Saturday, June 26, 2010

Elasticity Essay

Essay on Elasticity

What is elasticity? Elasticity is a property in certain substances that when one stretches it repeatedly, it will return to its original shape. However, there is an elasticity limit to every object, or how much stress can be applied to the object without it breaking or forming permanent deformation. When the stress applied to the elastic object is tolerable, it yields to the pressure and restores to its original shape when the stress is released; when the stress becomes too great, and it breaks or forms permanent deformation. The elastic property that allows elastic matter to return to its original shape is called elastic rebound. The elastic rebound and the elasticity directly varies as the temperature. Lots of materials have elasticity; a very good example is an elastomer, or rubber.

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Most rubber contains only one kind of molecule. The rubber molecules are interconnected by chemical bonds called springs in a vibrating structure. At a normal state, they are always in a vibrating state, called vibration heat. Although it vibrates, it has a solid structure and unlike liquid, it has a definite shape; and unlike gas, it has a definite volume. This property allows the molecules to move around easily and be in motion yet maintaining an overall shape. Due to this vibration, rubber can yield to stress unbearable to other substances and still return to its original shape. The molecules in the rubber are in a state of entropy, or having no specific order and structure, “they just wind and tangle around each other in one jumbled mess (Elastomers).”

When stretched, their chains will be forced to line up in an orderly fashion and be crystallized, or lose their state of entropy. When the stress is released, they are no longer crystallized by force and naturally return to their original structure of entropy. According to Hook’s Law, if a rubber is thrown to the ground, its structure stretches to yield to the force. Then, when the impact of the stress is absorbed, it returns to its original shape with a force. This force is what makes a tennis ball or basketball bounce. To get an even higher bounce, some elastomers have chains of molecules that are cross-linked, joining all the molecules into a single network.

The elasticity of elastomers, as well as all substances having the property of elasticity, is greatly affected by the temperature. The basic rules is that the hotter the elastomer is, the more elastic it is, assuming that the temperature is not hot enough to make the elastomer liquid or gas.

Elastomer molecules, as mentioned above, can withstand great stress because of their vibration heat, because the vibration almost gives the elastomer a liquid like quality. Thus, if the vibration heat cools off, the molecules will have a hard time moving around, and the elastomer will not be able to withstand as much stress. The temperature that allows elastomers to be cooled to the point where it acts like a rigid object such as a marble is called the class transition temperature, or Tg. When a rubber ball is at or near its Tg, it will behave like a marble when thrown to the ground; it will either not bounce, bounce a little, or shatter like glass. The ball will even have a different sound when bounced at different temperatures! A famous experiment demonstrating the relationship between temperature and elasticity is placing a rubber ball in liquid nitrogen, which is -160 Celsius, and breaking it on the ground.

The elasticity of elastomer is related to the temperature because the temperature controls the movement of the molecules in the elastomer and therefore determining its ability to yield to stress. If a rubber ball is place under cool temperature, then its elasticity will decrease.


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