Lesson 7.

SUMMARY OF IMPORTANT FACTS.

1.Principles of Electricity

A chemical element is built up from atoms. Each atom is made up of electrons, protons and neutrons. The electron has a negative charge and the proton a positive charge; the two electrical charges are equal and opposite, but the mass of proton is 1840 times greater than that of the electron. Current flow is a combination of electron flow (negative charge carries) and hole flow (positive charge carries).

A conductor offers low resistance to current flow, an insulator offers high resistance. The resistance of semiconductor is mid-way between the two extremes .

Voltage or e. m. f. is a measure of the potential of an electric circuit to produce current flow. The conductance (G) of an electrical circuit is measured in simians (S) and resistance (R in ohms).

A linear circuit element is one whose resistance is constant despite fluctuations of voltage and current. The resistance of a non-linear circuit element varies with fluctuations of voltage and current. A direct current or unidirectional current always flows in the same direc­tion around a circuit. An alternating current periodically reverses its direction of flow if the periodic time of an alternating wave is T.

1 second, the frequency of the alternating wave is f = Нz.

2. How Electrical Energy is Produced.

There are several methods of producing electricity for practical purposes. The battery of a pocket torch may be contrasted with the source of enormous energy represented by a larger power station. Both are examples of the application of electrical energy to a particular purpose, and in general the purpose determines the nature of the method used to produce the energy. Practical methods of producing electricity may be enumerated as follows:

1. Chemical, as represented by the various types of batteries or primary cells in which the electricity is produced by purely chemical actions.

2. Electromagnetic, forming the basis of operation of rotating generators in which the electricity is produced by conductors moving through a mag­netic field. This is the method employed in practice for generators of various sizes.

3. Thermo-electric, in which the heating of the Junction between two different metals produces a very small voltage, which may be used for purposes of temperature measurement and as a source of power.

4. Pieso-electric, in which a very small voltage is produced across certain faces of a crystal by application of mechanical pressure. This effect is used, for example, as a means of frequency control in radio oscillators or for gramophone pick-ups, but it is suitable for power supply.

5. Electronic, characterized by the flow of electrons through evacuated or gas-fi1 led tubes, and having the following forms:

a) Thermionic emission. In which electrons are produced by the heating of special materials.

b) Photo-electric emission, in which electrons are liberated at the sur­face of certain substances by the action of light.

c) Secondary emission, in which electrons are driven from a material by the impact of electrons or other particles on its surface.

d) Field emission, in which electrons are drawn from the surface of a metal by the application of very powerful electric fields.