SITEMAP   Physics Notes: Forces 4.4 Elastic potential energy model & elastic or inelastic?

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Forces 4: 4.4 A particle model of an elastic potential energy store - using it to explain the structure and behaviour of stressed elastic and inelastic materials

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4.4 A particle model of an elastic energy store

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.

Keywords, phrases and learning objectives for elastic potential energy

Be able to describe and explain a particle model description of an elastic potential energy store.

Using a particle model, be able to explain the difference between the elastic and inelastic behaviour and the structure of materials

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