Solids have a fixed shape due to the
strong attractive forces between the particles.
When the material is stretched so it
cannot immediately return to its original shape, the forces of attraction
still try to restore the particles to their original positions.
In this process work is done on the
particles in this separation and so potential energy is stored.
If the restriction is removed (applied
force released), particles can 'spring' back into position releasing
the elastic potential energy - a good example is wound up a clock
spring.
Note: If the applied force is too strong
and the interparticle forces are weakened too much, the particles cannot
return to their original positions - so the shape is permanently changed -
plastic deformation.
So, to sum up, when a solid is stretched, particularly if it is elastic,
work is done in pulling the particles very slightly apart and this
creates an elastic potential energy store as the particles are
trying to regain their original positions of minimum potential energy. A
good example is a stretched rubber band.
A
particle picture comparing elastic and inelastic materials
In terms of spacing in the lattice, the
particles (atoms, ions or molecules) in a solid are at equilibrium in terms of their
spacing. These fixed 'balanced' positions are determined by the balancing of
various forces of particle attraction (opposite charges) and repulsion (like
charges).
When you apply a force to a solid object you
are pressing/squeezing the particles closer together giving rise to an opposing
force of repulsion. To continue any compression requires a greater and greater
force. When the compression force is removed the particles repel each other and
move back to their original position if it is a truly elastic material.
When an object is stretched you are pulling
the particles apart from their normal stable positions and the attractive forces
between the particles try to resist the stretching - force of tension produced.
Therefore you are doing work on the system and storing energy in it as you
stretch the material.
If the stretching force is removed, and the material is
truly elastic, the object will return to its original shape and length. Rubber
materials have molecules that can actually be stretched at the particle level,
straining the chemical bonds. These bonds can relax back to normal length and
the object e.g. a rubber band or a steel spring return to their original shape
and length.
However, if the force is great enough, some
of the bonds are broken e.g. with an overstretched rubber band, or, the
particles are forced to shift position changing the structure in some way e.g. an over
stretched steel spring where layers of atoms can slide over each other. This
produces a permanent deformation when you go beyond the elastic limit of
proportionality and Hooke's law no longer applies.
