Basic concept and resistor

Basic concept abut atom, current , voltage and resistor :

The most fundamental electric component is resistor. In every electronic and electrical circuit, we find one or more resistors in various combinations. What does it do in circuit? Well it fundamentally does two major things in electronic circuit. One is, it resists the flow of current in a circuit and another is, it divides voltage in a circuit. Now two most frequently term, current and voltage come. So what is current? 
To understand current, we have to understand the atomic structure of material.  We will discuss very briefly about the related phenomena that will we need to understand our topics. All material consists of three fundamental particles and these are:
1.   Electron
2.   Proton and
3. Neutron

Electron is negatively charged particle, proton is positively charged particle and neutron is neutral particle. It has no charge. The amount of negative charge that an electron carry is -1.6×10-19C  and the positive charge that an proton carry is +1.6×10-19C  . Here C is the unit of charge and it is called Coulomb.

Proton and neutron are stay together at the center of an atom and it is called nucleus. Electron moves around the nucleus at different path according the energy it possess. According to the number of electron to the outer cell of an atom , materials are fundamentally classified into three groups:
1. Conductor
2. Semiconductor and
3.Insulator

The materials whose atoms have one , two or three electrons at their outer most cell are called conductors. These materials have numerous free electrons. Example of this type of materials is copper, iron etc.

The materials whose atoms have four electrons at their outer most cell are called semiconductors. These materials have some free electrons but not sufficient to use in practical use. Example of this type of materials are silicon ,  Germanium etc.

The materials whose atoms have five, six, seven electrons at their outer most cell are called insulators. These materials have very little or no free electrons. Example of this type of materials are glass ,  wood etc.

Now come to know about current and voltage.

Current: 

Current is the flow of electrons. We can’t see it , but can feel its existence by It’s action. When current flow through a bulb , it lit  up or when current flows through insulating material, it produce heat. The unit of current is Ampere (A). Mathematically
Current, I = q/t
q= charge
t = time


Voltage: 

Voltage is the difference of positive and negative charges in a place or pressure that pushes the free electron in a circuit or closed path. Its unit is volt (V).


Now come to the topics. What is resistor ?

Resistor: Resistor is a passive electronic component that resists the flow of electron in any closed path. And the property of a resistor to resist the flow of current is called resistance. The value of resistance depends on their length, resistivity and its cross sectional area. Resistivity is the resistance per unit area. The symbol of resistor is R and its unit is ohm (Ω).

There are various types of resistor, linear, non-linear, fixed and variable. Now come to the point , How do we measure resistance of  resistor theatrically and practically.

Measurement the value of a resistor ( Resistance):


There are two ways to measure the value of a resistor
1. Theoretical Measurement and
2.Practical Measurement

In Theoretical Measurement, we convert the color bands that are on the resistor body. For this we also called 4 band resistor, 5 band resistor and so on. Each color is called a band and each color has a numerical code that helps us to determine the values of resistor. There are usually 12 color and each color has a value. Fig-4 shows the color, their respective code. For example for black , the code is zero (0) and for green , the code is Five (5) and so on. Normally four or Five band resistors are available in market. If we consider a four band resistor, that is 4 colors are drawn in its body, then by the following formula we can easily calculate the value of the resistor.

Resistance  ( R ) = AB×10C ± D %
Here D is called Tolerance, which depends on the temperature.

Resistor color code for basic electronics
Fig: Resistor color code 


If we consider a resistor which has 4 band or color in its body and these colors are
Red, Green, Red, Gold , then the value of the following resistor will calculate as follows:

Here the respective code of the colors are
Red = 2
Green = 5
Red = 2
Gold = 5

Now if we put these values to the above formula, we get,

R = 25 × 102 ± 5 %  ohms (Ω )
R = 2500 ± 5 %  ohms (Ω )
R = (2375 – 2625 ) Ω

For Five band Resistor, the formula will be
Resistance  ( R ) = ABC×10D ± E %

If we consider the band as Red, Green, Black, Red, Gold then,
Here the respective code of the colors are
Red = 2
Green = 5
Black = 0
Red = 2
Gold = 5

Now if we put these values to the above formula, we get,

R = 250 × 102 ± 5 %  ohms (Ω )
R = 25000± 5 %  ohms (Ω )
R= (23750 – 26250) Ω


Practical Measurement : In Practical measurement , we use a ohm meter or multimeter.



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