VII. Talk on the properties of semiconductors and their practical application.

UNIT 8

I. Read the text.

ELECTRICITY AND MAGNETISM

TEXT 1

Electromotive Force

When free electrons are dislodged from atoms, electrical energy is released. Chemical reaction, friction heat and electromagnetic induction will cause electrons to move from one atom to another. Whenever energy in any form is released, a force called electromotive (e. m. f.) is developed.

If the force exerts its effort always in one direction, it is called direct; and if the force changes its direction of exertion periodically, it is called alternating. The chemical reaction in a dry cell, heat and friction are sources of a unidirectional force. Electromagnetic induction produces an alternating force. The direction of force depends on the direction in which the field is cut. Whenever an e. m. f. is developed, there is also a field of energy called an electrostatic field, which can be detected by an electroscope and measured by an electrometer.

TEXT 2

Electromagnetic Induction

An electromotive force is induced in the conductor when there is a change in the magnetic field surrounding a conductor. This induced electromotive force may be produced in several ways as follows:

a) A conductor may move in a stationary magnetic field of constant strength.

b) A stationary conductor may be exposed to a moving magnetic field of constant strength.

c) The strength of the field surrounding the conductor may change without any motion of conductor or magnetic circuit.

The electromotive force induced by motion of a conductor or a magnetic flux is the same when the conductor rotates and the flux is stationary or the flux rotates and the conductor is stationary. If both, conductor and flux, rotate in the same direction at the same speed, no electromotive force will be produced, if they rotate at the same speed but in opposite directions, the electromotive force induced would be twice as that which would be induced, if one of them was stationary. An electromotive force is not induced when a conductor is moved parallel to the lines of force, but only when it moves at an angle with these lines.

Any motion across the direction of the lines, however, will produce an electromotive force in the conductor. For this reason, the conductor is said to „cut" the lines of force. The actual electromotive force induced in the conductor depends upon the nature at which the flux is cut.

TEXT 3

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 is, the greater the resistance offered is.

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.Alternatingforce 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 mechanical 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, 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?

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