Definition of Resistance
When a voltage is applied across a substance there will be an electric current through it. The applied voltage across the substance is directly proportional to the current through it. The constant of proportionality is resistance. Hence resistance is defined as the ratio of applied voltage to the current through the substance.Where V is voltage, I is current and R is resistance.
Concept of Resistance
To understand the matter let us take examples of metallic substances. There are numbers of free electrons moving randomly in the crystal structure of a metallic substance. When a voltage is applied across the resistance due to the electric field the free electrons drift from lower potential point to higher potential point in the substance. During drifting motion, the free electrons continually collide with atoms of the substance and this phenomenon prevents the free motion of electrons and this causes resistance.
Unit of Resistance
From the definition of resistance, it can be said that the unit of electric resistance is volt per ampere. One unit of resistance is such a resistance which causes 1 ampere current to flow through it when 1 volt potential difference is applied across the resistance. The unit of electric resistance that is volt per ampere is called ohm(Ω) after the name of great German physicist George Simon Ohm.He is famous for his law called Ohm’s law which is applicable only on pure resistance. The unit ohm is normally used for moderate values of resistance but there may be a very large as well as a very small value of resistance used for different purposes. These values are expressed in giga-ohm, mega ohm, kilo-ohm, milli-ohm, micro-ohm even in nano-ohm range depending on the value of resistance.
|Unit Name||Abbreviation||Value in ohm (Ω)|
|Giga Ohm||G Ω||109 Ω|
|Mega Ohm||M Ω||106 Ω|
|Kilo Ohm||K Ω||103 Ω|
|Milli Ohm||m Ω||10 – 3 Ω|
|Micro Ohm||μ Ω||10 – 6 Ω|
|Nano Ohm||n Ω||10 – 9 Ω|
Resistance of Different Materials
Depending on the resistance value substances are divided into three categories.
- There are some materials mainly metallic substances which offer very low resistance to the current through them. These substances are referred to as conductors more precisely electrical conductors. Silver is an extremely good conductor of electricity but it is not widely used in electrical systems because of its high cost. Aluminum is a good conductor and it is commonly used conductor because of its low cost and plenty of availability. Copper is another good conductor commonly used in different electronics and electrical circuits and it is a better conductor than aluminum but at the same time, it is costlier than aluminum.
- There is another category of materials called semiconductor. These have a moderate value of resistance i.e. not very high as well as not very low at room temperature. There are endless uses of semiconductors for making electrons devices. Silicon, germanium are two mostly used semiconductor materials. In addition to these different compounds also behave as semiconductors.
- The materials offer extreme resistance to the current is known as the insulator or electrical insulation material. These materials are very bad conductor of electricity and mainly used to prevent leakage current in electric systems. Papers, dry woods, mica, porcelain, glass epoxy polyester, mineral oil, SF6 gas, Nitrogen gas, other gases, air, etc are very good examples of insulation materials.
Effect of Temperature on Resistance
In metallic substances with rising temperature the interatomic vibrations increase and hence offer more resistance to the movement of electrons causing the current. Hence, with increasing temperature the resistance of metallic substances increases. The temperature coefficient of resistance is positive for these materials. In semiconductors with increasing temperature the number of free electrons increases as at higher temperature more number of covalent bonds gets broken to contribute free electrons in the substance. This reduces the resistance of the substance. Hence semiconductors have a negative temperature coefficient of resistance.