Thermal expansion - demonstration experiments
The thermal expansion of
liquids is quite difficult to observe because
they do not expand much. The best way to see it is to use a large flask with a capillary tube
sticking out. The volume expansion of the liquid is then "magnified" by the movement up the
narrow tube – rather like a thermometer.
Other liquids to try are paraffin, turpentine,
glycerol and all sorts of oils (motor oil, sunflower oil, olive oil).
A few ideas about
thermal expansion using the
expansion of gases:
(a) any kind of pressurised
gas canister will explode if heated over a fire and so can be very dangerous
(b) the
gas/air mixture in the cylinder internal combustion engine "explodes" and expands when
heated by a spark from the spark plug
(c) the air in a hot air balloon expands when it is
heated so lowering its density and helping the balloon to rise
(d) an inflated sunbed or
rubber dinghy may explode if left in the sun for a long time
(e) a dent can be removed
from a table tennis ball by putting it in hot water
(f) cakes "rise" because of the gas in
them expanding
(g) Galileo's air thermometer works by the expansion of air
(h)
racing car tyres are kept warm to make them expand and become firm. This raises the cars
slightly off the road
(i) when a bullet is fired the air behind it is heated rapidly by the
explosion of the cartridge and the bullet is forced up the barrel of the gun
and now
some effects of
solid expansion:
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.
Bimetallic strip - this is made of two metals that expand by different amounts
when heated joined together. 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
See:
Expansion of solids