Spark image

The expansion of solids


Telephone wires are hung up slack in the hot summer weather so that they do not pull the telegraph poles over when they contract in the winter
Railway lines are laid with small gaps to allow for expansion in hot weather
Bimetallic strip - this is made of two metals joined together that expand by different amounts when heated. It is used in thermostats and fire alarms
Concrete roads are laid in sections with soft pitch between the sections
Girders in buildings and bridges are made with gaps at the ends
Glass to be used in cooking has to be a low expansion type such as Pyrex otherwise it would shatter as it got hot
High-speed planes are warmed by air friction and so get longer
Old buildings can be held together by a metal rod fixed through them and joined to plates on the walls
Rocks in deserts crack, bits fall off them and turn to sand in the end
Rivets are heated before they are put in place to hold two metal plates together

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.



Other example experiments:
(a) ball and ring
(b) bar and guage
(c) bimetallic strip see later


(d) bar breaker

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.


The bi-metallic strip

This is a strip of two different metals welded together, one side is brass (high expansion) the other is iron (low expansion). When the strip is heated it bends with the brass always on the outside of the curve.




Questions

1. Draw two diagrams to show what happens to the same bimetallic strip when it is:
(a) heated in a flame
(b) cooled in a freezer

2. What happens to the hole in a metal washer when the washer is heated? Does the hole get bigger, smaller or stay the same size?

3. Why will a hot, thick glass beaker shatter if plunged into cold water?

4. How will the expansion of the pendulum affect the running of a grandfather clock with a metal pendulum?


 
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© Keith Gibbs 2020