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INTRODUCTION TO THERMODYNAMICS: INTERNAL ENERGY 81

Worked Example 3.1 Use Equation (3.1) to demonstrate that U
Important: although
is negative for the condensation of steam if, say, U (final) = 12 J and
U (initial) = 25 J. we have assigned
numerical values to
U (final) and U (initial) ,it
Inserting values into Equation (3.1):
is, in fact, impossible
to know their values.
U = U (final) − U (initial)
In reality, we only
know the difference
U = (12 − 25) J
between them.
U =−13 J
So we calculate the value of U as −13 J. The change in U is negative
and, therefore, exothermic, as expected. The symbol ‘J’ here
We see that U is negative. We could have reasoned this result means joule,which is
by saying U (final) <U (initial) , and subtracting a larger energy from a the SI unit of energy.
smaller one generates a deficit.

Why do we sweat?

Endothermic reactions

We all sweat at some time or other, e.g. after running hard, living
We need the salt in
in a hot climate or perhaps during an illness when our temperature
sweat to decrease the
is raised due to an infection (which is why we sometimes say, we
water’s surface tension
have ‘got a temperature’). in order to speed up the
Producing sweat is one of the body’s natural ways of cooling evaporation process
itself, and it operates as follows. Sweat is an aqueous solution (we feel cooler more
of salt and natural oils, and is secreted by glands just below the quickly). The oils in
surface of the skin. The glands generate this mixture whenever the sweat prevents the
body feels too hot. Every time air moves over a sweaty limb, from skin from drying out,
a mechanical fan or natural breeze, the skin feels cooler following which would make it
evaporation of water from the sweat. susceptible to sunburn.
When we say the water evaporates when a breeze blows, we
mean it undergoes a phase transition from liquid to vapour, i.e. a
phase transition proceeding in the opposite direction to that in the
Evaporation is also
previous example, so Equation (3.2) occurs backwards. When we
called ‘vaporization’.
consider the internal energy changes, we see U (final) = U (water, g)
It is a thermodynamic
and U (initial) = U (water, l) , so the final state of the water here is more
process, because en-
energetic than was its initial state. Figure 3.2 shows a schematic ergy is transferred.
representation of the energy change involved.