CoachThe molecules of your seventh year are themselves built: every molecule is an assembly of atoms — nature’s alphabet, about a hundred kinds, whose combinations spell every substance. A water molecule: two hydrogen atoms and one oxygen. An atom, in turn, has architecture: a minuscule, heavy, positively charged nucleus at the center, and, far outside it, a cloud of electrons — almost weightless carriers of negative charge. Atom sizes run near ; their nuclei, near — a hundred thousand times smaller again.
Examples
Example 71.3 (The old joke, paid)
The electricity chapters owed you a small joke, and here it is. The marching charges in every wire of this book are electrons — and they are negative, so they march from the battery’s terminal to its : precisely opposite to the conventional direction fixed by physicists a century before anyone could ask the marchers. The convention was kept — every law of your circuits works perfectly with it — but the little irony stands: in every diagram you have drawn, the real crowd walks the other way.
Example 71.4 (And further down)
The nucleus has parts of its own, and their story — how they bind, why some nuclei split (the power station’s heat) or merge (the Sun’s) — runs physics down to metres and beyond, into the smallest structures humans have probed. The High School volume opens the nucleus properly; the university years go further still. The ladder’s lower rungs are still being built.
Example 71.7 (Ladder arithmetic)
How many atoms span a pencil’s width? Pencil, ; atom, : eight rungs — a hundred million atoms, shoulder to shoulder. How many Earths to the Sun? over : about — ten thousand marbles of the sports-field model, laid in a row. The ladder answers in seconds what raw kilometre-counts bury in zeros.