Radiophysics. Халюшева Г.Р. - 44 стр.

         11 Unit 11

         11.1 Text 11
                                                       What is an elecric current?

          The question is often asked: What is an electric current? "If we could examine the inside of a copper wire while a
current is flowing, we should see an electron, leaving one copper atom, moving over to the next copper atom and so on
.This stream of electrons moving along from atom to atom is called an electric current. The practical unit of current is called
the ampere.
          No one has ever seen an electric current. We only know of the existence of a current owing to its effects. A current
can heat a conductor, it can have a chemical action when passing through a solution, or it can produce a magnetic effect. We
can measure currents by observing their heating, chemical or magnetic effects.
         Two things are necessary to cause an electric current to flow: first – a complete
circuit, and second– a driving force, called the electromotive force (e.m.f.).
           If we were to put 100 free electrons on an insulated copper ball,what would they do? In this case they would try to
repel each other.
           In case we connected this charged ball to another ball of equall size by a copper wire, what would be the result?
The electrons would move along the copper wire until the number of electrons on each ball were the same. This is an
example electromotive force causing a current to flow.
           A battery has a surplus of electrons on one of its two plates; so we say that a battery furnishes an e.m.f. If a copper
wire is run from one plate to the other, a current flows in the complete circuit thus made. If a small bulb is placed in the
circuit, it will light up, giving evidence to a current flow.
           If the battery were disconnected and a generator substituted for it, we should have a typical lighting system. Both
batteries and generators are the most common sources of electromotive force. The practical unit of e.m.f. is the volt.
           Current will flow more readily in some substances than in others, that is, various substances offer less or greater
resistance to the flow of current. The practical unit of resistance is the ohm. An application of Ohm's law tells us that an
e.m.f. of 1 volt will produce a current of 1 ampere in a wire which has a resistance of 1 ohm.
           Symbolically, Ohm's law is often written
           R = V / I or resistance = (potential difference) / current
           Such substances as porcelain, ebonite, rubber, glass and the like having extremely high resistance are known as
insulators.
           Substances whose properties lie between those of conductors and insulators are called scmiconductors. Let us name
but a few most widely used at present, they are germanium, silicon, selenium and copper oxide. The importance of
semiconductors in our life cannot be overestimated. But for these tiny "workhorses" electronic industry would not have
achieved such a great progress.


         11.2 Words and word combinations to be remembered

         Electric current – электрический ток
         copper – медь; медный.
         wire – провод
         and so on – и так далее
         owing to – благодаря (чему–либо).
         solution – раствор
         to measure – мерить; измерять
         to observe – наблюдать
         to cause – быть причиной, вызывать
         a complete circuit – замкнутая цепь
         driving force – движущая сила
         electromotive force – электродвижущая сила
         to insulate – изолировать
         insulated – изолированный
         to repel – отталкивать
         to connect smth to – присоединять что–либо к
         surplus – излишек, избыток; остаток