### 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

( a ) Directly
proportional to length of conductor ( L )

( b ) 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

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

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