Учебные задания по английскому языку (90

3…is given by Ohm’s law.

4…is a significant step toward a far wider spectrum of applications. 5…to drive an electric current through a resistance.

6…have been used in diagnostic medical equipment. 7…to electric current produces heat.

5. Complete each sentence using one of the words from the box.

induces suspended liquid opposing keeps accordance floats lowered

Levitating Magnet

A small cylindrical magnet (1)______above a high temperature superconductor. The vapor is from boiling (2)______nitrogen, which (3)______the superconductor in a zero-resistance state. As the magnet is (4)______toward the superconductor, it

(5)_____an electric current, which creates an (6)______magnetic field in

(7)______with Ampere’s law. Because the superconductor has no electrical resistance,

this induced current continues to flow, keeping the magnet (8)______indefinitely.

UNIT THREE

ELECTRIC CURRENT CONDUCTION

READING

1. Read the text and choose from the list A-I the sentence which best fits each gap (1-8) in the article. There is one extra sentence which you do not need to use.

All electric currents consist of charges in motion. (1)_________________ When an electric current flows in a solid conductor, the flow is in one direction only, because the current is carried entirely by electrons. In liquids and gases, however, a two - directional flow is made possible by the process of ionization.

Conduction in Solids

The conduction of electric currents in solid substances is made possible by the presence of free electrons (electrons that are free to move about). Most of the

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electrons in a bar of copper, for example, are tightly bound to individual copper atoms. (2)________________

Ordinarily the motion of the free electrons is random; that is, as many of them are moving in one direction as in another. (3)______________________ This end is said to be at a higher potential and is called the positive end. The other end is said to be at a lower potential and is called the negative end. The function of a battery or other source of electric current is to maintain potential difference. A battery does this by supplying electrons to the negative end of the bar to replace those that drift to the positive end and also by absorbing electrons at the positive end.

(4)____________________ A perfect insulator would allow no charge to be forced through it, but no such substance is known at room temperature. The best insulators offer high but not infinite resistance at room temperature.

Some substances that ordinarily have no free electrons, such as silicon and germanium, can conduct electric currents when small amounts of certain impurities are added to them. (5)_____________________ Semiconductors generally have a higher resistance to the flow of current than does a conductor, such as copper, but a lower resistance than an insulator, such as glass.

Conduction in Gases

Gases normally contain few free electrons and are generally insulators. When a strong potential difference is applied between two points inside a container filled with a gas, the few free electrons are accelerated by the potential difference and collide with the atoms of the gas, knocking free more electrons. The gas atoms become positively charged ions and the gas is said to be ionized.

(6)_______________________ An electric current in a gas is composed of these opposite flows of charges.

Conduction in Liquid Solutions

Many substances become ionized when they dissolve in water or in some other liquid. An example is ordinary table salt, sodium chloride (NaCl). When sodium chloride dissolves in water, it separates into positive sodium ions, Na+, and negative chlorine ions, Cl-. If two points in the solution are at different potentials, the negative

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ions drift toward the positive point, while the positive ions drift toward the negative point. (7)________________ Thus, while water that is absolutely pure is an insulator, water that contains even a slight impurity of an ionized substance is a conductor.

Since the positive and negative ions of a dissolved substance migrate to different points when an electric current flows, the substance is gradually separated into two parts. (8)____________________

AThe electrons move toward the high-potential (more positive) point, while the ions move toward the low-potential (more negative) point.

BHowever, if a voltage is applied to the two ends of a copper bar by means of a battery, the free electrons tend to drift toward one end.

CSuch substances are called semiconductors.

DHowever, electric current is conducted differently in solids, gases, and liquids.

EThis separation is called electrolysis.

FThis phenomenon is widely applied in modern technology.

GAs in gases, the electric current is composed of these flows of opposite charges.

HInsulators cannot conduct electric currents because all their electrons are tightly bound to their atoms.

IHowever, some are free to move from atom to atom, enabling current to flow.

VOCABULARY PRACTICE

a. not in the form of a liquid or gas; b. specially for one person or thing;

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c.normal, usual, average;

d.not solid or gaseous;

e.acquiring a liquid state;

f.changed into electrically charged particles when dissolved;

g.unmixed with any other substance;

h.without restraint or control;

i.endless, without limits;

j.without any regular pattern;

k.small; not important.

2.Write out the passage using the most appropriate verb-form from the box in each gap.

(2)_______ an electric current. The two most common sources are generators and electrolytic cells.

Generators use mechanical energy, such as water (3)_______through a dam or the motion of a turbine (4)_______by steam, to produce electricity. The electric outlets on the walls of homes and other buildings, from which electricity

(5)_______lights and appliances is drawn, are connected to giant generators

(6)_______in electric power stations. Each outlet (7)_______two terminals. The voltage between the terminals (8)______an electric current through the appliance that is plugged into the outlet.

Electrolytic cells (9)______chemical energy (10)_______electricity. Chemical reactions within an electrolytic cell produce a potential difference between the cell’s terminals. An electric battery (11)_______of a cell or group of cells connected together.

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f. перемещаться; g. примеси;

h. тесно связанный; i. сталкиваться;

j. в движении;

k. постепенно разделяться.

SPEAKING

Make a presentation on a particular source of electric current. Use the following guidelines.

1.Outline the structure of your presentation.

2.Describe the product features and consumer benefits.

3.Describe the product’s target market.

4.Mention other competing products.

Decide on the structure of your presentation. These phrases will help you order

WRITING

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Produce a plan which would help you back up your project of using a particular electric power system for a certain purpose. Think of the advantages of this system over other systems.

MODULE II ASSESSMENT

Translate from English into Russian.

OTHER SOURCES OF ELECTRIC CURRENT

There are many sources of electric current other than generators and electrolytic cells. Fuel cells, for example, produce electricity through chemical reactions. Unlike electrolytic cells, however, fuel cells do not store chemicals and therefore must be constantly refilled.

Certain sources of electric current operate on the principle that some metals hold onto their electrons more strongly than other metals do. Platinum, for example, holds its electrons less strongly than aluminum does. If a strip of platinum and a strip of aluminum are pressed together under the proper conditions, some electrons will flow from the platinum to the aluminum. As the aluminum gains electrons and becomes negative, the platinum loses electrons and becomes positive.

The strength with which a metal holds its electrons varies with temperature. If two strips of different metals are joined and the joint heated, electrons will pass from one strip to the other. Electricity produced directly by heating is called thermoelectricity.

Some substances emit electrons when they are struck by light. Electricity produced in this way is called photoelectricity. When pressure is applied to certain crystals, a potential difference develops across them. Electricity thus produced is called piezoelectricity. Some microphones work on this principle.

MODULE III

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UNIT ONE

ELECTRIC CIRCUITS

READING

1. Read the text. Match headings A-F to paragraphs 1-5. There is one heading you will not need to use.

1 An electric circuit is an arrangement of electric current sources and

conducting paths through which a current can continuously flow. In a simple circuit consisting of a small light bulb, a battery, and two pieces of wire, the electric current flows from the negative terminal of the battery, through one piece of connecting wire, through the bulb filament (also a type of wire), through the other piece of connecting wire, and back to the positive terminal of the battery. When the electric current flows

example of such a circuit. When the switch is open, the connection is broken, electric current cannot flow through the circuit, and the bulb does not light. When the switch is closed, current flows and the bulb lights.

3The bulb filament may burn out if too much electric current flows through it. To prevent this from happening, a fuse (circuit breaker) may be placed in the circuit. When too much current flows through the fuse, a wire in the fuse heats up and melts, thereby breaking the circuit and stopping the flow of current. The wire in the fuse is designed to melt before the filament would melt.

4The part of an electric circuit other than the source of electric current is called the load. The load includes all appliances placed in the circuit, such as lights, radios, fans, buzzers, and toasters. It also includes the connecting wires, as well as switches, fuses, and other devices. The load forms a continuous conducting path between the terminals of the current source.

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complete the circuit. If excess current (6)_____(surge) through the circuit, the metal link will heat to its (7)_____(melt) point and break. This action (8)______(open) the circuit, stop the current flow, and thus protect the circuit. The cylindrical type of fuse is used mostly (9)______(protect) electrical equipment and appliances.

5. Find English equivalents to the following words and word combinations in bold.

a. перегорать; b. постоянно;

c. предотвращать;

d. соединительный провод; e. нагреваться, накаливаться;

f. последовательная цепь, схема; g. вентилятор;

h. размыкать цепь.

UNIT TWO

SERIES AND PARALLEL CIRCUITS

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