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Name: Kendra
Status: Student
Age: 12
Location: N/A
Country: N/A
Date: 2002


Question:
Where can I find out more about rubber bands and their elasticity?



Replies:
Your question does not have a simple answer. In order to understand the theoretical model of rubber bands, it is necessary to know something about the field of thermodynamics -- which in turn assumes that you know something about calculus. It may sound condescending, but at your age I suspect that you know little or nothing about either. So what can one say about rubber elasticity without math?

1. At a constant temperature, and for elongations not too large, a rubber band obeys Hooke's Law: The force, f = - K(X - Xo) where K is a constant, (X - Xo) is the elongation and the sign in negative because the force is in the direction opposite to the extension. That is the force, f, is trying to pull the rubber band back to its equilibrium length, Xo.

2. This behavior occurs because the polymer comprising the rubber band, when there is no load on it, that is the length is Xo, you can think of the polymer molecules as a tangled mass of very long spaghetti strands tacked together to one another in a few widely separated places along the strands. When the rubber band is stretched, this tangled mass of "spaghetti strands" stretch out and line up parallel to one another (more or less / mostly less). It is this lining up that allows the rubber band to stretch. However, there is a limit to how much the rubber molecules can be stretched because they are attached to one another in these few places. (The technical name for these links between the chains is "cross linked".) If too much force is applied these cross links will break and the rubber band will "snap".

3. There is very little change in the stretching process. The main effect is that the tangled mass of polymer strands become more ordered (less tangled) when the rubber band is stretched. This fact results in some peculiar behavior.

4. When you heat almost any solid, it expands. But not rubber bands!!! The reason is that heating the rubber molecules make them move around more. They become less aligned as a result and the rubber band SHRINKS instead of expanding when it is heated!!! The molecules become more tangled (and so contract ) like they were initially before you stretched the rubber band. You can actually do this experiment with a rubber band, a small weight that stretches the rubber band about 2 to 4 times its initial length. Suspend the rubber band in a glass tube, or something that will insulate it a little bit and heat the stretched rubber band with a hair dryer. It gets shorter, not longer, as you would expect from a "normal" solid material. Turn off the hair dryer and let the rubber band cool and it will stretch back to is initial stretched length (if you haven't heated it too much and degraded the rubber.) The amount the rubber band contracts when heated can be related to the way the order / disorder occurs, the force constant, K, and the amount of ordering that occurs when the rubber band is heated. It is this part, when you try to be quantitative about what is happening that you need thermodynamics and calculus.

Hope this helps.

Vince Calder



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