Environmental Engineering Reference
In-Depth Information
1
What Is Energy?
1.1 Aristotle in Times Square
Theterm'energy'hasbecomeubiquitous,aslikelytobeheardinayogaclassasataphysics
lecture. In its everyday use, it has become synonymous with force, vigour, well-being and
a certain kind of atmosphere. We talk about people or places having energy, a certain kind
of energy, or lacking it altogether. We've become so used to using the words 'energy' and
'energetic' as pliant descriptors that we're liable to overlook their scientific significance.
A first-time visitor to Times Square, the heart of one of the world's busiest cities, is
likely to first comment on the 'energy' of the place. But does this use of the term bear
any relation to its scientific meaning? The Greek term
ένέργεια
(
energeia
), the origin of the
English word, was probably coined by Aristotle. It combines the prefix
en
, meaning 'in' or
'at', with
ergon
, meaning 'action' or 'work'. According to Aristotle, all living beings are
defined by this attribute; they are 'at work', in contrast to inactive, inanimate objects. So
energeia
, for Aristotle, was intimately connected to movement. This philosophical concept
of energy remained for more than 2,000 years the main usage of the term. As late as 1737,
the philosopher David Hume wrote that there were “no ideas, which occur in metaphysics,
more obscure and uncertain, than those of power, force, energy or necessary connexion”.
Thefirstattemptstodefineenergyinscientifictermsdatebacktotheseventeenthcentury.
Isaac Newton established that the same force (gravity) which causes an apple to fall from
a tree also determines the movement of the planets around the sun. Newton's contemporary
Gottfried Leibniz identified something he called
vis viva
(literally 'living force'), the force
of any moving thing. Leibniz began the process of quantifying energy when he concluded
that while the force of a moving object depends both on its mass (weight) and its velocity
(speed), velocity was far more important than mass. In other words, a light but fast-moving
object has far more force than a heavy but slow-moving one. Just imagine catching a
basketball, which weighs about 600 grams, thrown by a teammate. Now compare this with
the impact of a 10-gram bullet fired from a gun.
The human understanding of energy took a huge leap forward during the Industrial
Revolution, pioneered by industrialists who were motivated as much by commercial
ambition as by scientific enquiry. For them, energy was not an abstract idea; it was the force
needed to drive the machines that were rapidly replacing human and animal labour. They
therefore redefined energy as the ability to perform work. This remains the most common
