Thermal Expansion

Thermal Expansion

When a solid is heated, its molecules gain extra energy and vibrate more violently and need more room for movement. The molecules try to push their neighbours slightly further away, against their mutual attraction. So heating a solid slightly increases the distance between the molecules and causes expansion in all directions.

The video shows two examples of expansion as a result of heating.

Example 1 - A metal bar with a pointer on the left side is heated. As the temperature of the bar increases and the molecules push against each other, it causes the bar to expand. The pointer, as a result moves as an indication of the expansion.

Example 2 - A ball is shown to be able to fit inside a ring when cool. After being heated for a few seconds, the ball has expanded and is unable to fit inside the ring. It must be cooled again, so that it can shrink to its original size to be able to fit in the ring once again.

[The video is in another language but you can watch the demonstration]

Applications of thermal expansion:

Bimetallic strip:

A bimetallic strip is made of two strips of different metals, e.g. brass and iron, welded together. When cold the double strip is straight. As it is heated the brass expands more than the iron and so the brass forms the outside of a curve and the iron the inside. Bimetallic strips are used in thermostats and many other mechanical switching circuits, but increasingly these are being replace by electronic circuits with moving parts.

Below is a representation of the bimetallic stip.

[The video below is a demo of a bimetallic strip [in another language but you can watch]

I think what he is trying to show, by repeating the experiment and placing the heat source at different locations, is that the result is the same. The strip will curve in one direction (the same direction) because one metal expands more than the other.