Does heat affect the elastic forces of a rubber band?

Stephen Sands P6 Science 8 Rb 25

Table of contents 1. 2. 3. 4. 5. 6. 7. Abstract Statement and Hypothesis Review of Literature Experimental Design Data Analysis Conclusion Bibliography

Abstractrubber is a substance that many of us have in front of us at an office desk or at home. However, it is one of the most unusual substances on earth as it shrinks when heated and expands when cooled. This is all because of entropy, the measure of disorder in a system. Rubber forms chains like a folded piece of paper, when heated, the rubber forms thicker chains and is harder to stretch. From my experiment, I concluded that rubber contracts when heated and is harder to stretch, clearly suppourting my research and hypothesis.

Statement and Hypothesis What is the effect of heat on the elastic force of rubber bands? If the temperature of the rubber band is increased, then it will be harder to stretch.

Review of literatureThe objective of my experiment is to find out how temperature affects the elastic forces of rubber bands. Since some rubber bands are different than others, I will be doing my experiments

using the same brand and size of rubber band. I will also be doing my experiments more than once to ensure an accurate result. I will be testing the elasticity by measuring the strength that each rubber band exerts while hooke's law is still in effect. Rubber is a strange substance, it shrinks when heated and expands when cooled. This is all because of entropy. Entropy is a way of measuring the amount of disordered molecules in a system. If all of the chains of molecules are lined up, we say that the entropy is very low. If they are all tangled up in a mess, we say that the entropy is high.

When rubber is stretched, the molecules must line up to maintain an orderly line. The more ways they can arrange themselves, the further they can stretch. When an object is heated, the molecules vibrate, causing them to have more entropy, also, since they're vibrating, they take up more space and cause the rubber bands to contract. When they're heated, they also have less space to arrange themselves so they also can't stretch as far.

There is a similar but opposite effect when it comes to force involved when stretching the rubber bands. Since rubber is a polymer, the chains of molecules in rubber have a natural elasticity. They form different bonds and act almost like a spring. When these chains are heated up, they compress and form thicker strands and are harder to pull apart, just like the force required to pull different thicknesses of rubber bands. It's the opposite effect when cooled, since they are vibrating less, they take up less space, and therefore are thinner and are easier to stretch. An example would be stretching a thick rubber band and a thin rubber band, the thin rubber band is easier to stretch because there are less chains of

molecules that need to be stretched Hooke's law states that the distance an object will stretch is in direct relation to the weight added to the object until the elastic limit is reached. Therefore, once the elastic limit is reached, the rubber band has reached a plastic state and cannot be restored to it's original length and is no longer elastic. 1. 2. 3. 4. 5. Introduction Heating rubber Entropy entropy hooke's law

6. Since 4'th grade, science teachers have told you that all substances expand when they are heated. However, I learned that there are several exception to this rule. For example, rubber contratcts when heated and expands when cooled. The objective of my experiments is to find out how this pehenomonan affects the elastic forces of a rubber band. Since some rubber bands by nature can be different than others, I will use each rubber band as it's own control, testing it twice. I will also only be stretching the rubber band to double it's length to make sure that hooke's law is still in effect.

When rubber is heated, it becomes smaller and thicker, making it harder to stretch. But to undersand why, we must first learn how rubber stretches in the first place. All solids are made of chains of molecules and these chains are what allow solids to stay together. Rubber however, also uses these chains for a different porpose, stretching. Rubber chains it's molecules together into short thick strands almost like a folded up piece of paper. When the rubber band is stretched, the paper is unfurled and is pulled outwords. However, the rubber band wants to return to it's original state and when the force applied to the rubber band is released, the rubber band snaps back into it's original size and shape. But why does the rubber band get thicker and contract when it's heated? This is because of entropy.

Entropy is the measure of the amount of ways a system can be arranged. When rubber is stretched, the molecules must form new chains that allow the rubber band to stretch without breaking. The amount of ways the rubber can arrange itself depends on the amount of entropy in the system. A greater amount of entropy is equal to a greater amount of disorder which in turn makes less ordered chains of molecules. When heated, the rubber band's molecules vibrate at a higher speed take up more space thereby making the rubber band thicker across and smaller in length.

When the rubber band is stretched while heated, the molecules have less space to allign themselves and the vibrations of the molecules disrupt the allignment of the rubber band's chains making thicker strands. Since this is happening, the rubber band is contracting and is therefore harder to stretch. While I am doing my experiments, I also learned that I cannon stretch the rubber band beyond the The elastic limit. The elastic limit is the amount of space a rubber band can be stretched without becoming permanently deformed. This is clearly illustrated in hooke's law, showing that an elastic object stretches at the same rate until it reaches a point where it can't be stretched anymore while still returning to the same original length. This shows why I am stretching the rubber band to double it's original length instead of stretching it until it breaks.

Experimental ProcedureMaterials Rubber bands x10 Water Heat source (hot enough to boil water) Pot Spring scale Chopsticks Ruler/meterstick Board w/ nail Steps: 1. 2. Get supplies Put boiling water in pot

3. 4. 5. 6. 7.

Submerge 10 rubber bands in water for 30 sec. Fish rubber band out with chopsticks Put one end of the rubber band on the nail, the other end on the spring scale Stretch the rubber band to double it's original length and look at the number on the scale. As the rubber band cools, observe what happens to the scale.

ConclusionMy hypothesis was that heating rubber bands would make them harder to stretch. My results clearly suppourt my hypothesis. When I stretched the heated rubber bands, the rubber bands became easier to stretch as they cooled down. The rubber bands became 10-11% easier to stretch. I think the tests I did ran smoothly and that the experiment can be repeated with the same results. However, if I did the tests again, I would have chosen a more controlled heat source than fire.

Bibliography 1. http://www.physics247.com/physics-tutorial/hookes-law.shtml 2. 3. Potter, Merle C. (2009) thermodynamics DeMYSTiFieD a self teaching guide New York, NY:McGraw-Hill