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Put two questions to each paragraph of the text and ask your fellow-students to answer them.

Find the infinitive constructions in the text and define them.

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LESSON SIX

ELECTRIC CIRCUIT

We know the circuit to be a complete path which carries the current from the source of supply to the load and then carries it again from the load back to the source.

Generally speaking, the current may pass through solid conductors, liquids, gases, vacuums or any combinations of these. It may flow in turn over transmission lines from the power station, through transformers, cables and switches, through lamps, heaters, motors, and so on,—back through other switches, cables, transformers and transmission lines to the generator in the power station.

There are many kinds of circuits, such as: open circuits, closed circuits, series circuits, parallel circuits and short cir­cuits.

If the circuit is broken or, as we generally say, "opened" anywhere, the current stops everywhere. Hence, we break the circuit when we switch off our electric, devices.

The path along which electrons travel must be complete or no electric power can be supplied to the load from the source. Thus, we close the circuit when we switch on our electric lamp.

A simple electric circuit is illustrated in Fig. 7. In this figure a 4-cell battery has been used, the switch being in an open position. If the switch is in a closed position, a current will flow around the circuit in the direction shown by the arrows.

To understand the difference between the following circuit connections is not difficult at all. When electrical devices are connected so that the current is not divided at any point they are said to be connected in series. Under such condition the current flow is the same in all parts of the circuit, as the is only a single path along which it may flow. Quite the opposite, the parallel circuit provides two or more paths for the passage of current. The circuit is divided in such a w«that part of the current goes through one path and pass through another.

 

Fig. 7. A simple electric circuit

We produce a short circuit or, as we sometimes call it, the "short" when we allow the current to return to the source of supply without control and without doing the work that we want it to do. In short, this phenomenon mostly results in cable faultand wire fault while under certain conditions it may even cause fires.

It is interesting to note that some substances like metals, for example, conduct electricity with ease; on the contrary, others, such as rubber, do not allow it to move freely. Thus, we obtain conductors and insulators, there being a marked difference between them, of course.

So far nothing was said about conductance, that is, the conductor's ability to pass electric charges. It appears to de­pend on four things, namely: its size, its length, the kind of material to be used, and its temperature.

It is not difficult to understand that a large water-pipe can pass much more water than a small one. We equally expect a large conductor to carry current' more readily than a thinner one. Fig. 8 illustrates this feature better than words alone! Indeed, we see that the larger the wire, the greater is its conductance because electricity meets less resistance then.

It is quite understandable too that to flow through a short conductor is certainly easier for the current than through a long one, in spite of their being made of similar materials. Hence, the longer the wire, the greater is its opposition, that is resistance, to the passage of current.

Fig. 8. Comparing water flow and current flow.

There is a great difference in the conductance of various substances. For example, almost all metals are supposed to conduct current. Nevertheless, copper appears to carry the current more freely than iron, silver conducting better than copper. It is therefore said to have a greater conductance than copper. The reader probably remembers that insulators also differ in their insulating properties.

As to temperature, we are familiar with the following feature. As the temperature rises, opposition to the passage of current increases as well. Hence, conductance depends on the temperature of the wire.

It is quite wrong to think that conducting materials are the only materials to play an important part in electrical engineering. As a matter of fact, to meet our everyday power requirements, we are certain to need both conductors and insulators.