Resistivity - r

The resistance of a length of wire is proportional to the length and inversely proportional to the cross sectional area.

r _= R A/l

Resistivity is a property of a material. The unit is the Ohm meter, the resistance between opposite faces of a cube a meter on a side. (The Ohm cm is sometimes quoted and is a more manageable unit).

The difference between the resistivity of silver and expanded polystyrene (or air) is enormous. Resistivity varies over a range of around thirty orders of magnitude!

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