OHM’S LAW

DEFINATION: -

“The current which is flowing through a conductor is directly proportional to the applied potential difference at constant temperature is called ohm’s law”.

Mathematically this law can be stated as,

                                              I∞ V

                                              I = V / R

                                Or              V = IR

Where R is constant of proportionality and is called resistance of the conductor. In S.I system the unit of resistance is OHM

OHM: - “One volt is potential difference is applied across any plates in one ampere current then the total resistance is one Ohm”.

              The conductors, which obey ohm’s law, is called OHMIC conductor and that conductor which does not obey ohm’s law is called NON-OHMIC conductor.

EXAMPLES: - Filament bulb, thermister and semiconductor diodes are the examples of NON-OHMIC conductor. Where metal wire is an example of OHMIC conductor.

ELECTRICAL RESISTANCE: -

                    “The measurement opposition offered to the flow of electric current is called resistance”. It arises as a result of continuous bombing of electrons with lattice atoms.

              Mathematically it can be expressed as,

                                R = V / I

Thus resistance is the ratio of potential difference to current. Its units is OHM and OHM is defined as, “one ohm is the resistance of a conductor through which a current of one ampere is flowing when the potential difference across it is one volt”.

                                    1Ω = 1 V A ^–1 
 

RESISTIVITY & ITS VARIATION WITH TEMPERATURE: - From observation we know that resistance of a conductor is directly proportional to the length “l” and inversely proportional to the area of cross-section “A” of the conductor.

              i.e.                           R ∞ L                                        (1)

                                              R ∞ L / A                    (2)

              Combining the above two equations we get

                                              R ∞ L / A

                                Or              R = ρ L / A                                 (3)

                                Or               ρ = R A / L                                (4)

Where ρ is constant of proportionality and is called resistivity of conductor and can be defined as,                    “the resistance of a conductor having unit length and unit area of cross-section”.

It is also called specific resistance and its unit is Ωm.

The reciprocal of resistivity is called conductivity. We represent it by σ and are given by

                                                                σ = 1 / ρ = L / R A