# 5.2.1 Resistivity

### Resistivity

• All materials have some resistance to the flow of charge
• As free electrons move through a metal wire, they collide with ions which get in their way
• As a result, they transfer some, or all, of their kinetic energy on collision, which causes electrical heating

Free electrons collide with ions which resist their flow

• Since current is the flow of charge, the ions resisting their flow causes resistance
• Resistance depends on the length of the wire, the cross-sectional area through which the current is passing and the resistivity of the material

Electrical resistance equation

• The resistivity equation shows that:
• The longer the wire, the greater its resistance
• The thicker the wire, the smaller its resistance

The length and width of the wire affect its resistance

• Resistivity is a property that describes the extent to which a material opposes the flow of electric current through it
• It is a property of the material, and is dependent on temperature
• Resistivity is measured in Ω m

Resistivity of some materials at room temperature

• The higher the resistivity of a material, the higher its resistance
• This is why copper, with its relatively low resistivity at room temperature, is used for electrical wires — current flows through it very easily
• Insulators have such a high resistivity that virtually no current will flow through them

#### Worked Example

Two electrically-conducting cylinders made from copper and aluminium respectively.
Their dimensions are shown below.

Copper resistivity = 1.7 × 10-8 Ω m
Aluminium resistivity = 2.6 × 10-8 Ω m

Which cylinder is the better conductor?

#### Exam Tip

• You won’t need to memorise the value of the resistivity of any material, these will be given in the exam question.
• Remember if the cross-sectional area is a circle e.g. in a wire, it is proportional to the diameter squared. This means if the diameter doubles, the area quadruples causing the resistance to drop by a quarter.

### Author: Ashika

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.
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