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Resistance:
                       According to Ohm’s Law, V = IR
                       R is a constant for the given metallic wire at a given temperature and is called
                       its resistance.
                   •  It is the property of a conductor to resist the flow of charges.
                   •  SI unit – ohm(Ω)
                   •  1 Ω resistance is 1 ohm when 1 ampere current flows through a conductor
                       provided 1 volt potential difference is applied across its ends.
               Factors on which Resistance of a conductor depends:
            1.  On its length
            2.  On its area of cross-section
            3.  On the nature of its material
            4.  Temperature


               o  Resistance of uniform metallic conductor is directly proportional to its length and
                  inversely proportional to area of cross-section.

               R∝l

               R∝ 1/A

               Therefore, R = ρ(l  / A),ρ(rho) is constant of proportionality.


               o  ρ is called electrical resistivity or specific resistance of the material of the
                  conductor. SI unit is Ωm.
               o  Resistivity of conductors is very low whereas the insulators have a very high
                  resistivity.
               o  Resistivity varies with Temperature.
               o  Alloys having higher resistivity than metals are used in electrical heating devices,
                  like iron and toasters, tungsten is used in filament of electric bulbs and copper
                  and aluminium are used for electrical transmission lines.
               Resistivity of Conductors < Resistivity of Alloys < Resistivity of Insulators
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