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,

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

Ø Read and translate the text, using the given vocabulary. Learn the words by heart.

Vocabulary

Ø Check pronunciation of the following words:

Basic Units of Electricity. Measuring Devices

There exist three basic electrical units in any electric circuit: ampere, ohm and volt.

The ampere is an electrical unit for measuring the strength electric current in a circuit. One ampere represents the amount of current generated by 1 volt acting through the resistance of 1 ohm. For measuring the intensity of an electric current in amperes a device called ammeter is used.

The ohm is an electrical unit for measuring the resistance or opposition to the flow of current. All substances show different resistances to the flow of electricity through them. With the increase in the temperature the resistance of all metals increases while the resistance of carbon, insulating materials, electrolytic solution decreases. One ohm represents such a resistance that a one volt addition to the potential produces 1 ampere of current. The device used for measuring resistance is called ohmmeter. The most commonly used device for measuring in­sulation resistance is megohmmeter or 'megger'. It is usually em­ployed for continuity, ground and short-circuit testing in general electri­cal power work.

The volt is an electrical unit measuring the external force applied to a circuit to overcome the opposition to the flow of current. This force is called voltage and is also referred to as electromotive force or elec­tric pressure. The electromotive force that causes a current of 1 ampere to flow through a resistance of 1 ohm equals 1 volt. The device used for measuring voltage is called voltmeter.

One more important unit of electrical measurement is the watt – the unit of power: the power of 1 ampere of current pushed by one volt of electromotive force. Devices used for measuring delivery of electric energy in watts are called wattmeter and watthourmeter.

Exercises

I. Complete the following definitions using the words from the list below:

ampere, ohm, megohmmeter, volt, watt, ammeter.

1. The ... is an electrical unit for measuring the resistance or opposition to the flow of current.

2. For measuring the intensity of an electric current a device called ... is used.

3. An electrical unit measuring the external force applied to a circuit to overcome the opposition to the flow of current is the ... .

4. The most commonly used device for measuring insulation resistance is ... .

5. The ... is an electrical unit used to measure the strength of electric current in a circuit.

6. The power of 1 ampere of current pushed by one volt of electromotive force is the unit of power called the ... .