Electromotive Force and Resistance

As was previously stated, there is always a disorderly movement of free electrons within all substances, especially metals.

Let us assume that there is a movement of electrons through the wire, say, from point A to point B. What does it mean? It means that there is an excess of electrons at point A. Unless there were a flow of electric current between A and B in any direction, it would mean that both the former and the latter were at the same potential. Of course, the greater the potential difference, the greater it the electron flow.

The electromotive force (e.m.f.) 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.

One need not explain to the reader that 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 here to another important factor that greatly influences electrical flow, namely, resistance. So, to resistance we shall turn our attention now. The student probably remembers that 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. Take glass and rubber as an example. They offer a very high resistance and, hence, they are considered as good insulators. Nevertheless, one must not forget that all substances do allow the passage of some current provided the potential difference is high enough.

Imagine two oppositely charged balls suspended far apart in the air. In spite of their having a difference of potential, no current flows. How can we explain this strange behaviour? The simple reason is that the air between the balls offers too great a resistance to current flow. However, the electrons could certainly flow from the negatively charged ball towards the positively charged one provided we connected them by a metal wire. As a matter of fact, it is not necessary at all to connect both balls in the manner described in order to obtain a similar result. All that we have to do is to increase the charges. If the potential difference becomes great enough, the electrons will jump through the air forming an electric spark.

One should mention in this connection that certain factors can greatly influence the resistance of an electric circuit. Among them we find the size of the wire, its length, and type. In short, the thinner or longer the wire, the greater the resistance offered. Besides, if we could use a silver wire, it would offer less resistance than an iron one.

V. Ответьте на вопросы:

1. What is the electromotive force?

2. What factors greatly influence the current flow in the circuit?

3. What does resistance depend on?

II вариант

I. Перепишите следующие предложения; подчеркните в каждом из них

глагол-сказуемое и определите его видо-временную форму и залог.

Переведите предложения на русский язык.

1. Today the changes in the global climate and water balance are bringing about

serious changes in the environment.

2. The students have not finished the translation of the article yet.

3. Goverment and public organizations will be analysing data on land, forest and

air for many years.

4. We have already solved the problem concerning the production of new types

of computers.

II. Перепишите следующие предложения; подчеркните в каждом из них

модальный глагол или его эквивалент. Переведите предложения на

русский язык.

1. One must study a lot to become a highly qualified specialist.

2. You ought to be more careful with electrical appliances.

3. In the next few years engineers are to develop computers of more than 2

billion operations a second.

4. You should carry out the experiment by yourself; you are quite able to do so.

5. We had to perform long computations to check the answer.

III. Перепишите следующие предложения с определительными и

дополнительными придаточными предложениями. Обратите внимание

на бессоюзные придаточные предложения, переведите предложения на

русский язык.

1. Students were informed that they would have industrial training in the third

year.

2. They thought she had graduated from a technical institute.

3. The material the conductors are made of must withstand high temperatures.

4. We know that it is possible to find the direction of the magnetic effect of the

current thanks to Ampere’s rule.

IV. Перепишите и письменно переведите текст на русский язык.

Electricity

It is impossible to imagine our civilization without electricity: economic and social progress would be turned to the past and our daily lives completely transformed.

Electrical power has become universal. Thousands of applications of electricity such as lighting, electrochemistry and electrometallurgy are longstanding and unquestionable.

With the appearance of the electrical motor, power cables replaced transmission shafts, gear wheels, belts and pulleys in the 19^thcentury workshops. And in the home, a whole range of various time- and laboursaving appliances have become a part of our everyday lives.

Other devices are based on specific properties of electricity, electrostatics, in the case of radar and television. These applications have made electricity most widely used.

The first industrial application was in the silver workshops in Paris. The generator – a new compact source of electricity – was also developed there. The generator replaced the batteries and other devices that had been used before.

Electric lighting came into wide use at the end of the last century with the development of the electric lamp by Thomas Edison. Then the transformer was invented, the first electric lines and networks were set up, dynamos and industrial motors were designed.

Since the beginning of the 20^th century, the successful development of electricity has begun throughout the industrial world. The consumption of electricity has doubled every ten years.

Today, consumption of electricity per capita is an indicator of the state of development and the economic health of a nation. Electricity has replaced other sources of energy as it offers improved service and reduced cost.

One of the greatest advantages of electricity is that it is clean, easily-regulated and generates no by-products. Applications of electricity now cover all fields of human activity from household washing machines to the latest laser devices. Electricity is the efficient source of some of the most recent technological advances such as the laser and electron beams. Truly electricity provides mankind with the energy of the future.

V. Ответьте на вопросы:

1. Where was the generator developed?

2. What is one of the greatest advantages of electricity?

3. When did electric lighting come into wide use?

III вариант

I. Перепишите следующие предложения; подчеркните в каждом из них

глагол-сказуемое и определите его видо-временную форму и залог.

Переведите предложения на русский язык.

1. Today the changes in the global climate and water balance are making the

environment worse.

2. The engineers have not finished the construction of the new device yet.

3. They were discussing this problem the whole evening yesterday.