Environmental Engineering Reference
In-Depth Information
forces that existed between certain minerals, which the Chinese used to build com-
passes as an aid to navigation. The Greeks noted, too, that amber, when rubbed
against furs, acquired the ability to attract small objects. By the 17th century, sci-
entists understood that materials rubbed against furs took on attractive as well as
repulsive forces. This understanding gave rise to the idea, introduced by Benjamin
Franklin in 1747, of positive and negative “electricity.” Franklin also clarified that
lightning is caused by “electricity” accumulated in clouds.
In 1785 Charles-Augustin de Coulomb began to measure these forces. What he
found was that the law of attraction (or repulsion) between two charges is similar
to the gravitational attraction existing between two bodies: it is proportional to the
amount of electricity in the two bodies and inversely proportional to the square of
the distance between them.
Heinrich Hertz discovered that rapidly oscillating electric charges generate elec-
tromagnetic waves that propagate in space and have an electric and a magnetic
component. His discovery opened the way to radio communication, and later to
television.
Nuclear
forces exist among the constituents of the nucleus of the atoms.
The basic constituents of matter are atoms, which have a structure similar to that
of solar system, in which planets turn around a central body (as the Earth circles
the Sun). In atoms, electrons (which have negative charges) are attracted and move
around the nucleus of the atom (which contains the positively charged protons) the
same way as planets circle around the Sun. As a whole, atoms are electrically neut-
ral. Typically atoms are 10
-8
cm in diameter. Molecules are made up by a combin-
ation of atoms.
The chemical elements are characterized according to the number of electrons
each has circling its nucleus: for example, hydrogen has 1 electron, helium 2, and
uranium 92. The nucleus has very small dimensions, typically 10
-13
cm in diamet-
er. Therefore, the repulsion among the protons is very strong. To counteract these
forces, there are nuclear forces that bind the protons together when they are separ-
ated by distances smaller than 10
-13
cm. In the nucleus there are also particles that
have no charge (called neutrons) that have a role in this binding process.
The expansive force of gases has also been known since antiquity, but the study
of the expansive force developed completely independent of the study of mech-
anics. The two studies were unified at the end of the 18th century when scientists
realized that mechanical work can be transformed entirely into heat. J. J. Thomson
(1753-1814) first noted this when observing the process of boring holes into iron
