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24.003 Current flow in a circuit

 

Ian McKenzie Smith
Hughes electrical technology, 1995
Pages 14-16, Current flow in a circuit

 

Fig. 2.3 Simple lamp circuit

2.3 Simple lamp circuit

 

2.4 Symbols used in Fig. 2.3

Fig. 2.4 Symbols used in Fig. 2.3

 

Electricity permits the source of energy to be remote from the point of application. Electrical engineering is concerned with the study of how this energy transmission takes place, but, before getting down to applying electric current to our use, it is necessary to become familiar with some of the basic electrical terms.

 


2.3 Electric charge

An electrical system generally transmits energy due to the movements of electric charge. Although we need not study electric charge in depth, we need to have some understanding in order to develop a system of measurement of electrical quantities and also to relate these to the measurement which we have reviewed in Chapter 1.

Electricity appears in one of two forms which, by convention, are called negative and positive electricity. Electric charge is the excess of negative or positive electricity on a body or in space. If the excess is negative, the body is said to have a negative charge and vice versa.

An electron is an elementary particle charged with a small and constant quantity of negative electricity. A proton is similarly defined but charged with positive electricity while the neutron is uncharged and is therefore neutral. In an atom the number of electrons normally equals the number of protons; it is the number of protons that determines to which element type the atom belongs. An atom can have one or more electrons added to it or taken away. This does not change its elemental classification but it disturbs its electrical balance. If the atom has excess electrons, it is said to be negatively charged. A charged atom is called an ion.

A body containing a number of ionized atoms is also said to be electrically charged. It can be shown that positively and negatively charged bodies are mutually attracted to one another while similarly charged bodies repel one another.

 


2.4 Movement of electrons

All electrons have a certain potential energy. Given a suitable medium in which to exist, they move freely from one energy level to another and this movement, when undertaken in a concerted manner, is termed an electric current flow. Conventionally it is said that the current flows from a point of high energy level to a point of low energy level. These points are said to have high potential and low potential respectively. For convenience the point of high potential is termed the positive and the point of low potential is termed the negative, hence conventionally a current is said to flow from positive to negative.

This convention was in general use long before the nature of electric charge was discovered. Unfortunately it was found that electrons move in the other direction since the negatively charged electron is attracted to the positive potential. Thus conventional current flows in the opposite direction to that of electron current. Normally only conventional current is described by the term current and this will apply throughout the text.

The transfer of electrons takes place more readily in a medium in which atoms can readily release electrons, e.g. copper, aluminium, silver, etc. Such a material is termed a conductor. A material that does not readily permit electron flow is termed an insulator, e.g. porcelain, nylon, rubber, etc. There is also a family of materials termed semiconductors which have a certain characteristics that belong to neither of the other groups.

 


2.5 Current flow in a circuit

For most practical applications it is necessary that the current flow continues for as long as it is required; this will not happen unless the following conditions are fulfilled:

1. There must be a complete circuit around which the electrons may move. If the electrons cannot return to the point of starting, then eventually they will all congregate together and the flow will cease.

2. There must be a driving influence to cause the continuous flow. This influence is provided by the source which causes the current to leave at a high potential and to move round the circuit until it returns to the source at a low potential. This circuit arrangement is indicated in Fig. 2.5.

 

2.5 Elementary circuit

Fig. 2.5 Elementary circuit

 

The driving influence is termed the electromotive force, hereafter called the e.m.f. Each time the charge passes through the source, more energy is provided by the source to permit it to continue round once more. This is a continuous process since the current flow is continuous. It should be noted that the current is the rate of flow of charge through a section of the circuit.

 


2.6 Electromotive force and potential difference

The e.m.f. represents the driving influence that causes a current to flow. The e.m.f. is not force, but represents the energy expended during the passing of a unit charge through the source; an e.m.f. is always connected with energy conversion.

The energy introduced into a circuit is transferred to the load unit by the transmission system, and the energy transferred due to the passage of unit charge between two points in a circuit is termed the potential difference (p.d.). If all the energy is transferred to the load unit, the p.d. across the load unit is equal to the source e.m.f.

It will be observed that both e.m.f. and p.d. are similar quantities. However, an e.m.f. is always active in that it tends to produce an electric current in a circuit while a p.d. may be either passive or active. A p.d. is passive whenever it has no tendency to create a current in a circuit.

Unless it is otherwise stated, it is usual to consider the transmission system of a circuit to be ideal, i.e. it transmits all the energy from the source to the load unit without loss. Appropriate imperfections will be considered later.

Certain conventions of representing the e.m.f. and p.d. in a circuit diagram should be noted. Each is indicated by an arrow as shown in Fig. 2.6. In each case, the arrowhead points towards the point of high (or assumed higher) potential. It is misleading to show an arrowhead at each end of the line as if it were a dimension line. An arrowhead is drawn on the transmission system to indicate the corresponding direction of conventional current flow.

 

Fig. 2.6 Circuit diagram conventions

2.6 Circuit diagram conventions

 

It will be seen that the current flow leaves the source at the positive terminal and therefore moves in the same direction as indicated by the source e.m.f. arrow. The current flow enters the load at the positive terminal, and therefore in the opposite direction to that indicated by the load p.d. arrow. Energy is converted within the load unit and, depending on the nature of this conversion, the p.d. may be constituted in a variety of ways. It is sufficient at first to consider the p.d. as the change in energy level across the terminals of the load unit. This is termed a volt drop since the p.d. (and e.m.f.) are measured in volts.

 

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