Anumher of recent articles14 have supported the ideaof introducing the theory ofrubber elasticity as an alternate thermodynamic system of interest. Far the past few years we have been running an experiment on rubber elasticity in our physical chemistry laboratory which has proven to be quite successful with our student body. Rubber has a number of unusual "Aha" properties which make it a most interesting system. Students are continually amazed when a thin strand free of suoerficial surface nicks can be stretched to well over five times its normal leneth without snapping. The thcrmnl effert bf having a rather h r a load ~ slowlv h e w raised by a hnnd stretched warly to its breaking point is a mnbmcing demmstrstiun of the tangible r r u l t oia c h a o g ~in
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As shown in the figure, theapparatusisquitesimplr.Thehousing isa 70-cm straight-hme Liebig condenser (an organic lab smple~with the butwm rut ofia few centimeters below the end uf t h water ~ locket. From the t w , d ~ n e - h u lrubber e stopper supports a stitf wire nhich h d d r paper clips (A), a1engthofrubb;r ( B ) ,and a threadsupporting apan (C)made from the plastic lid of a food tin, -10 cm in diameter. The student can obtsin the rubber band from the pile usually found on the -~~~~ reeretarv's desk. and -5 cm of a straieht leneth is cut from it. The oiece of rubber ~" ia tied het.ween two nnner d i m with the reltltiehtenine knot as shown in the inset. Usine the entire ~~~~~
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are placed on the pan. The apparatus is designed to run a t two constant temperatures, that of cold tap water entering through the bottom inlet, and steam entering from the top. From the stress-strain measurements it is possible to determine the equation of state and the Gough-Joule relationship, and, by applying both the cyclic and chain rules, to evaluate all the first-order partial derivatives of interest. Incidentallv. .. the S.I. svstem turns out t o he a natural as all the numbers fall out as simole values. In the mathematical analvsis emohasis is olaced on the neeessitv of constant so that all the iimt-orricr derivaiivrs twll apply I*, the same keeping partic& state. This important concept isone of the mapr asseta: of t h experimenl. ~ A complete pa
