Electromotive force and resistance

The electromotive force is the very force that moves the electrons from one point in an electric circuit towards another. In case this e. m. f. is direct, the current is direct. On the other hand, were the electromotive force alternating, the current would be alternating, too. The e. m. f. is measurable and it is the volt that is the unit used for measuring it. A current is unable to flow in a circuit consisting of metallic wires alone. A source of an e. m. f. should be provided as well. The source under consideration may be a cell or a battery, a generator, a thermocouple or a photocell, etc.

In addition to the electromotive force and the potential difference reference should be made to another important factor that greatly influences electrical flow, namely, resistance. All substances offer a certain amount of opposition, that is to say resistance, to the passage of current. This resistance may be high or low depending on the type of circuit and the material employed. Glass and rubber offer a very high resistance and, hence, they are considered as good insulators. All substances do allow the passage of some current provided the potential difference is high enough.

Certain factors can greatly influence the resistance of an electric circuit. They are the size of the wire, its length, and type. In short, the thinner or longer the wire, the greater is the resistance offered.

II. Give the English equivalents for the words below. Find in the text the sentences with these words and translate them

1) трение; 2) электродвижущая сила; 3) элемент; 4) параллельное соединение; 5) сопротивление; 6) электромагнитная индукция; 7) переменный ток; 8) постоянное напряжение; 9) фотоэлемент.

III. Guess the meaning of the following international words and translate them: reaction, electrostatic, electrometer, electroscope, volt, metallic.

IV. Say whether these sentences are true or false:

1. Alternating force always exerts its effort in one direction.

2. Alternating force is produced by electromagnetic induction.

3. The electromotive force is induced by motion of a conductor.

4. Resistance is an important factor that greatly influences electrical flow.

5. The type of the material employed doesn’t influence the resistance.

V. Answer the questions:

1) What factors cause the motion of electrons from one atom to another?

2) When is the electromotive force developed? 3) When does an electrostatic field appear? 4) How is the electromotive force induced? 5) What unit is used for measuring the electromotive force? 6) What are the sources of electromotive force? 7) What is called “resistance”? 8) How do the types of circuit and material influence the resistance? 9) Name the factors that influence the resistance of an electric circuit.

UNIT 9

I. Read the text

DYNAMOS

The term «dynamo» is applied to machines which convert either me-

chanical energy into electrical energy or electrical energy into mechanical energy by utilizing the principle of electromagnetic induction. A dynamo is called a generator when mechanical energy supplied in the form of rotation is converted into electrical energy. When the energy conversion takes place in the reverse order the dynamo is called a motor. Thus a dynamo is a reversible machine capable of operation as a generator or motor as desired.

A generator does not create electricity, but generates or produces an induced electromotive force, which causes a current to flow through a properly insulated system of electrical conductors external to it. The amount of electricity obtainable from such a generator is dependent upon the mechanical energy supplied. In the circuit external to a generator the e. m. f. causes the electricity to flow from a higher or positive potential to a lower or negative potential. In the internal circuit of a generator the e. m. f. causes the current to flow from a lower potential to a higher potential. The action of a generator is based upon the principles of electromagnetic induction.

The dynamo consists essentially of two parts: a magnetic field, produced by electromagnets, and a number of loops or coils of wire wound upon an iron core, forming the armature. These parts are arranged so that the number of the magnetic lines of force of the field threading through the armature, coils will be constantly varied, thereby producing a steady e. m. f. in the generator or a constant torque in the motor.

II. Fill in the gaps with the words given below:

to convert, generator, reversible, obtainable, induction, loops

1. The term “dynamo” is applied to machines which.....either mechanical energy into electrical or on the contrary electrical energy into mechanical energy.

2. A dynamo is a..... machine capable of operation as a generator or motor as desired.

3. The amount of electricity..... from such a generator is dependent upon the mechanical energy supplied.

4. The action of a generator is based upon the principles of electromagnetic.......

5. The dynamo consists of two parts: a magnetic field, produced by electromagnets, and a number of.....or coils of wire.

III. Find the Russian equivalents for the following English words and word combinations:

1) to be applied to smth.; 2) to convert smth. into smth.; 3) rotation; 4) to utilize; 5) a properly insulated system; 6) internal (external) circuit; 7) capable of operation; 8) positive (negative) potential; 9) reverse order;

10) energy conversion.

IV. Answer the questions

1. What term can be applied to machines converting mechanical energy into electrical and vice versa?

2. What kind of machine is a dynamo?

3. What is the function of a generator?

4. What is the action of a generator based upon?

5. What parts does the dynamo consist of?

V. Talk on the dynamo action

UNIT 10

I. Read the text

GENERATORS

The powerful, highly efficient generators and alternators that are in use

today operate on the same principle as the dynamo invented by the great English scientist Faraday in 1831.

Dynamo-electric machines are used to supply light, heat and power on a large scale. These are the machines that produce more than 99.99 per cent of all the world's electric power.

There are two types of dynamos – the generator and the alternator. The former supplies d. c. which is similar to the current from a battery and the latter provides a. c. To generate electricity both of them must be continuously provided with energy from some outside source of mechanical energy such as steam engines, steam turbines or water turbines.

A generator is an electric machine, which converts mechanical energy into electric energy. There are direct-current (d. c.) generators and alternating-current (a. c.) generators. Their construction is much alike. A d. c. generator consists of stationary and rotating elements. The stationary elements are: the yoke or the frame and the field structure. The yoke forms the closed circuit for the magnetic flux. The function of the magnetic structure is to produce the magnetic field.

The rotating elements are: true armature and the commutator. They are on the same shaft. The armature consists of the core and the winding. The winding is connected to the commutator. With the help of the brushes on the commutator that conduct the electric current to the line the winding is connected to the external circuit. The stationary element of an a. c. generator is called a stator. The rotating element is called a rotor.

The essential difference between a d. c. generator and a. c. generator is that the former has a commutator by means of which the generated e. m. f. is made continuous, i. e. the commutator mechanically rectifies the alternating e. m. f. so that it is always of the same polarity.

D. c. generators are used for electrolytic processes such as electroplating.

Large d. c. generators are employed in such manufacturing processes as steel making. The d. c. generator of small capacities is used for various special purposes such as arc welding, automobile generators, train lighting systems, etc. It also finds rather extensive use in connection with communication systems.

II. Give the Russian equivalents for the following English words and word combinations:

1) generator; 2) alternator; 3) steam turbine; 4) water turbine; 5) armature; 6) rotor; 7) stationary; 8) commutator; 9) stator; 10) yoke; 11) brushes; 12) core; 13) frame; 14) winding.

III. Fill in the blanks

1. A generator is an electric machine, which a) ----- mechanical energy into electrical energy.

2. A direct-current generator consists of b) -----.

3. The dynamo was invented by c) ----- in 1831.

4. The d.c. generator is used for various purposes such as d) -----.

IV. Work out the plan of the text

V. Speak on the following points:

1. The construction of a generator.

2. The direct current generators and their industrial application.

 

UNIT 11

I. Read the text