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