## 10/12/2017

### Resistance It is a defined as property of substance which oppose the flow of current through it.  The SI unit of resistance is ohm ( Ω ).  One ohm is defined as the resistance of conductor when one voltage applies between conductor resulting one ampere current flows through it.  The resistance is expressed in milli – ohm , kilo ohm or mega ohm. Law of resistance The resistance of conductor depends upon length of the conductor, area of conductor, type of conductor material and operating temperature. The resistance of the conductor is         ( 1 ) Directly proportional to length of conductor ( L )         ( 2 ) Inversely proportional to area of the conductor ( a )         R a ( L / a )         R = ρL / a Where ρ = Resistivity of the material or specific resistance of the material Unit of Resistivity         ρ = Ra / L            = ohm – meter2 / meter            = ohm – meter The unit of resistivity is ohm – meter. Specific Resistance If we consider a = 1 meter2 and L = 1 meter, the resistance is equal to specific resistance. The specific resistance of a material is defined as a resistance of material having unit length and unit cross section area.                                    OR It is defined as the resistance between opposite faces of unit cube material. Conductance It is defined as reciprocal of resistance.  The symbol of conductance is G. Its unit is ohm-1 or mho.  The other unit is Siemens ( S ).  The conductance is defined as inducement offered by the conductor for the flow of current. Conductance G = 1 / R                          = 1 / ( ρL / a )                          = a / ρL                         =  ( 1 / ρ )( a / L )                         =  σa / L Conductivity The σ ( sigma ) is called as specific conductance or conductivity of the material.  Therefore the conductivity is reciprocal of resistivity or specific resistance.  The unit of conductivity is siemen / meter.        G = σa / L        σ = GL / a           = Siemen – meter / meter2           = Siemen / meter

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