90 The Poetry of Physics and The Physics of Poetry
this book and try to compress it. After squeezing out the air between the
pages, you will find little if any give. You are pushing the paper
molecules against one another and they are resisting the force you are
creating. The attractive force between the molecules can be felt by trying
to tear a solid apart. This force is not always overwhelming as can be
demonstrated by tearing a piece of paper in half.
Although there exists an equilibrium position between molecules
where the molecular force vanishes, the molecules of a solid do not sit
placidly in this position. In fact, the molecules are oscillating wildly
about their equilibrium positions. It is this motion, which we identify as
heat. It is the kinetic energy of this internal motion of the molecules,
which explains why it takes energy to transfer heat to matter. The
temperature of a solid represents the rate at which the molecules are
moving. The heat of a solid represents the total amount of internal energy
due to its internal molecular motion. As the temperature of a solid
drops, the molecular motion slows down and hence the amount of heat
contained in the solid diminishes. The lowest temperature possible is
the point at which all molecular motion ceases. This point known as
absolute zero (0 Kelvin) although impossible to achieve exactly, occurs
at –273.15° Centigrade.
As the temperature of a solid increases the molecular agitation
becomes greater and greater. As the temperature reaches the melting
point, which for water is 0° Centigrade, the molecular motion becomes
so violent that the molecular forces can no longer maintain the structure
necessary for the substance to remain a solid.
The substance at this point passes into the liquid phase in which a
large number of the molecular bonds have been broken. A number of
molecular bonds still remain, however, in the strings of molecules that
form the structure of a liquid. These strings of molecules easily pass by
each other, which explains the fluid nature of liquids. This ability to flow
is also possessed by granulated forms of solids such as sand and salt, and
provides a macroscopic picture of the fluid nature of liquids.
The electric forces between molecules in a liquid manifest themselves
in a number of ways. The short-range repulsive interaction is responsible
for the incompressibility of liquids in much the same manner as it is
with solids. The attractive aspect of the molecular interaction on the
other hand is responsible for the property of liquids to coalesce. This
property is observed as the tendency of water to form itself in drops.
At the surface of a drop or any other aggregate of a liquid one observes