Law of resistance, Specific resistance,conductance, conductivity

Law of Resistance:-
Resistance of a substance/ wire :
-          Directly proportional to the length, l
-          Inversely proportional to the area of cross section.
-          As the length of wire increases there is a chance of increase in number of electrons collusions resulting in increase of resistance.
-          As the length of wire decreases, number of electrons collusions decreases resulting in decrease of resistance.
-          As the cross sectional area of substance/wire increases, electrons get more area to move through resulting in decrease of resistance.
-          As the cross sectional area of substance/wire decreases, electrons should move through a congested area resulting in increase of resistance.
-          Rαl
-          Rα(1/A)
-          Combining both the above
Rα(l/A)
-          Where l is the length of substance/wire and A is the cross sectional area of substance/wire.
-          Here, the proportionality constant is resistivity/specific resistance which depends upon the nature of substance/wire.
-          By removing proportionality constant and putting equality constant
R=ρ*(l/A)


Specific resistance/resistivity:-
-          Resistance between opposite faces of a unit cube of that material.
-          Measured in ohm-metres


Conductance:-
-          Reciprocal of resistance
-          Encouragement of substance to flow of electrons through it.
-          G=(1/R)
-          Unit : mho (Ʊ)


Conductivity:-
-          Reciprocal of resistivity.
-          Conductance between the two opposite faces of a unit cube.
-          σ =(1/ρ)
-          Unit: mho/metre or siemen/metre

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