Defining energy turns out to be more difficult
than what one might think. The high school physics definition of energy “The
ability to do work” does not take us very far, for what is ability? And what is
work? What is the stuff (or non-stuff) that allows this to occur? Robert Romer
wrote a good physics textbook which was about using energy concepts to
understand all the conventional material of physics because “all physics is
about energy.” Yet even he admitted that he was unable to give a satisfactory
definition of energy. He said we can see energy's effects, we can measure them,
but we do not really know what it is.
Physicist
Jacques Treiner recognizes the difficulty in defining energy precisely. He says
“Energy, in the scientific context we are dealing with, is an abstract notion,
of mathematical nature, which allows us to quantify the transformations of
matter.” Usually we detect energy containing or transforming materials (food or
petroleum and oxygen) and energy being used because something is moved: a car,
a basketball player, chemicals against a gradient, and so on. Hence energy can
be thought of, not quite precisely, as that which causes motion. For our
day-to-day experiences, energy is mostly found associated with either photons
coming from the sun or the oxidation of fuels such as wood or food or oil that
generates work (i.e., moves something) at some point in space and time. These
are things we can experience and understand pretty well, even if the physicists
cannot define them exactly, at least in a way that most of us can readily
grasp. One of the things that makes energy easy for me to understand fairly
well is that, having measured biological energy a great deal, I am impressed
with the general sense and repeat-ability of the measurements. But I understand
that I am not measuring energy itself, but rather its effects.
Physicist Jacques Treiner has thought a lot about energy. He says
we do not live on energy. We live on transforming matter around us (and, C.H.
adds, extracting the energy within it). One way of visualizing the notion is to
ask how much matter has to be processed in order to deliver or consume a given
amount of energy? The answer depends of course on the type of energy and the
interactions involved in the process. The more intense the energy interaction involved;
the less matter will be necessary.
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