Resistivity: From R=ρ1/A ; If l = 1m, A = 1 m2 then R = ρ i.e. Resistivity is numerically equal to the resistance of a substance having unit area of cross-section and unit length.
(i) Unit and dimension: It's S.I. unit is ohm ´ m and dimension is [ML3T-3A-2]
(ii) It's formula: ρ = m/ne2t
(iii) Resistivity is the intrinsic property of the substance. It is independent of shape and size of the body (i.e. l and A).
(iv) For different substances their Resistivity is also different e.g. ρ minimum = 1.6 *10-8 Ω-m and ρ maximum =1016 Ω-m
(v) Resistivity depends on the temperature. For metals ρt = ρo(1+αΔt) i.e. Resistivity increases with temperature.
(vi) Resistivity increases with impurity and mechanical stress.
(vii) Magnetic field increases the Resistivity of all metals except iron, cobalt and nickel.
(viii) Resistivity of certain substances like selenium, cadmium, sulphide is inversely proportional to intensity of light falling upon them.
Conductivity: Reciprocal of resistivity is called conductivity (s) i.e. σ = 1/ρ with unit mho/m and dimensions[M-1L-3T3A2]
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Conductance: Reciprocal of resistance is known as conductance C=1/R.
It's unit is 1/Ω or Ω-1 or "Siemen".
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Filament of electric bulb: Is built up of tungsten which has high melting point, high resistivity
Element of heating devices: Is made up of nichrome which has high Resistivity and high melting point.
Resistances of resistance boxes: Are made up of alloys (manganin, constantan or nichrome) these materials have moderate Resistivity which is practically independent of temperature so that the specified vaue of resistance does not alter with minor changes in temperature.
Fuse-wire: Is made up of tin-lead alloy (63% tin + 37% lead) It should have low melting point and high resistivity. It is needed in series as a safety device in an electric circuit and is designed so as to melt and thereby open the circuit if the current exceeds a predetermined value due to some cause. The operation of a fuse is independent of its length.
Safe current of fuse line relays with it's radius as .
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Thermistors: A thermistor is a heat sensitive resistor usually prepared from oxides of various metals such as copper, nickel, cobalt, iron etc. These components are also semi-conductor. For thermistors a is very high which may be negative or positive. The resistance of thermistors modifies very rapidly with change of temperature.
Thermistors are used to detect small temperature change and to measure very low temperature.
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