Unit 5. Electric circuits

The concepts of electric charge and potential are also essential in the study of electric currents. When an extended conductor has different potentials at its ends, the tree electrons of the conductor itself are caused to drift form one end to the other. In order for this flow to continue it is necessary that the potential difference be maintained by some electrical source such as an electrostatic generator, or, much more frequently, a battery or a direct-current generator. The wire and the electrical source together form an electric circuit, the electrons drifting around it as long as the conducting path is maintained. In effect such a flow of electrons constitutes an electric current.

Batteries and direct-current generators are sources of potential difference which urge the electrons around a circuit continually in one direction, producing a unidirectional current. For this reason such a source is said to have a fixed polarity, one terminal being called positive and the other negative. If it is desired to reverse the flow, then the terminals of the circuit must be reversed with respect to the source.

From the early days of electrical science, current has been regarded as a flow of electricity from the positive terminal to the negative terminal in the external circuit connected to a source. Now we know a current through a conductor to be actually a movement of electrons, and since these have negative charges, they travel around the external circuit from the negative terminal to the positive terminal. The electron flow is, therefore, opposite to the conventional direction of current, making it necessary, in order to avoid confusion, to distinguish one from the other by name.

EXERCISES I. Find in the text:

1. Nominative Absolute Constructions;

2. Sentences with Complex Subject and Complex Object with the Infinitive.

2. Find in the text synonyms for:

In support, constantly, to wish, often, hence, really, standard, movement.

2. Translate paying attention to the different meanings of the wolds in italics:

1. Many different kinds of tubes have been invented since the introduction of the two-electrode tube. 2. Silver is the best conductor since it offers the least resistance to the flow of current of electricity. 3. Since the commutator is connected to the armature coil, it rotates with it. 4. An electromotive force causes the electrons to move in some definite direction. 5. The moving of the magnet causes the electron flow. 6. Force and motion always go together; one is a cause, the other a result. 7. Different kinds of instruments are used to measure electric current.8. A volt is a measure of electromotive force.

2. Retell the text according to your own plan.

3. Translate into English:

Струм в електричному колі є потоком електронів.Його величина обумовлюється швидкістю цього потоку, яка пов’язана з величиною

е.р.с., що докладається. Одиницею виміру струму є ампер.Відомо, що потік електронів у колі створює магнітне поле навколо провідника,вздовж якого вони рухаються.