Задание для дистанционного обучения для студентов колледжа (специальность Электроснабжение).

Прочитать тексты из Unit 2, Unit 4, письменно выполнить упражнения после текстов.

UNIT 2

I. Read the text

ELECTRIC CURRENT

The electric current is a quantity of electrons flowing in a circuit per second

of time. The unit of measure for current is ampere. If one coulomb passes

a point in a circuit per second then the current strength is 1 ampere. The symbol

for current is I.

The current which flows along wires consists of moving electrons. The

electrons move along the circuit because the e .m. f. drives them. The current

is directly proportional to the e. m. f.

In addition to traveling through solids, however, the electric current can

flow through liquids as well and even through gases. In both cases it produces

some most important effects to meet industrial requirements.

Some liquids, such as melted metals for example, conduct current without

any change to themselves. Others, called electrolytes, are found to change

greatly when the current passes through them.

When the electrons flow in one direction only, the current is known to be

d. c., that is, direct current. The simplest source of power for the direct current

is a battery, for a battery pushes the electrons in the same direction all the time

(i.e., from the negatively charged terminal to the positively charged terminal).

The letters a. c. stand for alternating current. The current under consideration

flows first in one direction and then in the opposite one. The a. c. used for

power and lighting purposes is assumed to go through 50 cycles in one second.

One of the great advantages of a. c. is the ease with which power at low voltage

can be changed into an almost similar amount of power at high voltage and vice

versa. Hence, on the one hand alternating voltage is increased when it is necessary

for long-distance transmission and, on the other hand, one can decrease it to

meet industrial requirements as well as to operate various devices at home.

Although there are numerous cases when d. c. is required, at least 90 per

cent of electrical energy to be generated at present is a. c. In fact, it finds wide

application for lighting, heating, industrial, and some other purposes.

V. Say whether these sentences are true or false:

1. The symbol for current is I.

2. The electric current can flow only through liquids.

3. The current can be of two types: direct current and alternating current.

4. The alternating current flows in one direction.

5. A battery is the simplest source of power for the direct current.

6. Direct current finds wider application than alternating current.

7. Electrolytes don’t change greatly when current passes through them.

8. One of the great advantages of alternating current is the ease with

which voltage can be changed.

UNIT 4

I. Read the text

Electric Circuits

The concepts of electric charge and potential are very important in the study

of electric currents. When an extended conductor has different potentials at its

ends, the free electrons of the conductor itself are caused to drift from one end to

the other. The potential difference must be maintained by some electric source

such as electrostatic generator or a battery or a direct current generator. The wire

and the electric source together form an electric circuit, the electrons are drifting

around it as long as the conducting path is maintained.

There are various kinds of electric circuits such as: open circuits, closed

circuits, series circuits, parallel circuits and short circuits.

To understand the difference between the following circuit connections is

not difficult at all. If the circuit is broken or «opened» anywhere, the current is

known to stop everywhere. The circuit is broken when an electric device is

switched off. The path along which the electrons travel must be complete otherwise

no electric power can be supplied from the source to the load. Thus the

circuit is “closed” when an electric device is switched on.

When electrical devices are connected so that the current flows from one

device to another, they are said «to be connected in series». Under such conditions

the current flow is the same in all parts of the circuit as there is only a

single path along which it may flow. The electrical bell circuit is considered to

be a typical example of a series circuit. The “parallel” circuit provides two or

more paths for the passage of current. The circuit is divided in such a way that

part of the current flows through one path and part through another. The lamps

in the houses are generally connected in parallel.

The “short” circuit is produced when the current can return to the source

of supply without control. The short circuits often result from cable fault or

wire fault. Under certain conditions the short circuit may cause fire because the

current flows where it was not supposed to flow. If the current flow is too great

a fuse is used as a safety device to stop the current flow.

VI. Say, whether these statements are true or false

1. When an extended conductor has the same potential at its ends, free

electrons are drifting from one end to another.

2. The wire and the electric source together form an electric circuit.

3. A path of any material will allow current to exist.

4. Silver, copper and gold oppose very strongly.

5. The slighter the opposition is, the better the insulator is.

6. There is only one type of electric circuit.

7. We close the circuit when we switch on our electric device.