Geology Reference
In-Depth Information
Box 1.1 What is energy?
the concept of energy is fundamental to all branches of
science, yet to many people the meaning of the term
remains elusive. In everyday usage it has many shades of
meaning, from the personal to the physical to the mystical.
Its scientific meaning, on the other hand, is very precise.
to understand what a scientist means by energy,
the best place to begin is with a related - but more
tangible - scientific concept that we call
work
. Work is
defined most simply as
motion against an opposing force
(atkins, 2010, p. 23). Work is done, for example, when a
heavy object is lifted a certain distance above the ground
against the force of gravity (Figure 1.1.1). the amount of
work this involves will clearly depend upon how heavy
the object is, the vertical distance through which its
centre of gravity is lifted (Figure 1.1.1b), and the strength
of the gravitational field acting on the object. the work
done in this operation can be calculated using a simple
formula:
equation 1.1.1 shows is equivalent to kg × m × m s
-2
=
kg m
2
s
-2
(see table a2, appendix a). alternative forms of
work, such as cycling along a road against a strong oppos-
ing wind, or passing an electric current through a resistor,
can be quantified using comparably simple equations, but
whichever equation we use, work is always expressed in
joules.
the weight suspended in its elevated position
(Figure 1.1.1b) can itself do work. When connected to suit-
able apparatus and allowed to fall, it could drive a pile into
the ground (this is how a pile-driver works), hammer a nail
into a piece of wood, or generate electricity (by driving a
dynamo) to illuminate a light bulb. the work ideally recov-
erable from the elevated weight in these circumstances is
given by equation 1.1.1. If we were to raise the object
twice as far above the ground (Figure 1.1.1c), we double
its capacity for doing work:
Work
=× ×
mhg
2
(1.1.2)
Work
=××
mhg
J kg m ms
2
(1.1.1)
−
alternatively if we raise an object three times as heavy
to a distance
h
above the ground (Figure 1.1.1d), the
amount of work that this new object could perform would
be three times that of the original object in Figure 1.1.1b:
where
m
represents the mass of the object (in kg),
h
is
the distance through which its centre of gravity is raised
(in m - see footnote)
2
, and
g
, known as the
acceleration
due to gravity
(metres per second per second = m s
-2
), is a
measure of the strength of the gravitational field where
the experiment is being carried out; at the earth's sur-
face, the value of
g
is 9.81 m s
-2
. the scientific unit that
we use to measure work is called the joule (J), which as
Work
=××
3
mhg
(1.1.3)
the simple mechanical example in Figure 1.1.1 shows only
one, simply understood way of doing work. Mechanical work
can also be done by an object's motion, as a demolition
crew's 'wrecking ball' illustrates. electric current heating the
element of an electric fire represents another form of work,
as does an explosive charge used to blast a rock face in a
mine.
m
in italics represents mass (a variable in this equation); m in
regular type is the abbreviation for metres (units).
2
molecules in liquid water are more mobile than those in
ice - that is, they have higher
kinetic energy
- the
enthalpy of water (
H
water
) is greater than that of an
equivalent amount of ice (
H
ice
) at the same temperature.
The difference can be written:
between the initial (solid) and final (liquid) states of
the compound the H
2
O. It represents the work (Box 1.1)
that must be done in disrupting the chemical bonds
that hold the crystal together. Δ
H
symbolizes the
amount of heat that must be supplied from the sur-
roundings for the crystal to melt completely; this is
called the
latent heat of fusion
, or more correctly the
enthalpy of fusion
, a quantity that can be measured
experimentally or looked up in tables.
∆
HH H
=
−
ice
(1.2)
water
The Δ symbol (the Greek capital letter 'delta'), when
written in front of
H
, signifies the
difference
in enthalpy
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