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Physics unit 2 page 2 class XII



                page 2

   

resistance:  it is the property of a conductor by which it oppose the flow of charge or current through the conductor .R=V/I  hence resistance of conductor is defined as the ratio of the potential difference applied across the conductor to the current. 


S.I unit:  ohm 


hence resistance of a  is conductor is said to be 1 ohm if 1 ampere current flowing through it after applying 1 volt potential difference. 


dimension formula: [ML^2T^-3A^-2]


 specific resistance or resistivity:  the resistance of a conductor is directly proportional length (l)  of conductor and inversely proportional to its cross sectional area (a). 


 R{l/a  or R=pl/a


where p is constant called resistity or specific resistance of the material of the conductor. 
it's value depend on the nature of the material of the conductor and physical condition like temp., & pressure. 
it doesn't depend upon shape & size of the conductor 
If l=1, A=1 then p=R.
the resistivity of the material of the conductor is equal to its the resistance offered by its unit length and having unit cross sectional area  
Conductor: those substance which have low resistivity are conductor like metals
non conductor: those substance which have high resistivity are non conductor like insulator such as mica  glass, bakelite.etc
semiconductor:  those substance which have resistivity in between conductor and insulator are called semiconductor. like carbon ,germanium, silicon etc
conductance(G):  the reciprocal of the resistance of the conductor is called conductance .
G=1/R
Electrical conductivity: the reciprocal of resistivity of a material is called electrical conductivity .it is denoted by sigma.
where p is resistivity.
s.i unit: ohm^-1 metre^-1 or mho metre^-1or sieman metre^-1
Dimension formula [M^-1L^-3T^3A^2]







S.I unit Ohm metreDimension: [ML^3T^-3A^-2]

S.I unit ohm^-1 or mho or siemen(s). Dimension formula: [M^-1L^-2T^3A^2]

 sigma=1/p


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