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Name: Simon
Status: Other
Grade: Other
Location: Australia
Country: Australia
Date: May 2006


Question:
When a metal paper clip is straightened out why is it difficult to bend back to its original shape?



Replies:
Because you have gone over the materials yield strength. Materials have a property known as yield strength. It is the point in which a material will be permanently deformed. As long as you stay well below the yield strength, you should be able to return to the original shape.

Thanks for using NEWTON.
Christopher Murphy, P.E.
Air Force Research Laboratory


When a metal paper clip is straightened out, it does not return to its original state, like a rubber band does because the metal atoms have been permanently displaced from their original location and that information has been permanently lost to the paper clip. It cannot "remember" where it was, to use everyday terminology. There is a more accurate, but subtle, reason if you compare the behavior of the metal and a rubber band. When the rubber band is stretched its entropy (the number of molecular configurations available to it in the stretched state) actually decreases. This is reflected in the observation that if you place a rubber band on your lips and stretch it you can feel it cool slightly. So when you let go the rubber band molecules seek the state that maximizes the number of molecular configurations available to it -- that is its original 'unstretched' state -- and so it returns to that state spontaneously. In contrast, if you place a paper clip on your lips when you distort it, you increase the number of atomic states available to it. This is reflected in the observation that you can feel the paper clip warm slightly. When you let go the paper clip seeks the state than maximizes the number of atomic configurations available to it -- its entropy increases.

The second law of thermodynamics states that (approximately isolated objects) spontaneously go to the state that maximizes the number of atomic/molecular configurations available to it. In the case of the paper clip it is the distorted shape. In the case of a rubber band it is the original unstretched state.

Vince Calder



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