Converting Sunlight , to Mechanical Energy A Polymer Example of Entropy Lon J. Mathlas Department of Polymer Science, University of Southern Mississippi, Hatliesburg, MS 39406 Entropy is a difficult concept to grasp and relate to realworld examples. This experimentldemonstration provides elementary through high school science students with hands-on ex~eriencewith polymer entropy. Construction of a simple machine for convkrting light into mechanical energy is descrihed using inexpensive, everyday materials. This "light engine" is virtually foolproof in construction and performance, spinning a t rates up to 50-60 rpm. Extension experiments can be designed using the engine to investigate the effects on the rate of rotation of color, light intensity, angle of incidence, and light transmission through clear glass or plastic. Background
Entropy and enthalpy are the two components of the Gihh's free energy expression, AG = AH - TAS.Repeated reference to this equation is made throughout science, either explicity or as a "hidden" rationale, to describe "driving force" and "spontaneity" of chemical and physical processes such as phase changes and chemical reactions. While enthalpy is relatively easy to understand in terms of heat flow that can actually be felt, entropy is difficult to grasp and demonstrate. Entropy deals with disorder, or more accurately, a change in order that is often opposite in direction to the enthalpic change. Generally, the overall AG is less than zero for a "snontaneous" Drocess. That is, the combination of the enthalpy change and the entropy term (which also rontains the temverature in KeI\.in) utthe proress sum t o a negative value. 1; absolute magnitude, enthalpic changes are usually larger than the associated entropic change. In many cases, the two terms cancel such that AG = 0, which requires that AH = TAS. Examples of entropy in the everyday world are hard t o see because of the larger magnitude of enthalpy. Shaking a box of marbles or makine". ooncorn miaht serve, although the best . example-increased freedom of motion of gas molecules on expansion of a sealed container-is just not easily visualized by young people. Polymer entropy is one of the few easily seen and visually dramatic examples available for illustrating entropy. The behavior of elastomers has been treated in depth ( I ) , and an inexpensive physical chemistry experiment has been descrihed in which the entropy change of a rubber band is accurately determined (2). The decrease in length of a stretched rubber band on heating has recently been put forth as a visual demonstration of entropy (3) but it lacks a dynamic aspect. A classic demonstration exists in the literature of a "rubber-band engine" powered by the expansion and contraction of tightly stretched elastomers on cooling and heating which causes rotation of a wheellike frame (4). Speeds of several hundred rpm have been reported (5),and, while visually very effective, it has been our experience that these engines require skill and patience to balance properly and to keep running. A workable alternative to the rubber-hand engine was recently described that uses inexpensive, readily available materials to construct a reliable solar-powered engine (6). The following is a condensed and modified version of the original description. Additional background material is
me "light engine available on the thermodynamics of elastomers and especially entropy-driven elongation and contraction (1-6).Construction is simple enough t o carry out in one evening. In fact, because you will find the demonstration to be of interest to friends and colleagues, you might want to gather enough material for several. Making two requires very little additional effort and gives you an extra one to serve as a snare. Havine extra arts is handv. anvwav - in case of damwe. ~ e p l a c e m e norienied i strips are a must because they will break, especially if you get them too hot with a sun lamp. Materials and Equlpmeni
The following is enough for one complete engine, although, since the cost is minimal, i t is just as easy to assemble several at once. Variations in some of the dimensions are all right, making this a very "forgiving" endeavor. In fact, strivine in cuttine and eluine is wasted effort: -for nerfection . " the system is self-correcting in the final step. Just be careful cuttine"the vlastic strins and usine the various kinds of due. . Rulher cement is touih to yet ofkngers, although the; are the best a ~ ~ l i c a t i otools n in this rase. With forethouaht and common sense, children can even make a workahie solar engine with some adult supervision.
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Materials List 1 black garbage bag
(other colors will not work)
1single-edge razor blade or an Exaeto knife 1 'I4-in. wooden dowel, 12-14 in. long, sharpened at one end 2 12-16.0~ Styrofoam cups (must be Styrofoam or expanded poly-
styrene, not thin plastic) 1 l/8-L/4-in.Styrofoamtray or flat plate big enough to cut out several
circles to fit the cups 16-8-in. plastic lid (e.g.,coffeecan top)
several straight pins 1pointed-nose pliers 2 pint cans or 500-mLbeakers boa of paper clips 5-minute epoxy cement 1 tube contact cement pencil, sandpaper,scissors, ruler, flat file, '14-in. drill bit as needed or not, depending on your skill Volume 64
Number 10
October 1987
889
instructions 1. Cut 40-50 l-in. X 3-4-in. strips of black plastic. Before cutting all strips cut one parallel and one perpendicular to the bag's top. Stretcheach one first to see which draws out better. Usually parallel t o the top works best. Cut the rest of the strips. 2. Slowly orient thestrips by drawing them out. The plastic mill thin out and hecame stronger. If you pull too fast, they will break, and if you do not pull far enough, they will not work. 3. Cut two foam circles t o fit inside the eups about '/4-1/-'12 in. Sand as needed t o get good fit. 4. Slowly and carefully push the sharpened dowel through each afthecup bottomsso that theyfaceoutfrom themiddleofthe dowel (see figure). If they break or split too much, drill out a small hole by hand first with the drill hit. These need t o be snug fits. 5. Fit one of the cup end inserts onto the dowel, and remove it for later gluing. 6. Mix up enough 5-minute epoxy to glue the eups onto the dowel. Cup position on the dowel is important: symmetrical with hoth hottoms about 2-3 in. apart and with 2-3 in. of dowel showing past both lips (see figure, lower right). Best procedure is t o put a little glue on the dowel just where the foam will he and slide the cup onto that spot. Doping on more epoxy to make a firm seal is desirable: this is the pressure bearing point in the system. Stand the glued ensemble upright tomaintain reasonable centering of the dowel inside the middle of the open cup ends. 7. When the cup-dowel joints are dry, mix up enough epoxy t o coat hoth the inside and the outside of the cup lips to a depth of ahout 'I2in. and t o put a m a t on each of the inserts around and on the edge that will touch the cups. This will he the glue base for the contact cement, which will otherwisedissolve the Styrofoam. Allow to dry thoroughly. 8. Cut about 20 strips so that they stretch from lip t o lip on the cups parallel to the dowel. There should be about 'I2in. a t each end to bend around and into the cup on gluing. 9. Using your finger and working quickly with each strip, apply contact cement to hoth ends and fasten in place on bothcups. Glue to hoth the inside and outside of the cup lip to ensure a good bond, hut do not try to readjust or align too carefully. While it is important t o make sure they are reasonably parallel to the dowel, do not worry about getting them all the same tightness from end to end. They will automatically adjust on heatine later. The strios should he ahout as far m a r t as the"
off center on the dowel. 11. Cut a 1-2 in. circle in the middle of the other insert. This one will not he touching the dowel when glued in place. Again use contact cement to attach inside the other cup end. 12. Before finishing the solar engine, let the sun or a sun lamp shineon thestrips forafewminutes whileslowly rotatingitto
890
Journal of Chemical Education
allow uniform annealing and alignment. This should tighten all the plastic strips but may cause the center of the openended cup t o move off center some. This will he corrected in the next step. 13. Cut a hole about 1-2 in. in the middle of the plastic lid, and glue it t o the cup that has the insert with the large hole. Carefully center thelid with respect to the doloel, not thecup. This will help balance the engine while it turns. 14. Now insert the straight pins in the center of the dowel ends so that the engine can rotate freely. The pins will rest in small notches filed in the pint canlips, or better in the pour spout of the 500-mL beakers. Spin the engine. I t will wobble some and then stop with the heavy side down. Carefully attach paper clips to the opposite side on the plastic lid until the engine balances and spins smoothly with a gentle push. 15. Ta operate the solar engine, rest the assembled product on a window ledge or out of doors in direct sunlight. Give it a gentle spin in one direction to get i t started, and i t should gradually accelerate to about one revolution per second. For indoor demonstrations, use a hand-held sun lamp in a elamp-on hase, but be careful: its very easy t o bring this lamp too close and melt the plastic strips. Be sure t o start the engine spinning before bringing the sun lamp close. Normal operating distance is about 2 ft above the engine with a 150-W lamp, although you should check this distance yourself. Just do not try to get too close or you will have t o replace some of the strips. This is easy, by the way; just pull off as much of the damaged strips as you can without breaking the cup, and glue new ones on. The only problems you may run into involve gradual aging of the strips or getting thelamp too close and causing melt-down. Be sure to show anyone else who plans to use it how to start i t first. We have had several of these engines last for months with occasional use and repeated abuse in demonstrations and workshops. Any demonstration of the solar engine should involve asking the audience questions to help them understand, first of all, how the engine works, and second, why it works. Thermal contraction of the strips (the opposite of what most members of the audience would respond) throws the open-ended cupoff center. Gravity then pullsit down, exposing new strips to heat and allowing the heated ones to cool. Try visualizing for the audience how increased need for space to accommodate heat-induced motion in the polymer molecules causes the ends of the strips t o pull in. Now ask them if the engine would work in the space shuttle: if you havenot overemphasized the role of gravity up to this point, it will take them a few seconds to reason out the answer. The key to the ability ofthe engine to work is that it uses both sunlight and gravity to generate mechanical work.
1. Narh. I..
K. J. Chrm. Educ. 1979.56.363.
