Principles of Analog Electronics

61

Nature of Matter

Electrical and electronic engineering essentially originate from exploiting the

motion of electrical charges. These constitute fundamental parts of matter, which

makes it essential to know its basic elements, namely the atoms. In the past, many

theories regarding the structure of an atom has been hypothesized; let’s consider

here the most relevant.

3.1 ATOMIC MODEL

We can start with the model of Democritus (Democritus, Greek philosopher,

460–370 BCE) who hypothesized that all matter was discontinuous, so in trying

to divide it iteratively, there was a limit beyond which the matter itself could not

be decomposed anymore. Therefore, he thought a tiny, fundamental basic unit

existed and called it the atom (from the greek atomos: not divisible).

The theoretic-philosophical approach of Democritus was proved experimentally

when Dalton(John Dalton, English chemist and physicist, 1766–1844, Figure 3.1)

observed that in compounds, that is substances formed by the union of basic ele-

ments, the proportion between the masses of the individual elements is always

constant. This result is possible only if the hypothesis of the existence of the

atoms was true.

Therefore, matter shall be constituted by a basic component with a well-dened

mass.Dalton hypothesized the atom as a unique sphere.

Afterwards, Thomson (Sir Joseph John Thomson, English scientist, 1856–1940,

Figure 3.2), thanks to experiments with cathode rays (luminescent rays com-

ing from the negative terminal, named the cathode, of a tube lled with gas to

3

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Principles of Analog Electronics

62

which was applied a difference of voltage) discovered

that the atom had to be made of moving negatively

charged particles, electrons, having a mass 2000

times smaller than the lightest known atom (hydro-

gen). But, being neutral matter, those negative charges

had to be balanced by positive charges.

The atomic model proposed was again spherical,

but not indivisible anymore. Rather it had itsinternal

negative particles immersed in a “dust” of thin posi-

tively charged matter.

The successor of Thomson at the Cavendish labora-

tories of Cambridge University was Rutherford (Ernest

Rutherford, New Zealand physicist, 1871–1937, Figure

3.3), who used the alpha particles (rays emitted by

radioactive materials) as “bullets” to penetrate atoms

of a thin sheet of gold.

If the model of Thomson was correct, as alpha

particles are 7400 times heavier than electrons, they

would have crossed the thin sheet of gold without

any problems. However, certain particles were unex-

pectedly diverted. The correction to the Thomson

model led to the new hypothesis of the Rutherford

model: the atom is still a spherical structure, but

with a relatively small core, the nucleus, containing

all the positive charges and most of the mass (and

therefore also constituted by particles without charge,

neutrons), while the electrons are distributed in the

rest of the atom and orbiting around the nucleus like

planets around the sun (Figure 3.4).

The solar system model by Rutherford was also

replaced, as it did not justify subsequent observa-

tions regarding the light that was emitted from atoms

excited from electric discharge.

Bohr (Niels Henrik David Bohr, Danish physicist

and mathematician, 1885–1962, Figure 3.5), a stu-

dent of Rutherford, formulated the assumption that

the electrons in atoms can move only along certain

circular orbits, allowed each to correspond to a very

specic energy level of electrons. The orbits are said

to be discrete, that is, having a nite or countable,

not continuous, set of values.

Accordingly, each orbit corresponds to a specic

value of energy, so the energy is discrete as well.

The electrons can “transit” from one energy level to

another (from one orbit to another), by the release or

absorption of what is called photon of energy, adis-

crete value, equal to the “energy drop,” that is, the

difference between the initial and the nal energy. A

transition from a higher energy level to a lower level,

FIGURE 3.1 J. Dalton.

FIGURE 3.2 J.J.Thomson.

FIGURE 3.3 E.Rutherford.

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