4 Chapter 1. What the ancients knew[[Student version, December 8, 2002]]
Figure 1.1:(Diagram.) Various ways to get up a hill. Each arrow represents an energy-conversion process.its mechanical energy is lost. Nobody has ever seen a rock sitting in warm mud suddenly fly up
into space, leaving cold mud behind, even though such a process is perfectly compatible with the
conservation of energy!
So even though energy is strictly conserved,somethinghas been wasted when we let the rock
plop. To make a scientific theory of this something, we’d like to find an independent, measurable
quantity describing the “quality” or “usefulness” of energy; then we could assert that sunlight, or
the potential energy of a rock, has high quality, whereas thermal energy (heat) has poor quality.
Wecould also try to argue that the quality of energy always degrades in any transaction, and in
this way explain why the conversions indicated by arrows in Figure 1.1 are so much easier than
those moving against the arrows. Before doing these things, though, it’s worthwhile to recall how
the ancients arrived at Idea 1.1.
1.1.2 Just a little history
Physicists like a tidy world with as few irreducible concepts as possible. If mechanical energy can
beconverted to thermal energy, and (partially) reconverted back again, and the sum of these forms
of energy is always constant, then it’s attractive to suppose that in some sense these two forms
of energy are really the same thing. But we can’t build scientific theories on æsthetic, culturally
dependent judgements—Nature cares little for our prejudices, and other eras have had different
prejudices. Instead we must anchor Idea 1.1 on some firmer ground.
An example may help to underscore this point. We remember Benjamin Franklin as the great
scientist who developed a theory of electricity as an invisiblefluid.Franklin proposed that a
positively charged body had “too much” of this fluid^1 ,and a negative body “too little.” When such
(^1) Franklin’s convention for the sign of charge was unfortunate. Today we know that the main carriers of charge—
electrons—each carry anegativequantity of charge in his convention. Thus it’s more accurate to say that a positively
charge body has too few electrons, and a negatively charged body too many.