Geoscience Reference
In-Depth Information
3.4
Thermal energy and mechanical work
3.4.1
Thermal systems
conduction. In fluid systems, the heat can then be trans-
ported by convection. Heat transfer at a distance through
the gases of the atmosphere occurs by radiation.
We saw previously (Sections 1.5 and 2.2) that many Earth
layers are defined by their temperature contrast with neigh-
boring layers. The flow behavior of such thermal systems
depends upon temperature-dependent properties such as
density and viscosity. Air and shallow water change density
and pressure because of variations in temperature caused by
solar heating. Erupting volcanoes discharge lava which
cools and changes viscosity during gradual descent. The
venting of a “black smoker” is another obvious example.
These are all physical systems in thermal disequilibrium ;
that is, their temperature is varying with time. By way of
contrast, the main mass of lake or ocean waters may be said
to be in thermal equilibrium where temperature changes
are very slow over a time span large in relation to their
speed of travel. Clearly, small volume thermal systems
and/or those having thermal contact with other thermal
systems are more likely to suffer changing temperature than
large volume systems: we call these latter thermal reservoirs .
The oceans are good thermal reservoirs, due to both their
large volume and the high thermal capacity (Section 2.2) of
water. The idea of thermal contact is a useful one with
which to view the transfer of heat by conduction, convec-
tion, and radiation between thermal systems. As we shall
see in more detail later (Sections 4.18-4.20), when two
thermal systems directly touch, the heat is transferred by
3.4.2
Thermal variables of state
These are pressure, p , temperature, T , volume, V , and mass,
m ; relationships between the variables are expressed as the
equation of state. To illustrate this with an everyday exam-
ple, if you compress air from a bicycle pump into a tire, the
increase of pressure in the pump is accompanied by volume
decrease and temperature increase. As we shall see in a later
section, we are pushing the widely spaced air molecules
together, causing them to collide more frequently, both
with themselves and with the walls of the pump and its
moving piston. Boyle first experimented on this interesting
problem, though not with a bicycle pump, and showed that
the increased pressure times the decreased volume was pro-
portional to the increased temperature (Fig. 3.13). In
other words, for a constant mass of air, the three variables
vary in proportion as pV
T . When applied to dilute (low
density) gases, the relationship becomes pV
nRT the
Ideal Gas Law , where the parameter n is the number of
moles of gas present and R is the universal gas constant .
This important equation has many applications; we use it to
calculate the density of air (Box 3.1).
This line is the trace of the constant product pV
as we vary these at constant T. The curve is an
isotherm and at all times T is constant and p 1 V 1 = p 2 V 2
Boyle
p 1
Isotherm A represents a > T than isotherm B.
This is because for a variable T , p is proportional
to T / V
p 2
Note the shapes of the
isotherms, they are
hyperbolae. This follows
because if pV = k , then p
must be proportional to
1/ V , that is, increasing p
leads to decreasing V
A
B
Boyle
V 1
V 2
Volume, V
Fig. 3.13 Boyle's Law tells us that for a given concentration and volume of dilute gas at a given or fixed temperature, the product pV
is constant.
Search WWH ::




Custom Search