9.7 Resistance and resistivity

Describe the conductivity of gold:

• It has a number density almost as high as copper and is much less reactive, so it corrodes very little over time.

• Sometimes used as contacts

What are the factors that affect resisistance?

• Temperature

• Material of the wire

• Length, L, of the wire

• Cross sectional area of the wire

What is the difference between resistance and resistivity?

• Resistance ony refers to a specific component.

• Resistivity is used to describe the electrical property of a material.

• Eg. different components made from copper may have different resistances, but copper itself has its own unique.

What’s the relationship between resistance and length?

• Doubling the length doubles the p.d.

• The resistance of the wire is directly proportional to its length

What’s relationship between resistance and cross-sectional area?

• When the cross-sectional area of the wire increases, the resistance decreases.

• Doubling the cross-sectional area will double the current in the wire, so the resistance must have halved.

• The resistance R of a wire is inversely proportional to its cross sectional area.

How to calculate resistance from resistivity?

R = ρL/A

The unit for resistivity ohm metre (Ω⋅𝑚)

This equation may be used for a known constant temperature. 

What is resistivity?

ρ = RA/L 

Resistivity of a material varies with temperature in the same way as the resistance of most components varies with temperture. As the material gets hotter, its resistivity increases.

What is the resitivity in Conductors, semiconductors, and insulators?

• Good conductors: have a resistivity of the order 10^-8 ohmeteres.

• Insulators have a value of the order of 10^16.

• Semiconductors are between these extremes.