Professor Dave here, let's talk about heat.
We are all quite
familiar with the concept of heat and
can describe the sensation of feeling
too hot or too cold, but understanding
what things like heat and temperature
really are on the molecular level can be
pretty tricky business.
Nevertheless, we must fundamentally
understand them as a prerequisite to
learning about thermodynamics so let's
dig into this abstract concept. We can
start with temperature, like the
temperature outside today. It might be hot,
it might be cold, but in science we want
to quantify everything that we can so
saying something is hot or cold just
isn't enough. We want to be able to say
how hot or cold with numbers. But that
begs the question, what is it that
temperature is physically measuring?
Temperature is measuring the amount of
heat energy available for work in a
system and heat energy is related to the
average kinetic energy of the atoms and
molecules within that system. We know
that greater kinetic energy means
objects will move faster and this is
true of molecules in the atmosphere just
as it is of macroscopic objects, so
higher temperatures simply means faster
moving particles. This kinetic energy of
motion is distributed amongst
translational or side-to-side motion
rotational or spinning motion and
vibrational motion where the covalent
bonds in a molecule bend and stretch.
Heat will flow from hot objects to cool
objects because as the particles move
they will impart some of their kinetic
energy onto other particles through
collisions. This will continue to occur
until the whole system is that the same
temperature in a state called thermal
equilibrium. This is how thermometers
measure temperature. Heat from the
environment will be in contact with the
thermometer and flow into the
thermometer fluid causing the atoms to
gain kinetic energy and the column will
expand until it is in thermal
equilibrium with the environment
thus allowing us to read the temperature
in one of our man-made units. We must
bear in mind that all phases of matter
expand with higher temperature whether
solid liquid or gas.
This phenomenon is called thermal
expansion. Energy is transferred as heat
in a wide variety of processes. When a
collision occurs there is usually some
heat generated. Friction can cause heat as
well which is why your hands warm up
when you rub them together vigorously.
In each case we are considering the kinetic
energy of the particles and the transfer
of some of this energy to other
particles as the definitions of heat and
heat transfer. Heat is therefore a kind
of energy in transit, and will be
measured in joules just like any other
kind of energy.
Furthermore the transfer of heat energy
will always alter the temperature of an
object, as heat will always flow from the
higher temperature object to the lower
temperature object until thermal
equilibrium is reached.
This is why hot objects feel hot and
cold object feel cold, because a hot cup
of coffee, which is hotter than your hand
will transfer heat into your hand while
a cold piece of ice, which is colder than
your hand, will cause heat to be
transferred out of your hand and into
the ice. These events produce the hot
and cold sensations we feel in our
day-to-day life. Now that we better
understand heat and temperature,
let's check comprehension.
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