Bent Metal Returning to Original Shape
Date: May 2006
When a metal paper clip is straightened out why is
it difficult to bend back to its original shape?
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
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.
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Update: June 2012