Resistance and laws of resistance

Resistance.

The property of a substance by which it restricts the flow of electricity (electrons) through it. Metals,acids and salts solutions are good conductors. Among metals, silver, copper and aluminium are very good conductors of electricity.When electrons flow through the molecules or the atoms of the conductor, collide to the other atoms and electrons and generate heat. Those elements which offer relatively greater difficulty  to the passage  of these flowing electrons are said to be poor conductors of electricity.

Dry wood, rubber,PVC (polyvinyl chloride) and glass are poor conductors examples. Good insulators are fibrous substances like paper and dry cotton, mineral oil free from acid and water.

Unit of Resistance.

Unit of resistance is ohm, it can be defined as " a conductor having a resistance of one ohm when it permits one ampere current to flow through it when one volt is impressed across its terminals. For high resistance insulators bigger units  are used. Its symbol is (omega) Ω.

Laws of resistance.

The resistance R offered by a conductor depends upon the following factors which are known as laws of resistance, which are 

1: It change direction as its length l.

2: It varies inversely as the cross-section A of the conductor.

3: It depends on the nature of the material.

4: It depends on the temperature of the conductor.

So we can say 

                             R = ρ . l/A

where ρ is a constant depending on the nature of the material of the conductor and is known as specific resistance or resistivity.

Resistivity

Specific resistance or resistivity of a material may be defined as the resistance between the opposite forces of a meter cube of that material.

S.I unit of resistivity is ohm-meter.(Ω.-m)

 Mathematically

                           ρ = A (meter²) x R (ohm)/ l (meter)

                      = AR/l