Definition: The characteristics of substance by virtue of which it opposes the flow of current through it, is known as the resistance.

Formula of resistance: For a conductor if l = length of a conductor, A = size of area of cross-section of conductor, n = No. of free electrons per unit volume in conductor, t = relaxation time then resistance of conductor R = ρ1/A; where r = Resistivity of the material of conductor.

Unit and dimension:  It's S.I. unit is Ohm(W)or Volt/Amp..

Also 1 ohm = 1 volt/1Amp = 10⁹ emu of resistance

Its dimension is[ML2T^-3A^-2].

Dependence of resistance: Net resistance of a conductor depends upon the following factors:

(i) Length of the conductor: Resistance of a conductor is directly proportional to it's length i.e. R ∝ I and inversely proportional to its area of cross-section i.e. R ∝1/A  

(ii) Temperature: For a conductor

Resistance ∝  temperature

If          R₀ = resistance of conductor at 0°C

            R_t = resistance of conductor at t°C

and a, b = temperature co-efficient of resistance

then    and

If R₁ and R₂ are the resistances at t₁^oC and t₂^oC  respectively then .

The value of a is different at specific temperature. Temperature sign of resistance averaged over the temperature range t₁^oC to t₂^oC is given by  which givesR2=R1[1+at₁- t₂]. This formula gives an approximate value.

Variation of resistance of some electrical material with temperature

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