more about stress and strain

What is true of a metal wires loading and unloading curves if the elastic limit is not exceeded?

They are the same, the wire returns to its original length when unloaded

What is the difference between elastic limit and limit of proportionality?

Limit of proportionality is the point beyond which the material no longer obeys hookes law (stress strain graph no longer straight)

Elastic limit is the point beyond which the material no longer returns to its original length when load is removed

What happens to a metal wires unloading curves if it’s elastic limit is exceeded?

The unloading line is parallel to the loading line, but the wire is slightly longer when unloaded since it has permanent extension

What is permanent extension?

When a material is stretched beyond its elastic limit and does not return to its original length upon unloading

How does the unloading curve of a rubber band compare to its loading curve?

The unloading curve is below the curve for a given extension (except at zero and maximum extensions) meaning the extension during unloading is greater than during loading

Does a rubber band return to its original length after unloading?

Yes, it returns to the same unstretched length, so it remains elastic

What are the elastic properties of a rubber band?

It is elastic (returns to original length) but has a low limit of proportionality (does not obey hookes law over a large range)

What two properties characterise a polythene strip under tension?

A low limit of proportionality and plastic deformation. It does not return to its original length when unloaded

What is the key difference between rubber and polythene when unloaded?

Rubber returns to its original length (elastic) whereas polythene does not (plastic deformation)

What does the are between the loading and unloading curves of a rubber band represent?

Energy lost as heat during the stretching and releasing cycle/ the difference between energy stored when stretched and useful energy recovered when unstretched

What does the area under the loading curve of a rubber band represent?

The work done to stretch the rubber band

What does the area under the unloading curve of a rubber band represent?

The work done by the rubber band when it is unloaded (useful energy recovered)

Why does the energy difference in rubber bands occur?

Some of the energy stored in the rubber band becomes the internal energy of the molecules when the rubber band unstretches

What does the area between the loading and unloading curves represent for polythene?

Work done to permanently deform the material, as well as internal energy retained by the polythene when it unstretches.