
Resistivity and Conductivity
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Resistivity and Conductivity
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 = ρl/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.
As conductance is the reverse of resistance, so conductivity is the reverse of resistivity.
So conductivity,
Conductivity is measured in Sm-1