Fig. 4. 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 fault and 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. 2 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.

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.

 

 

Fig. 5. Comparing water flow and current flow

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.

 

Exercises

 

Translate the following sentences and define the functions of the infinitive.

1. The current is known to flow when the circuit is complete.

2. To stop the flow of current is to break the circuit in some point.

3. Copper and silver are considered to be the best conductors in electricity.

4. Various switches are generally used to open or to close the circuits.

5. Ampere supposed the current to flow from the positive pole of the source to the negative pole.

6. It is quite possible to generate a.c. and then transform it into d.c.

7. Yablochkov was the first to apply a.c. in practice.

8. A battery is the simplest device to produce direct current.

9. We know the circuit to be a path of an electric current.

10. We may expect a short circuit to result in wire fault and cable fault.

 

Answer the following questions.

1. What do we deal with in this article?

2. What is an electric circuit?

3. What kinds of circuits do you know?

4. When is the “short” produced?

5. What feature of the conductor is illustrated in Figure 2?

6. What does conductance depend on?

7. What do we mean by the term “short circuit”?

8. What does the term “closed circuit” mean?

9. Who determined the difference between the electric current and the static charges?

10. Why does the current flow when the circuit closed?

 

Define the meaning of the following words.

Circuit, electron, conductor, insulator, switch, current, conductance.

 

Compare.

1. Conductors and insulators.

2. Closed circuits and open circuits.

3. Series circuits and parallel circuits.

 

Ask and answer questions.

1. If heat is a form of energy.

2. If all substances are made up of molecules.

3. If the molecules are travelling in various directions in a substance.

4. If the circuit is a complete path which carries the current from the source and back to the source.

5. If all metals conduct electricity with ease.

6. If we close the circuit when we switch off the electric light.

7. If we open the circuit when we switch on the electric light.

8. If conductance depends on the temperature of the wire.