Geology Reference
In-Depth Information
will be to show that thermodynamics, even at a simple
and approachable level, can contribute a lot to our
understanding of chemical reactions and equilibrium in
the geological world.
Energy changes in chemical systems are most easily
introduced (as in Box  1.1) through analogy with
mechanical forms of energy, which should be familiar
from school physics.
Secondly, an object in a gravitational field possesses
energy (i.e. can do work) by virtue of its position in
that field, a property known as potential energy . The
water held behind a hydroelectric dam has a high
potential energy: under the influence of the Earth's
gravitational field it would normally flow downhill
until it reached sea-level, but the dam prevents this
from happening. The fact that the controlled downward
flow of this water under gravity can be made to drive
turbines and generate electricity indicates that the
water held behind the dam has the potential for doing
work, and therefore possesses energy.
The potential energy E p of an object of mass m at a
height h above the ground is given by:
Energy in mechanical systems
The energy of a body is defined as its capacity for doing
work (Box 1.1). As we have discovered, 'work' can take
various forms, but in simple mechanical systems it
usually implies the movement of a body from one pos-
ition to another against some form of physical resist-
ance (friction, gravity, electrostatic forces, etc.). Then:
Em h
p =××
() ( (
J gmsm
) ()
where g is the acceleration due to gravity (9.81 m s 2 ).
Similar equations can be written representing the
potential energies of bodies in other types of force
field, such as those in electric and nuclear fields.
An important aspect of potential energy is that its
value as calculated from Equation 1.5 depends upon the
baseline chosen for the measurement of height h . The
potential energy calculated for an object in a second-
floor laboratory, for example, will differ according to
whether its height is measured from the laboratory floor,
from the ground level outside, or from sea-level. The last
of these alternatives seems at first sight to be the most
widely applicable standard to adopt, but even that refer-
ence point fails to provide a baseline that can be used for
the measurement of height and potential energy down a
deep mine (where both quantities might have negative
values measured relative to sea-level). This ambiguity
forces us to recognize that potential energy is not some-
thing we can express on an absolute scale having a
universal zero-point, as we do in the case of temperature
or electric charge. The value depends upon the 'frame of
reference' we choose to adopt. We shall discover that
this characteristic applies to chemical energy as well. It
seldom presents practical difficulties because in thermo-
dynamics one is concerned with energy changes , from
which the baseline-dependent element cancels out (pro-
vided that the energy values used have been chosen to
relate to the same frame of reference).
In general, a body possesses kinetic and potential
energy by virtue of its overall motion and position.
work done
J oules
Nnewtons kgms
distance body ismoved
So, for example, the work done in transporting a train-
load of iron ore from A to B is the mechanical force
required to keep the train rolling multiplied by the dis-
tance by rail from A to B . The energy required to do this
is generated by the combustion of fuel in the engine.
One can distinguish two kinds of mechanical energy.
Firstly, an object can do work by means of its motion. A
simple example is the use of a hammer to drive a nail
into wood. Work is involved because the wood resists
the penetration of the nail. The energy is provided by
the downward-moving hammer-head which, because
of its motion, possesses kinetic energy (a name derived
from the Greek kinetikos , meaning 'setting in motion').
The kinetic energy E k possessed by a body of mass m
travelling with velocity v is given by:
E mv
k =
() ( (
1 2
Thus the heavier the hammer ( m ) and/or the faster it
moves ( v ), the more kinetic energy it possesses and the
further it will drive in the nail. For similar reasons, a
fast-moving stream can carry a greater load of sediment
than a slow-moving one.
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