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236 PHYSICS Eq. (11.1), the total heat absorbed equals the 11.10 REVERSIBLE AND IRREVERSIBLE work done by the system. PROCESSES Imaginesomeprocessinwhichathermodynamic 11.9 SECONDLAWOFTHERMODYNAMICS The First Law of Thermodynamics is the system goes from an initial state i to a final state f. During the process the system absorbs heat Q principle of conservation of energy. Common from the surroundings and performs work W on experience shows that there are many it. Can we reverse this process and bring both conceivable processes that are perfectly allowed by the First Law and yet are never the system and surroundings to their initial states with no other effect anywhere ?Experience observed. For example, nobody has ever seen suggests that for most processes in nature this a book lying on a table jumping to a height by is not possible. The spontaneous processes of itself. But such a thing would be possible if the principle of conservation of energy were nature are irreversible. Several examples can be the only restriction. The table could cool cited. The base of a vessel on an oven is hotter than its other parts. When the vessel is removed, spontaneously, converting some of its internal heat is transferred from the base to the other energy into an equal amount of mechanical energy of the book, which would then hop to a parts, bringing the vessel to a uniform height with potential energy equal to the temperature (which in due course cools to the temperature of the surroundings). The process mechanical energy it acquired. But this never happens. Clearly, some additional basic cannot be reversed; a part of the vessel will not principle of nature forbids the above, even get cooler spontaneously and warm up the base. though it satisfies the energy conservation ItwillviolatetheSecondLawofThermodynamics, principle. This principle, which disallows ifitdid.Thefreeexpansionofagasisirreversible. many phenomena consistent with the First The combustion reaction of a mixture of petrol Law of Thermodynamics is known as the and air ignited by a spark cannot be reversed. Second Law of Thermodynamics. Cooking gas leaking from a gas cylinder in the The Second Law of Thermodynamics gives kitchen diffuses to the entire room. The diffusion a fundamental limitation to the efficiency of a process will not spontaneously reverse and bring heat engine and the co-efficient of the gas back to the cylinder. The stirring of a performance of a refrigerator. In simple terms, liquid in thermal contact with a reservoir will it says that efficiency of a heat engine can convert the work done into heat, increasing the never be unity. For a refrigerator, the Second Law says that the co-efficient of performance internal energy of the reservoir. The process cannot be reversed exactly; otherwise it would can never be infinite. The following two amount to conversion of heat entirely into work, statements, one due to Kelvin and Planck violating the Second Law of Thermodynamics. denying the possibility of a perfect heat engine, and another due to Clausius denying the Irreversibility is a rule rather an exception possibility of a perfect refrigerator or heat in nature. Irreversibility arises mainly from two causes: pump, are a concise summary of these one, many processes (like a free expansion, or observations. an explosive chemical reaction) take the system Kelvin-Planck statement to non-equilibrium states; two, most processes No process is possible whose sole result is the involve friction, viscosity and other dissipative absorption of heat from a reservoir and the effects (e.g., a moving body coming to a stop and losing its mechanical energy as heat to the floor complete conversion of the heat into work. Clausius statement and the body; a rotating blade in a liquid coming to a stop due to viscosity and losing its No process is possible whose sole result is the mechanical energy with corresponding gain in transfer of heat from a colder object to a the internal energy of the liquid). Since hotter object. It can be proved that the two statements dissipative effects are present everywhere and can be minimised but not fully eliminated, most above are completely equivalent. processes that we deal with are irreversible. Reprint 2026-27