The variables involved are fthe tension of the rubber band measured in newtons l. A rubber band is composed of a collection of tangled, crosslinked polymers. Clearly understandidentify what is being asked for draw a sketch showing interactionsstates and identify a solution strategy. Theoretical background the thermodynamic identity for a rubber band is du. So you are doing positive work on the rubber, and it is doing negative work on you its surroundings. For an ideal solution the entropy of mixing is assumed to be. Exploring the thermodynamics of a rubber band oregon state. For the love of physics walter lewin may 16, 2011 duration. This allows students to experimentally check the predictions of the entropic spring model for elastomers. But avoid asking for help, clarification, or responding to other answers. Most rubber bands are manufactured out of natural rubber or, especially at larger sizes, elastomer, and are sold in a variety of sizes. Lt, where j is the tension in the rubber band, l is its length, t is the absolute temperature and. Statistical physics and thermodynamics ss 2017 problem sheet 10 hand in.
If f 0 when the band is pulling inward, then work f dl is done on the band when it is stretched through a distance dl0. A resource for teachers a script for classroom demonstration illustrating dg dh tds. Materials such as these, which are capable of undergoing large reversible extensions and compressions, are called elastomers. According to thermodynamics a reaction system always proceeds to equilibrium at which the concentration of the reagents and products have a fixed relation described by the equilibrium. We can safely assume that the volume change of the rubber band is negligible for small stretches, in which case for an isothermal stretch. The same thing happens when you relax the elastic band. Contraction of the rubber band is entropydriven and enthalpyopposed. So, from the first law of thermodynamics, if the process is adiabatic, the internal energy of the rubber is increasing. A rubber band in some regions known as a binder, an elastic or elastic band, a lackey band, laggy band, lacka band or gumband is a short length of rubber and latex formed in the shape of a loop and is commonly used to hold multiple objects together. When a rubber band is unstretched relaxed state the molecule chains are all twisted up figure 1a. Selecting this option will search all publications across the scitation platform selecting this option will search. An example of such a material is natural rubber, obtained from the plant hevea brasiliensis. Then measure the length of the rubber band within the marks just drawn.
This option allows users to search by publication, volume and page selecting this option will search the current publication in context. Rubber band thermodynamics module 8 university of minnesota chemistry 4501. We describe an upperdivision experiment in thermal physics where students measure the tension of a rubber band as a function of temperature and length and use a maxwell relation to find the change in internal energy and entropy for an isothermal stretch. Rubber band thermodynamics thread starter llabesab16.
Individual 4carbon backbone units are forced into rotational conformations that have longer endtoend distances and this restricts the number of conformations states that are thermally accessible. Entropy cannot go down, so the entropy becomes thermal motion of the atoms, i. To determine how the entropy of a rubber band changes as the rubber band is stretched. A rubber band is a single molecule as is a latex glove. Thermodynamics and equilibrium 145 here is a simple experiment. Sup2 rubber bands and entropic elasticity ucsd nano 4 darren lipomi duration. A new theory handles large stretches of any rubbery material by including extra entropy. But for now lets dive into the thermodynamics of a rubber band.
It is an amusing coincidence, but perhaps confusing to many, that my article showing disorder to be a misleading concept happened to appear in the same. A weight is hung from the ceiling with a rubber band. Consider a rubber band under tension, f, with length per. This setup allows the rubber band to be completely submerged when the tube is. A rubber band also known as an elastic band, gum band or lacky band is a loop of rubber, usually ring shaped, and commonly used to hold multiple objects together. First law of thermodynamics problem solving video khan. Isothermal heat engines violating the second law of thermodynamics and representing, perhaps, the most primitive biological motors their.
When you stretch the elastic you force the constituent chains into a more ordered state, but you havent stretched the chains themselves. Bertrand, professor of chemistry thermodynamics in two dimensions surface tension. So, when you stretch a rubber band, ill take my rubber band, ill stretch it a bit, there is a restoring force, lets call it f. Think of it this way, when an elastic polymer is in its relaxed state, these molecules are all tangled up with each other and have no particular direction to them, but when you apply a force, i. When the elastic band is not stretched, these molecules are all tangled up with each other and have no particular direction to them, but when you stretch the elastic they all become lined up with one another, at least to some.
In the following you will calculate the temperature and entropy change of stretching and relaxing an elastic rubber band. First, second, and third laws of thermodynamics, energy minimization and. A heat source generates thermal energy that brings the working substance. Solving thermodynamics problems solving thermodynamic problems can be made significantly easier by using the following procedure. Turn off the heater and allow the rubber band to cool. The whole subject of thermodynamics depends essentially upon the following kind of consideration. Thanks for contributing an answer to physics stack exchange. Rubber band heat engines do violate the second law of thermodynamics. Standard theory cant predict the force produced by a rubber band stretched more than a small amount. Before going to start this problem, lets understand. Place the rubber band flat on a table and mark two lines from where the rubber bands elasticity will be measured. Nov 20, 2018 a relaxed rubber bands chains are a tangled mess. Gale rhodes department of chemistry university of southern maine portland, maine 041049300. Perform the following experiments with the supplied rubber band.
According to this law, a system or body will move from a state of order to another state of disorder naturally. Thermodynamics part i problems stanford university. The rubber band was patented in england on march 17, 1845 by stephen perry. Thermodynamics problems real world physics problems. As the rubber is stretched they begin to untangle themselves as shown in figure 1b. The rubber band has lower entropy, is more orderd when stretched. Students stretch a rubber band from a hook in a stopper at the bottom of a glass tube to a chain connected to a force meter at the top of the tube. The criterion for spontaneity for an isolated system is.
The individual molecules that make up the rubber band are called polymers. Exploring the thermodynamics of a rubber band david roundy department of physics, oregon state university, corvallis, or 97331 michael rogers department of physics, ithaca college, ithaca, ny 14850 we describe an upperdivision experiment in thermal physics where students measure the tension. It is an amusing coincidence, but perhaps confusing to many, that my article showing disorder to be a misleading concept happened to appear in the same issue. Thermodynamics demonstration originally prepared for the coursera mooc. The ball is usually made from 100% rubber bands, but some instructions call for using a marble 20 or a pingpong ball 21 as a starting point. Entropy is a way of measuring the amount of disorder in a system. The thermal properties of rubber will be explored in this demonstration. A stretched rubber band contracts when heated under constant tension. It has to do with the tension in a rubber band and the work required to stretch or compress the rubber band. And entropy can be defined as lack of order in a system. A rubber band ball is a sphere of rubber bands made by using a knotted single band as a starting point and then wrapping rubber bands around the center until the desired size is achieved.
Its temperature increases when stretched adiabatically. Statistical physics and thermodynamics ss 2017 problem. Why do rubber bands contract when heated instead of expanding. As the temperature of the room changes describe what happens to the length of the rubber band. The tension in the rubber band must equal the suspended weight, so f is constant throughout the heating and cooling. Rubber band thermodynamics this problem deals with an extension of ideas that you learned in class about equations of state and work.
This one says, lets say youve got this problem, and it said 60 joules of work is done on a gas, and the gas loses 150 joules of heat to its surroundings. L f bt, where l is length, f is tension, b is a constant, and t is temperature. Trenton vogt answer rubber bands are unusual because they are made from looselypacked chains of atoms. Statistical physics and thermodynamics ss 2017 problem sheet 10. Examining a rubber band rubber is a long chain of polymerized isoprene monomers chain can stretchcompress upon thermal changes changes in internal energy and entropy rubber contracts with heat, increasing force expands with cooling. Summarize given data in own words, leave out unneeded information 2. Dwrev3d pdv, and occasionally with work in onedimension. When a rubber band is stretched, some of the network chains are forced to become straight. Lt, where j is the tension in the rubber band, l is its length, t is the absolute temperature. Note whether the rubber band becomes longer or shorter when it is heated.
To measure partial derivatives experimentally and test the validity of the cyclic rule of partial differentiation. Will this process be spontaneous at all temperatures. The key lies in the reason for the stretchiness of the rubber band. You can easily measure the force produced by a drastically stretched rubber band. Rubber band heat engines do violate the second law of.
But such a procedure can cause problems when there is noise in the. Physics stack exchange is a question and answer site for active researchers, academics and students of physics. The classical theory of rubber elasticity, developed in the 1940s, adds together the entropies of the free sections between the fixed crosslinks. It does this by bringing a working substance from a higher state temperature to a lower state temperature.
In thermodynamics and engineering, a heat engine is a system that converts heat or thermal energyand chemical energyto mechanical energy, which can then be used to do mechanical work. Under tension, the molecules in a rubber band line up and the arrangement becomes much more ordered, lowering the entropy. For reversible processes we have for the rubber band tds dq rev c l dt jdl. Exploring the thermodynamics of a rubber band david roundy department of physics, oregon state university, corvallis, oregon 97331 michael rogers department of physics, ithaca college, ithaca, new york 14850 received 27 february 2012. It also allows students to quickly alter the length of the rubber band by changing which. Exploring the thermodynamics of a rubber band article pdf available in american journal of physics 811. Stretching the band causes the chains to untangle and line up in an orderly fashion. Browse other questions tagged thermodynamics or ask your own question. No, as t decreases, d g becomes less negative, and at some t, the d h term will dominate, and d g will be positive.
Selecting this option will search all publications across the scitation platform selecting this option will search all publications for the publishersociety in context. The rubber band is stretched under constant tension j0 to a length 2l0 while in contact with a heat reservoir of temperature tb. We can model a rubber band as a onedimensional chain of n polymer links each of length l, where. Stretched rubber snaps back to its original state to maximize its entropy, as thermodynamics demands. Jan 27, 2016 rubber band thermodynamics module 8 university of minnesota chemistry 4501. The thermodynamics of regular and ideal solutions ideal and regular solutions. Pdf exploring the thermodynamics of a rubber band researchgate.
An elastomer has rather unusual physical properties. Chemistry stack exchange is a question and answer site for scientists, academics, teachers, and students in the field of chemistry. Thats present when you stretch the rubber band longer than its equilibrium length, that is, the length when its just sitting there. Stretching forces those polymers to align themselves more closely to the axis of elongation, leading to greater ordering and decreased entropy thus, heating the rubber band should increase its restoring force. To leave a comment or report an error, please use the auxiliary blog. To determine the number of polymeric chains per unit volume in a rubber band.
The internal energy of rubber is known from observations to be nearly a function only of temperature. By stretching the band you are decreasing its entropy. The equation of state for an idealized rubber band is j. Jan 27, 2016 for the love of physics walter lewin may 16, 2011 duration.
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