# Resistance of a copper wire

#### Archimedess

Homework Statement
A copper wire has an internal diameter $d_1$ of $10$ $cm$ and an external diameter $d_2$ of 20 $cm$. If the wire is $20m$ long what is the resistance of the wire?
Homework Equations
$R=\rho\frac{l}{\Sigma}$
What i did is: $R=\rho\frac{l}{\Sigma}=\rho\frac{l}{d_2^2\pi-d_1^2\pi}$

The problem is that I don't have $\rho$. Is there a way to find $R$ without knowing it?

Many thanks.

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#### willem2

The problem is that I don't have ρρ\rho. Is there a way to find RRR without knowing it?
If you know the material, you can look it up

#### Archimedess

If you know the material, you can look it up
This is from a past exam of my course (we can't use any table during exam), a friend of mine says that you don't need it and you can find it anyway.. I doubt it.

#### Orodruin

Staff Emeritus
Homework Helper
Gold Member
2018 Award
If you know the material, you can look it up
Indeed. In real life, problems do not come with all the input data that is required to solve them. In fact, half the work of problem solving (if not more) is identifying the input needed and looking for a way of obtaining this information. In the case of finding the resistivity of Cu, a quick Google search should suffice if you do not have a table of material properties readily at hand (the Google search may be quicker anyway).

#### Orodruin

Staff Emeritus
Homework Helper
Gold Member
2018 Award
This is from a past exam of my course (we can't use any table during exam), a friend of mine says that you don't need it and you can find it anyway.. I doubt it.
You can easily show that quantities of dimension length are not sufficient to determine resistance by using dimensional analysis. Using lengths only it is impossible to build a quantity that has the correct physical dimension.

"Resistance of a copper wire"

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