What is an electric current?

The question is often asked: "What is an electric current?" No one has ever seen it. We only know of the existence of a current owing to its effects. A current can heat a conductor, it can have a chemical action when passing through a solution, or it can produce a magnetic effect. We can measure currents by observing their heating, chemical or magnetic effects. The practical unit of current is called the Ampere.

Two things are necessary to cause an electric current to flow: the first - a complete circuit, and the second - a driving force¹ called the electromotive force (e. m. f.).

If you put free electrons on an insulated copper ball, what would they do? In this case they would try to repel each other. In case you connected this charged ball to another ball of equal size by a copper wire, what would be the result? The electrons would move along the copper wire until the numbers of electrons on each ball were the same. This is an example of electromotive force causing a current to flow.

A battery has a surplus of electrons on one of its two plates; so you say that a battery furnishes an e. m. f. If a copper wire is run from one plate to the other, current flows in the complete circuit thus made. If a small bulb is placed in the circuit, it will light up, giving evidence to a current flow. If the batteries were disconnected and a generator substituted for it, we should have a typical lighting system. Both batteries and generators are the most common sources of electromotive force. The practical unit of e. m. f. is the Volt.

Currents will flow more readily in some substances than in others, that is, various substances offer lesser or greater resistance to the flow of current. Such substances as porcelain, ebonite, rubber, glass and the likes having extremely high resistance are known as insulators. The practical unit of resistance is the Ohm.

Substances whose properties lie between those of conductors and insulators are called semiconductors. Let us name but a few most widely used at present, they are germanium, silicon, selenium and copper oxide. The importance of semiconductors in our life cannot be overestimated. But for these tiny "workhorses" electronic industry would not have achieved such a great progress.

Notes on the text

¹driving force (here) = electromotive force (e. m. f.) — электродвижущая сила

²and the like — и другие подобные (вещества)

Ex.7 Answer the questions to the text.

1. What is the unit of current?

2. What can an electric current do?

3. What is necessary to cause an electric current to flow?

4. What are the most common sources of electromotive force?

5. How does current flow in various substances?

6. What is the unit of resistance?

7. What substances do we call insulators?

8. What semiconductors do you know?

Words to be learnt.

charge v — заряжать circuit n — цепь common a — распространенный, общий complete a — зд. замкнутая current n — ток evidence n — доказательство insulate v — изолировать insulator n — изолятор

like a — похожий, подобный owing to — благодаря plate n — плата, пластинка readily adv — легко, быстро semiconductor n — полупроводник silicon n — кремний substitute (for) v — заменять thus adv — таким образом tiny a — крошечный

Ex.8 Translate the words.

verb	nouns		Adjectives or Participle II
	device	process
conduct	conductor	conduction	conductive conducting conducted
		conductivity
generate	generator	generation	generative
induce	inductor	induction	inductive
		inductance
insulate	insulator	insulation	insulated
receive	receiver	reception	received
resist	resistor	resistance	resistive
transmit	transmitter	transmission	transmitted

Ex.9 Translate words and word – combinations.

а) radio and radar equipment; city water-supply system; high-speed production pro-cess; ground-space communication station; high-voltage transmission lines; radio-

wave length stabilization

б) left-right movements; up-and-down movements; not-very-distant future; left-hand plates; right-hand plates

Ex. 10 Use the verbs in the correct form.

1. If the solid is heated, it (to expand).

the motion of electrons (to increase).

2. In case they created the device it (to be put) into operation at once.

they (to conduct) interesting observations.

3. Had he examined the motor no overheating (to take place).

he (not to switch) it on.

1 We shall continue research if you (to help) us.

the necessary data (to be received).

2. The reactor would be brought into operation provided

all the preparations (to be completed). the designers (to come) next week.

3. We could have used this we (to know) about it before.

method if the technology of the application (to be

received).

Ex.11 Translate the sentences paying attention to the underlined words.

а) 1. The government has provided all the necessary conditions for successful development of science.

2. The number of workshops and whole plants provided with automatic control is rapidly increasing in our country.

3. Ohm's law provided the possibility of determining resistance provided the voltage and current are known.

4. Provided the Earth's mass were twice as great as it is, it would attract any body twice as strongly.

б) 1. Radio waves are like light waves.

2. We like working in our institute laboratories because we can always get help there in case we need it.

3. We should like to know if (whether) you have interrupted this research or not.

4. Like charges repel each other, while unlike charges attract each other.

5. Unlike solids and liquids all gases have nearly the same coefficient of expansion.

в) 1. If some current flows through a thin wire and then the same amount of current is sent through a thicker one, a different amount of heat will be developed in both these wires.

2. We may say that on the Moon the weight of some body would be only 1/6 as much as it is on the Earth, its mass, however, would be certainly the same.

Ex.12 Translate the sentences paying attention to conjunctions.

1. Between conductors and insulators there are many materials which are neither good conductors nor good insulators.

2. Neither of the engines will give the efficiency desired.

3. In many cases heat plays an important role, either starting or accelerating a chemical reaction.

4. Either of the methods of producing electricity may be used under these conditions.

5. There is no doubt that both magnetic and electrostatic forces play a part in the formation of all molecules.

Ex.13 Translate the sentences.

but — но; только, лишь; кроме

but for — если бы не

1. The proton and neutron have almost the same weight, but the electron weighs only l/2,000th as much as a proton or neutron.

2. The nucleus of hydrogen contains but one proton.

3. An electric current is but a procession of charges.

4. Whether other substances but uranium possessed the property of emitting radiations was of great interest to scientists.

5. It seems that the electron is nothing but a charged particle.

6. But for the atmosphere no life would be possible on the Earth.

Ex.14 Translate and state the type of conditionals and the tense.

1. In case there is no current in a conductor, there can be no electric field around it.

2. Should a permanent magnet be moved in and out of a coil of wire, both the magnet and the coil would be used as a simple electric generator.

3. Had there been no Earth's gravitation, the artificial satellites would have moved through airless space in a straight line at a uniform speed.

4. It would have been impossible to launch space satellites and space rockets provided we had not had the necessary polymeric materials and synthetic fuels.

5. Unless the temperature rises, the speed of the motion of the molecules will not increase.

6. Were the temperature increased, the velocity of the molecular motion would also be increased.

7. If you were asked: "How can electric charges be made to flow and what factors influence their flow?" what would you answer?

Ex.15 Read and translate the text. Find conditional sentences.