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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 – meter^{2} / meter
= ohm – meter
- The
unit of resistivity is ohm – meter.

Specific Resistance
- If
we consider a = 1 meter
^{2} 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 / meter^{2}
= Siemen / meter

^{2}/ meter

^{2}and L = 1 meter, the resistance is equal to specific resistance.^{-1 }or mho.^{2}

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