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Resistance | What is resistance and factors on resistance depends | Specific resistance or Resistivity | Full details

In this article, we will discuss what is resistance is widely and all its aspect from every area all the possible information available on me I delivered to you hope it helps you.



RESISTANCE

The opposition offered by a substance to the flow of electric current is called its resistance.

Since the current is the flow of free electrons, resistance is the opposition offered by the substance to the flow of free electrons. This opposition occurs because atoms and molecules of the substance obstruct the flow of these electrons. Certain substances(e.g. metals such as silver, copper, aluminum, etc.) offer very little opposition to the flow of electric current and are called conductors.


On the other hand, those substances which offer high opposition to the flow of electric current (i.e. flow of free electrons) are called insulators e.g. glass, rubber, mica, dry wood, etc.


It may be noted here that resistance is the electric friction offered by the substance and causes the production of heat with the flow of electric current. The moving electrons collide with atoms or molecules of the substance; each collision resulting in the liberation of a minute quantity of heat.


Unit of resistance :

The practical unit of resistance is ohm and is represented by the symbol W.


👉It is defined as under :

A wire is said to have a resistance of 1-ohm a potential difference of 1 volt across its ends causes 1 ampere to flow through it. There is another way of defining ohm. the wire is said to have a resistance of 1 ohm if it

releases 1 joule (or develops 0.24 calories of heat) when a current of 1 A flows through it for 1 second.



A little reflection shows that the second definition leads to the first definition.


 Thus 1 A current flowing for 1 second means that the total charge flowing is Q = I × t = 1 × 1 = 1 coulomb. Now the charge flowing between A and B (See Fig. 1.10) is 1 coulomb and energy released is 1 joule (or 0.24 calories).


Obviously, by definition, the potential difference between A and B should be 1 volt.


Factors Upon Which Resistance Depends


  • The resistance R of a conductor is directly proportional to its length i.e.
                          R ∝ l
  • Resistance is inversely proportional to its area of X-section i.e.
R ∝ 1a
  • Resistance depends upon the nature of the material.
  • Resistance depends upon temperature. 


From the first three points (leaving temperature for the time being), we have,


R ∝ l/a

or R =l ρ a

where ρ (Greek letter ‘Rho’) is a constant and is known as resistivity or specific resistance of the

material. Its value depends upon the nature of the material.


Specific Resistance or Resistivity


We have seen above that
R =l ρ a
Hence specific resistance of a material is the resistance offered by 1 m length of wire of
material having an area of cross-section of 1 m2.

Specific resistance can also be defined in another way. Take a cube of the material having each
side 1 m. Considering any two opposite faces, the area of cross-section is 1 m2 and length is 1 m

Hence specific resistance of a material may be defined as the resistance between the opposite faces of a meter cube of the material.

Unit of resistivity: We know R = ρal or  ρ = R a
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