We are now in a position to make a statement regarding the Second law of thermodynamics. The essence of a formulation of this law as given by Thomson and Planck is as follows:
It is impossible to construct a heat engine working between a source and a sink which draws heat from the source and converts the whole of it into useful work without producing any other effects. Such a formulation does not violate the First Law since the total energy remains conserved, but it does tell us heuristically that there can be no way by which heat can be converted fully into useful mechanical energy without producing any other effects. Here 'any other effects' mean delivering any heat to a sink. In other words, what the Second law states is that there can be no machine which will convert the whole of the heat drawn from a source to useful work without delivering any of it to a sink. In the reversible Carnot cycle, the engine delivers to the sink as much heat as required to keep the entropy constant. Lessening the amount of heat to be delivered to the sink below this limit due to leakage by such physical processes as friction, turbulence, conduction, diffusion, etc., which is quite a common occurrence, makes the transfer process irreversible, resulting in increase of the entropy of the system. Such irreversibility and unavailability of useful work are exceedingly numerous in nature and in everyday experience. The following are some examples:
(i) A glass cylinder containing some water is rotated rapidly and then suddenly left to itself. The rotating water comes to a halt after a while due to friction, with a slight rise of temperature. The warming of the water is due to conversion of its kinetic energy into heat. However, the process is irreversible because water cannot be made to rotate again simply by extracting some heat from the water.
(ii) A high-speed bullet fired from a gun hits a target at a distance. The temperature rises at the target due to conversion of kinetic energy into heat. It is simply impossible to make the bullet to fly back simply by extracting some heat from the target. The process is clearly irreversible. The heat generated is unavailable for any useful work.
(iii) In a transparent cylinder, place a few layers of white marbles and then place a few layers of red marbles on top of the white ones. Shake the cylinder so that both types of marbles get mixed. It is impossible to get the red marbles back on top again simply by re-shaking the cylinder. In this case, order has given way to disorder. Experience tells us that a lot of external work will be needed to restore the original arrangement of the marbles. So the process is clearly irreversible.
(iv) A decorated glass filled with water is placed at the edge of a high table. It accidentally falls down and breaks into pieces with water splashed all over the floor. It is impossible to get the glassful of water, as it was originally, back on the table again. The process is clearly irreversible.
(v) A large ocean wave on striking the coastline breaks down into irregular whirls and foams. It is impossible to get back the energy of the wave by re-assembling the dissipated parts of the wave which have been lost to friction.
Numerous such examples may be cited about the way in which energy during transformations from one form to another becomes unavailable for useful work or for that matter order easily turns into disorder or organization into disorganization in physical processes, though the total of all forms of energy remains invariant. It is this unavailability of energy for work, tendency of nature to go from order to disorder or from organized state to a disorganized state that is highlighted by the Second Law of thermodynamics. In all such physical processes, entropy increases.
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