November 2021 2 20 Report
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1. The word "atom" means “indivisible”, and it was once thought that
аtoms were the ultimate, indivisible constituents of matter, that is, they were
regarded аs elementary particles. One of the principal achievements of
physics in the 20th century was the revelation that the atom is not indivisi-
ble or elementary, but has а complex structure. In 1911 Ernest Rutherford
showed that the atom consists of а small, dense nucleus surrounded by a
cloud of electrons. It was later revealed that the nucleus itself can be broken
down into discrete particles, the protons and neutrons, and since then а
great many related particles have been identified. During the past few dec-
ades it has become apparent that those particles, too, are complex rather
than elementary.
2. They are now thought to be made up of simpler things called quarks. А
solitary (одиночный) quark has never been observed, in spite of many at-
tempts to isolate one. Nonetheless, there are excellent grounds for believing
they do exist. More important, quarks seem to be truly elementary.

3. Subatomic particles can be classified into large families according to
the kinds of interactions they participate in, or according to the kinds of
forces they "feel." These forces are: gravitation, electromagnetism, the
strong force and the weak force. All particles except the photon are classi-
fied according to their response to the last two forces. Those that feel the
strong force are called hadrons (from the Greek word meaning "large'");
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those that do not feel the strong force but do respond to the weak force are
called leptons. There are just six of them: the electron and the electron neu-
trino, the muon and the muon neutrino, the tau lepton and the tau neutrino
(and their six antiparticles). The leptons give every indication of being ele-
mentary particles. The electron, for example, behaves as а point charge, and
even when it is studied at the energies of the largest particle accelerators, no
internal structure саn be detected.

4. The hadrons, on the other hand, seem complex. They have а measur-
able size of about 10-13 centimeters. Moreover, there are hundreds of them,
and most of them are massive and unstable. It was in an effort to explain
this great variety of particles that the quark hypothesis appeared. It was in-
troduced independently in 1963 by Murray Gell-Mann and by George
Zweig, both of the California Institute of Technology.

5. The quark model states that a quark is an elementary particle and a
fundamental constituent of matter. Quarks combine to form particles called
hadrons, the most stable of which are protons and neutrons, the components
of atomic nuclei. Quarks are never found in isolation; they can only be
found within hadrons. When the quark hypothesis was first proposed, there
were supposed to be three kinds of quark. The revised version of the theory
requires 18 kinds (and 18 anti-quarks). In the terminology that has evolved
for the discussion of quarks they are said to come in six flavors, and each
flavor is said to come in three colors. ("Flavor" and "color" are, of course,
arbitrary labels; they have no relation to the usual meanings of those
words).
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Задание ответить на вопросы ниже
Exercise 5. Read the text again and answer the questions below.
1. Who was the first to show that the atom consists of a small
dense nucleus surrounded by electrons?
2. What other particles have been identified since Rutherford’s
discovery in 1911?
3. Are these particles really elementary? What are they made up of?
4. Into what two families can subatomic particles be divided?
5. According to what principle can they be classified?
6. In general, how many forces are there?
7. What force do hadrons feel?
8. What force do leptons respond to?
9. How many leptons are there?
10. Why are leptons regarded as truly elementary particles?
11. Why did the quark hypothesis appear?
12. Who was it introduced by and when?
13. What does the quark hypothesis state?
14. How many kinds of quarks does the revised version of the hy-
pothesis require?
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