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Friday, 2 November 2012

Resistance & conductivity


          
           Resistance & conductivity


The electrical resistance of a wire would be expected to be greater for a longer wire, less for a wire of larger cross sectional area, and would be expected to depend up on the material out of which the wire is made. Experimentally, the dependence upon these properties is a straightforward one for a wide range of conditions, and the resistance of a wire can be expressed as


                                                      R=(ρL)/A

                                                      ρ=Resistivity
                                                      L=Length
                                                      A=Cross sectional area

The factor in the resistance which takes in to account the nature of the material is the resistivity. Although it is temperature dependent, it can be used at a given temperature to calculate the resistance   of the wire of given geometry.

The inverse of resistivity is called conductivity. There are contexts where the use of conductivity is more      convenient

                              Electrical Conductivity= σ =(1/ρ)                                                                                                                                                                                

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