BASIC CONCEPTS AND UNITS

CHAPTER ONE
BASIC CONCEPTS AND UNITS
1.1 Introduction
The study of an electrical engineering involves the analysis of the energy transfer from one form to another or from one point to another. So before beginning the actual study of an electrical engineering, it is necessary to discuss the fundamental ideas about the basic elements of an electrical engineering like electromotive force, current, resistance etc. The electricity is related with number of other types of systems like mechanical, thermal etc. To analyze such transfer, it is necessary to revise the S.I. units of measurement of different quantities like work, power, energy etc. in various systems.
1.2 The Structure of Matter
The structure of matter plays an important role in the understanding of fundamentals of electricity. The matter which occupies the space may be soild, liquid or gaseous. The atom is composed of three fundamental particles: neutron, proton and the electron.
The small electrical effort, externally applied to such conductor makes all such free electrons to drift along the metal in a definite particular direction. This direction depends on how the external electrical effort is applied to the conductor. Such physical phenomenon is represented in the Fig.1.2.
The free electrons as are negatively charged get attracted by positive of the cell connected.
The flow of electrons from negative to positive of the cell.
This movement of electron is called an Electric Current. The movement of electrons is always from negative to positive while movement of current is always assumed as from positive to negative. This called direction of conventional current.

1.3 Relation between Charge and Current
The current is flow of electrons. Thus current can be measured by measuring how many electrons are passing through material per second. This can be expressed in terms of the charge carried by those electrons in the material per second.

Mathematically we can write the relation between the charge (Q) and the electric current (I) as,
i=dq/dt (1.1)
Where current is measured in amperes (A), and
1 ampere = 1 coulomb/second
The charge transferred between time t0 and t is obtained by integrating both sides of Eq. (1.1). We obtain
1.4 Resistance
When the electrons begins flow in the metal. The ions get formed which are charged particles as discussed earlier. Now free electrons are moving in specific direction when connected to external source of e.m.f. So such ions always become obstruction for the flowing electrons. So there is collision between ions and free flowing electrons. This not only reduces the speed of electrons but also produced the heat. The effect of this is nothing but the reduction of flow of current. Thus the material opposes the flow of current.


The resistance is denoted by the symbol R and is measured in ohm symbolically represented as ?. We can define unit ohm as below.

1.5 Factors Affecting the Resistance
1. Length of the material: The Length is denoted by l .
2. Cross-section area: The cross sectional area is denoted by a .
3. The type and nature of the material:
4. Temperature: The temperature of the material affects the value of the resistance.
1.6 Effect of Temperature on Resistance
The resistance of the material affected as temperature of a material change. As example, Atomic structure theory says that under normal temperature when the metal is subjected to potential difference, ions i.e. unmovable charged particles get formed inside the metal. The electrons which are moving randomly get aligned in a particular direction as shown in the fig. 1.3. If temperature increases, the ions gain energy and start oscillating about their mean position and cause collision and obstruction to the flowing electrons. Due to collision and obstruction due to higher amplitude of oscillations of ions, the resistance of material increases as temperature increases. But this is not true for all materials. In some cases the resistance decreases as temperature increase.
1.6.1 Effect of Temperature on Metals
The resistance of all the pure metals like copper, iron, tungsten etc. increases linearly with temperature. This is shown in the Fig. 1.4.
For good conductors, an increase in temperature will result in an increase in the resistance level. Consequently, conductors have a positive temperature coefficient.
1.6.2 Effect of Temperature on Carbon and Insulator
The effect of temperature on carbon and insulators is exactly opposite to that of pure metals. Resistance of carbon and insulators decreases as the temperature increase. The result is a negative temperature coefficient.
1.6.3 Effect of Temperature on Alloy s
The resistance of alloys increase as the temperature increase but rate of increase is not significant. In fact, some of alloys show almost no change in resistance for considerable change in the temperature like Manganin (alloy of copper, manganese and nickel), Eureka (alloy of copper and nickel) etc. Due to this property alloys are used to manufacture the resistance boxes. Fig.1.5 shows the effect of temperature on metals, insulating materials and alloys.

1.9.4 Effect of Temperature on Semiconductors
The materials having conductivity between that of metals and insulators are called semiconductors such as silicon, germanium etc. At absolute zero temperature, the semiconductors behave as perfect insulators.
For semiconductor materials, an increase in temperature will result in a decrease in the resistance level. Consequently, semiconductors have negative temperature coefficients.
The thermistor and photoconductive cell are excellent examples of semiconductor devices with negative temperature coefficients.


1.10 Resistance Temperature Coefficient (R.T.C.)
From the discussion up till now we can conclude that the change in resistance is,
1) Directly proportional to the initial resistance.
2) Directly proportional to the change in temperature.
3) Depends on the nature of the material whether it is a conductor, alloy or insulator.
Let us consider a conductor, the resistance of which increases with temperature linearly.
Let R0= Initial resistance at 0 Co, R1= Resistance at t1 Co, R2=Resistance at t2 Co
As shown in the Fig. 1.7. R2>R1> R0
Definition of R.T.C.: The resistance temperature coefficient at t Co is the ratio of change in resistance per degree Celsius to the resistance at t Co. the unit of R. T.C. is 1/Co.