Current electricity

 


Current: Suppose q amount of charges flows through a wire or a medium at time interval of t, then current flowing through it is



e. m. f. :   e. m. f. of a cell is defined as the maximum potential difference between the two electrodes of a cell or battery when the cell is in open circuit.

S.I.  unit of e.m.f. is joule per coulomb (JC -1)



Ohm’s Law: Ohm’s law states that the current I flow through a conductor is directly proportional to the potential difference V applied across the two ends of the conductor.


Where R is a proportionality constant called resistance of the material.

S. I. unit of resistance is ohm denoted as Ω.

 


Resistivity or specific resistance:


1) In experiment, it was found that the resistance R is directly proportional to the length l of the conductor & also resistance is inversely proportional to the area A of cross-section of the conductor.



Therefore



2) Where ρ is a proportionality constant, this proportionality constant is called resistivity or specific resistance of the material or the conductor.


When l = 1   &    A = 1   we get that

R = ρ


3) The specific resistance or resistivity of a material is equal to the resistance of the material having unit length and unit area of cross-section of that material.


The S.I. unit of specific resistance or resistivity is ohm meter or Ωm.

 








Conductance:


The conductance of a material is just the reciprocal of electrical resistance of the object.

Suppose R is the resistance of a conductor then conductance which is denoted as G is


The S.I. unit of conductance is mho or Siemen, the symbol is S.



Conductivity:


Conductivity is different from conductance. The conductivity of a material is defined as the reciprocal of resistivity ρ or reciprocal of specific resistance.



Conductivity is denoted as σ and



the S.I. unit of conductivity is