# Aluminium vs Copper in power lines

Hi,

I have a question concerning the use of aluminium in power lines. I know that it's lighter than copper and thus cheaper, which is the main reason we prefer it above copper. But in fact alumunium has a lower conductivity than copper. Why is it then that one kg of aluminium can carry twice as much electricity as one kg of copper ? This seems to be contradictory to me.

Thank you :)

collinsmark
Homework Helper
Gold Member
Here's some information that should be useful in the process of considering the question objectively:

Resistivity, $\rho_e, \mathrm{(\Omega \cdot m)}$:
Copper: $1.68 \times 10^{-8}$
Aluminum: $2.86 \times 10^{-8}$​

Conductivity, $\sigma, \mathrm{(S / m)}$:
Copper: $5.96 \times 10^{7}$
Aluminum: $3.50 \times 10^{7}$​

Density $\rho_m, \mathrm{(g/cm^3)}$ [Edit: corrected copy-paste error and a typo.]
Copper: 8.96
Aluminum: 2.70
By the way, resistivity and conductivity are not independent. They are merely reciprocals. $\rho_e = 1/\sigma$. Both resisitivity and conductivity are given in terms of spacial dimensions, such as unit cross-sectional area and unit length.

If $A$ is the cross sectional area of the conductor, $\ell$ is the conductor's length, then

Resistance, $r = \frac{\rho_e \ell}{A}$

Conductance, $G = \frac{1}{r} = \frac{\sigma A}{\ell}$

Mass, $m = A \ell \rho_m$

Last edited:
rbelli1
Gold Member
Density ρm,(g/cm2) \rho_m, (g/cm^2)
Copper: 2.70
Aluminum: 8.96

You got those swapped. Copper is the higher density material.

BoB

• collinsmark
Nidum
Gold Member
Think about two wires of equal length . One made from 1 kg of Copper and the other from 1 kg of Aluminium .

collinsmark
Homework Helper
Gold Member
And the price of metals depends on many factors besides just their densities (their weights). Lead is not cheaper than silver because it is lighter (it isn't: lead is heavier than silver); it's cheaper because it is more abundant and easier to extract.

collinsmark
Homework Helper
Gold Member
You got those swapped. Copper is the higher density material.

BoB
Thanks. It was a copy-paste error. Correction made.