The concept of entropy was introduced for the study of thermodynamic by Rudolph Clausius in 1856. It was introduce to give a quantitative basis for the second law. It provides another variable to describe the condition of a system to go along with pressure, volume, temperature and internal energy. If a system undergoes a reversible process during which it absorbs heat Q at absolute temperature dT, then increasing in the states variable is called Entropy. It is denoted by S and it is written by, dS=Q/T. It is a change in entropy of the system which is important as like potential energy or internal energy.
Change in entropy
Change in entropy is positive when the heat is added to the system and change of entropy is negative when heat is removed from the system. Suppose, an amount of heat Q flows a reservoir at temperature T1 through a conducting rod to reservoir at temperature T2, when T1> T2. The change in entropy of the reservoir, at temperature T1, which loses heat, decreases by Q/T1 and of the reservoir at temperature T2, which gains heat increases by Q/T2. Hence, the total change in entropy=Q/T2-Q/T1 is positive (+). It follows that in all the processes, where heat flows from one system to another, there is always a total increases in entropy. This is another statement of 2nd law of thermodynamic. According to this law, If a system undergoes a natural process, this will go in the same direction that causes the entropy of the system plus the environment to increase.
It is observed that, natural process always tends to proceed towards a state of greater disorder. Thus, there is a relationship between entropy and molecular disorder. For example an irreversible heat flows from a hot substance to a cold substance of system, it increased disorder because the molecules are initially sorted out in hotter and cooler regions. This order is lost, when a system comes to the thermal equilibrium. If we add heat to a system, it increases its disorder because of increase in average molecular speeds and therefore, the randomness of molecular motion. Similarly, free expansion of gas increases its disorder because the molecules have greater randomness of position after expansion than before expansion. Thus in both example, entropy is said to be Increased. We can conclude that only those process are probable for which the entropy of the system increases or remains constant. In reversible process, entropy remains constant; whereas for all irreversible process, entropy of the system usually increases.
Conversion of heat
The opportunity to convert some heat into work is lost, when every time entropy increases. For example, there is an increase in entropy when hot water and cold water are mixed with each other. Then warm water which results cannot be separated into a hot layer and a cold layer. There has been no loss of energy but some of the energy is no longer available for conversion into work. Therefore, increase in entropy means change of energy from a higher level where more work can be extracted to a lower level at which minimum or no useful work can be done. The energy in a sense is degraded, going from more orderly form to less orderly form, usually ending as thermal energy. In all real process where heat transfer occurring, the energy which is available for doing work decreases. In other word the entropy increases, if the temperature of some system decreases. Therefor decreasing the entropy, it is at the expense of total increase in entropy for some other system. When all the system are taken as the universe, the entropy of the universe will always increases.
Environmental Crisis As Entropy Crisis
The second law of thermodynamics provides us the basic key for understanding our environmental crisis as well as for understanding how we should deal with this crisis. From a human standpoint, the environmental crisis result from our attempts to the nature for our comforts. From a physical standpoint, however, the environmental crisis is an entropy or disorder crisis resulting from our useless efforts to ignore the second law of thermodynamics. According to the second law, any increase in the order of a system will always produce an even greater increase in entropy or disorder in the environment. An individual impact may not have a major problem but an impact of large number of all individuals disorder producing activities may be affect the overall life support system. The energy process which we use are not very efficient. As a result many energy is lost as heat to the environment. Although we can improve the efficiency but 2nd law eventually produce an upper limit on improvement. Thermal pollution is an inevitable consequence of 2nd law of thermodynamic and heat is the finally death of any form of energy. The increase in thermal pollution of environment means increase in the entropy and that causes greater concern, even in small temperature's change in the plant and in animals. This can cause serious problem of the overall ecological balance. In addition to the thermal pollution, the most of the energy transformation process such as heat engines used to transportation and for power generation cause air pollution. In effect of this, all forms of energy production have some unfavorable result and in some cases all the problems may not be anticipated in advance.