ENTROPY

a term introduced into physics by Clausius as the name of cne of the two important thermodynamical properties of a substance which depend on its ‘state.’ Suppose we have 1 lb. of water, at atmospheric pre ure and 212° F., say, and suppose we apply heat to the water and change it into 1 lb. of steam at 212° F. The temperature does not change during this proce , while the heat whicL must be added is the latent heat of the steam, namely, about 960 British Thermal Units. The increase of entropy from the first state to the second state is got by dividing the heat given to the substance, namely, 960 B.Th.U., by the absolute temperature at which that heat was given to the substance, namely, 461 + 212 = 673 degrees absolute, i.e. the ina crease of entropy is 700 = 1-43 units. If the temperature changes with the addition of heat, as it would usually do, we have to imagine the heat to be supplied in small quantities, and to take the average temperature of the body at which these tiny quantities of heat ave supplied. The quotient heat+temperature is taken for each small quantity of heat, and the results are added together. The summation is defined as the increase in entropy between the initial and the final states;. In mathematical language the increase in entropy between state A and state Bisgiven by (6s—¢a)= ue * dQ A fi where ¢ stands for the entropy, Q for the heat received by the body, and-T for the absolute temperature at which > 196 ENVELOPES the heat is received. The importance of the entropy function, namely, the dQ integral a in thermodynamics is due to the fact that it is, like the internal] energy of the substance, a function of the state of the substance only, and consequently in any temperature cyclic change in which the final state of the substance is the same ag its initial state A, the total change, either of its internal energy or of its entropy, is zero. The fact that the tétal change in the internal energy of thé substance is zero is practically €quivalent to the First Law of Thermodynanvics, while the fact that the total cliange of the entropy of the substance ig zero is equivalent to the Second Law of Thermodynamics.—BIB1L10GRaPHY: J. H. Poynting and J. J. Thomson, Textbook of Physics (vol. 3): Heat.