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IV. Get ready to retell Text 1B: divide the text into several parts, make up a plan and write down the key words.
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Text 1C
Read and translate the text, using the given vocabulary. Learn the words by heart.
Vocabulary
| quantity | количество |
| unit time | единица времени |
| gaseous | газообразный |
| mobile | подвижный |
| conducting medium | проводящая среда |
| electromotive force | электродвижущая сила |
| existence | существование |
| resistance | сопротивление |
| d.c. generator | генератор постоянного тока |
| rectifier | выпрямитель |
| circuit section | участок цепи |
| voltage | напряжение |
| source of supply | источник питания |
| inductance | индуктивность |
| capacitance | ёмкость |
| both | как …, так и … |
Check pronunciation of the following words:
ion, mobile, effect, heat, pulsating, rectifier, supply, inductance, anode.
Electric Currents and Their Properties
Electric charges in motion form electric current. Electric current is equal to the quantity of electric charges passing a given point in unit time. Conduction is the name given to a movement or flow of charges. The charges may be either positive or negative. In electrical circuits the flow of electric charges is considered to be a flow of electrons (negative charges) along a conductor or through a conducting medium. But charges may also be ions when the conduction takes place in gaseous or liquid conductors in which the ions are mobile.
To make electrons move through a metal conductor, there exists a driving force called electromotive force (e.m.f.), which moves electric charges from one point in the circuit to another. When the e.m.f. is applied to the ends of the wire, the free electrons move in one direction. The greater the number of participating electrons, the greater is the flow of current.
As it is impossible to see electric current, its existence is proved by 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. The currents can be measured by observing their heating, chemical or magnetic effects.
There exist several kinds of current: direct current, alternating current, pulsating current.
Direct current (d.c.) is an electric current that flows in one direction only and doesn't change its direction and magnitude at a constant resistance of a circuit.
The sources of direct current may be either galvanic elements, or thermo elements, or d.c. generators and accumulators. Besides, direct current may be obtained by converting alternating current into direct one with the help of rectifiers.
Almost all current calculations in d.c. circuits are based on Ohm's law. According to this law, any circuit section, connecting two points of this circuit and not having electromotive force, contains current
I = U / R,
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where U – voltage or potential difference between these two points, R – resistance of the circuit section connecting them. Ohm's law can be applied to the whole circuit in case there exists only one source of supply in it:
I = E / R,
where E – electromotive force of d.c. source of supply; R – resistance of the entire circuit, including internal resistance of the source of supply.
Alternating current (a.c.) is an electric current that changes both its direction and magnitude at fixed intervals of time. The strength of the current in alternating circuits, unlike that in direct ones, depends not only on the resistance of the entire circuit and the voltage applied to it, but on the inductance and the capacitance of the circuit sections as well.
Pulsating current is an electric current, alternative in its magnitude but stable in its direction. It is produced when the conversion of an alternating current into a direct current in current rectifiers, in anode circuits of electronic lamps, etc. takes place.
Exercises
I. Make sure you can translate the following words from the text correctly:
проводимость, электродвижущая сила, ион, подвижный, двигать, сопротивление, раствор, преобразовать, выпрямитель, в том случае, весь, индуктивность, в отличие от, ёмкость, внутренний, напряжение.
II. Fill in the missing words:
1. A movement or flow of charges is called.... 2. Charges may be ions when the conduction takes place in gaseous or... conductors. 3. Electric current existence is proved by its.... 4. There exist several kinds of current: direct, alternating and.... 5. The sources of direct current may be d.c. generators and.... 6. Alternating current is an electric current that changes both its direction and... at fixed intervals of time. 7. Pulsating current is produced when conversion of an alternating current into a direct current in current... takes place.
III. Here are the answers. Make up questions, write them down.
1) Electric charges in motion form electric current.
2) It is called electromotive force.
3) Direct current flows in one direction only.
4) It may be obtained by converting alternating current with the help of rectifiers.
5) The strength of the current in alternating circuit depends on the inductance and capacitance of the circuit sections.
IV. Get ready to retell Text 1C: divide the text into several parts, make up a plan and write down the key words.
Text 1D
Read and translate the text, using the given vocabulary. Learn the words by heart.
Vocabulary
| unit | единица измерения |
| measuring device | измерительный прибор |
| strength, intensity | сила, напряжённость |
| ammeter | амперметр |
| electrolytic solution | электролит |
| carbon | угольный электрод |
| addition | прибавление |
| potential | эл. потенциал, напряжение |
| insulation resistance | сопротивление изоляции |
| megohmmeter | мегомметр |
| continuity | непрерывность |
| ground | заземление |
| short circuit | короткое замыкание |
| external force | внешняя сила |
| overcome opposition | преодолеть сопротивление |
| electric pressure | электрическое напряжение |
| cause a current to flow | заставлять ток течь |
| power | мощность, энергия |
| delivery | подача, снабжение |
| wattmeter | ваттметр |
| watthourmeter | электросчётчик |
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