All these effects can be explained by the expansion
of solids when they are heated.
When a material gets hot it expands - this is
because the molecules in it are moving about more vigorously and so need more
room.
As a solid is heated the molecules vibrate more violently and the solid expands in
all directions. We will just look at the increase in length for simplicity. The hotter it gets and
the longer it was to start with the more the solid expands. Different materials expand by
different amounts for the same rise in temperature.
The amount of expansion depends
on
(a) what material it is
(b) how big the temperature rise is
(c) how long it was
to start with
Solids do not expand very much and so we have to find a way of
magnifying this expansion. You can do this in the laboratory with a bar of iron or steel and a
slide projector, the bar need only be about 50 cm long. The bar is fixed in a retort stand and
the end of the bar is placed so that it sticks into the beam of the projector where the slide
carrier would have been. The shadow of the bar is focused onto a wall on the other side of
the laboratory.
The bar is now heated strongly in a Bunsen flame and you will see
the shadow grow. If you measure the width of the bar and the shadow you can work out the
magnification of the projector and then the actual expansion of the bar.
This piece of apparatus shows that it is very difficult to stop metals from expanding when you heat them and shrinking when you cool them. A strong steel bar is fixed in the frame of the apparatus by a large nut at one end a cast iron peg at the other. When the bar is heated the peg breaks because of the huge force in the bar. It is also possible to make the peg break when the bar contracts on cooling by tightening the nut when the bar expands.
