Resistivity

The resistance of a wire depends on three factors:

• the length; double the length, the resistance doubles.

• the area; double the area, the resistance halves.

• the material that the wire is made of.

Resistivity is a property of the material.  It is defined as the resistance of a wire of the material of unit area and unit length.

The formula for resistivity is:

In physics code we write this as r = AR/l

  There are three bear traps

• The unit for resistivity is ohm metre (Wm), NOT ohms per metre. 

• Notice too that the physics code r (rho, a greek letter 'r') is the same as that for density.  Resistivity has NOTHING to do with density.

• The area is in square metres.  Real wires have areas in square millimetres; 1 mm² = 1 x 10^-6 m²