Resistivity What factors affect the resistance of a material? a) Length - the further electrons have to travel through material, the more collisions they will have so the higher the value of resistance. b) Area - a bigger area means that in any 1 second more electrons will be able to travel through a piece of wire. More electrons means more current which means less resistance. c) Material - if you swapped all the copper wire in a circuit for wood you'd notice a lot less current and a lot more resistance in the circuit. The ability of a material to resist a current is called its resistivity, ρ. Resistivity is measured in ohm-metres ( Ω m). d) Temperature - but we've covered that in 'current-voltage graphs'. So:. R ∝ l R ∝ l/A R ∝ ρ These can be combined to give: R = ρ l/A Where:. R = resistance (Ω) ρ = resitivity of the material (Ω m) I = length of wire (m) A = cross-sectional area of the wire (m2) Here's one for you to try: A 20 cm long metal wire has a cross-sectional area of 1 mm2. /**/ Conductivity As conductance is the reverse of resistance, so conductivity is the reverse of resistivity. So conductivity, Conductivity is measured in Sm-1