process, Tear 1s, , a y the system, then in the same step in reverse process,, same amount of heat will be given out by the substance., , OR, , A thermodynamic process (from initial to final state) is, reversible if the process can be turned back such that both the, system and surroundings returns to their original states, with, no other change any where else in universe.

Page 2 : 276, , , , , , Sie ve) 1S, fs wate 1 » direct proce:, If some work is done by the system ™ the Ae aes A, then an equal amount of work muSLDE done on sy', rever, ome UPTV. Ek, Za “he process should be carried oul exthel ., » * ; : : id oe ' : ny he mus, (i.e., it must be quasi static). For this the system 4., , in J : (librium., remain in mechanical, thermal and chemical equilib, , The pressure difference between the working eee, and surroundings at any stage of the operation of cycle |, , pe be very small., 3. The various parts of system must be free from the, lasticity, viscosity,, , dissipative forces such as friction, ine, __eectrical resistance etc., 4. There should be no loss of energy due to conduction,, , convection and radiation during the operation. 3., , Examples, 1. Working substance taken in complete Carnot’s cycle., , 2. An infinitesimal slow compression and expansion of an, ideal gas at constant temperature. 4, , 3. The gradual compression and extension of an elastic, spring is approximately reversible., , In fact a complete reversible process is an idealised concept, , as it can never be realised in practice because dissipative forces, , , , , , , , can’t be cempteteby-ehmuinated. Unit, A process which cannot be retraced in the reverse direction, exactly is called irreversible process. Most of processes occuring, in nature are of irreversible process sae, 1: Diffusion of two gases EXA, Rusting of iron a gas, , , , 3. Exchange of heat between bodies at different | its ve, , temperatures, 4. Work done against friction, 5, Heat generation during flow of current through a resistor, , or conductor, 6. Sudden expansion or contraction of gas., , RK, elated Queitions, , 1. Using first law of thermodynamics, obtain the adiabatic, relation’ between pressure and volume. Also write, relations between P and T also between V and T., , 2. Obtain an expression for work done during isothermal, process., , 3. Qbtain an expression for work done during adiabatic, process.

Page 4 : ssure 1s |, , ically, , 7 cal, , 30 kPa,, , , , rocess 1S, , y = 5/3,, , |, , , , ai, , , ERMO- |, , essential, al work,, work are, | work 1s, lowever,, , , , 81, , 1. It does not tell us about the directions of transfer of, heat., , We know that heat flows from a body of higher temperature, to a body at lower temperature, it does not tell us, why heat, can’t flow from body of lower temperature to the body at, higher temperature., , Examples. (i) When a bullet strikes against a target, kinetic, energy of the bullet is converted into heat energy. But heat, energy developed in the target cannot be converted back into, mechanical energy of the bullet enabling it to fly back., , (ii) When a moving car is stopped by applying brakes,, work done against friction is converted into heat. When the car, , cools down, jt@oes not start moving with the conversion of all, of its heat€nergy into mechanical work., , , , , , . It does not tell us how much extent or amount of the, , , , , given qua of heat is converted into work., “Tt does#ot tell us the conditions under which heat energy, is converted to work., , 4: This law gives no information about the source of heat,, 1.e., Whether it is a hot or a cold body., , 24. HEAT ENGINE, , In order to convert heat into work we need heat engine., Heat engine is a device which is used for converting heat, energy into mechanical energy., , Principle : It is based on principle that, A system whose, various parts are at different temperatures always tends to, change towards the state of thermal equilibrium. This tendency,, when properly channelised, delivers some energy (work) to an, external agent (i.e., wheels of machinery), , Types of Heat Engine, , (i) External Combustion Engine : The engine, in which, heat is produced by burning the fuel in a chamber outside the, main body of engine. (cylinder and piston arrangement), , For example, steam engine. The theoretical value of, efficiency of steam engine at 20 atmospheric pressure and 250°C, is about 32%. However, practical thermal efficiency of steam, engine varies from 12% to 16%, , (ii) Internal Combustion Engine : The engine, in which, heat is produced by burning the fuel in a chamber with in the, main body of the engine. For example petrol engine, diesel, engine etc,, , The theoretical efficiency of internal combustion engine is, nearly 52%, But practical efficiency of petrol engine is 26%, and the efficiency of diesel engine is about 40%, , Actually petrol engine is small in size, it is used in scooter,, , car, aeroplane etc, Diesel engine is heavy and is used in bus,, truck, tractor, etc,, , Construction : A heat engine consists of following parts, , (i) Source ; It is a hot body maintained at a very high, temperature (T)), It is also called as reservoir, It supplies the, heat energy which is to be converted into work. In petrol engine, source of heat is the combustion of petrol vapours and air,

Page 5 : nally, heat, , State,, , T> 18, , Ip of, , \\ ot, , relngere, , F THERMODYNAMICS, Sw that first law of thermody Trcs 18 the principle, Yet there are many processes 1D, but they are never observed., phenomena consistent, , VC KN, of conservation of energy., which the energy 18 conserved, , iple which disallows certain, , The prince : | |, is known as second law of, , with first law of thermodynamics, , thermodynamics., According to second law of thermodynamics,, , heat flows, at higher temperature to another, , spontaneously from a substance ne, flow spontaneously in, , at lower temperature. Heat does not, reverse direction., , Sccond law of thermodynamics has been stated in two, forms, , (i) Kelvin Plank statement : “// is impossible to get a, continuous supply of work by cooling a body to a temperature, lower than that of the coldest of its surroundings. OF, , “It is impossible to construct a heat engine which would, absorb heat from a reservoir and convert 100% of the heat, absorbed into work.”, , This law implies that a perfect heat engine with 100%, , efficiency can never he constructed. Aettally presence of si, is essential for continuous conversion of heat into work., , , , “It is impossible for a self acting machine unaided by any, external agency to transfer ,heat#}rom one body at lo, temperature to another, at } igher te; mperature. ”’, , This statement, , Lr, , , , Lo as