# Homework Help: Changes in Resistance due to Temperature

1. Apr 7, 2008

### dari09

1. The problem statement, all variables and given/known data

When a metal rod is heated, not only its resistance but also its length and its cross-sectional area change. The relation R = L/A suggests that all three factors should be taken into account in measuring at various temperatures.
(a) If the temperature changes by 5.0 C°, what percentage changes in R, L, and A occur for a copper conductor? The coefficient of linear expansion is 1.7 10-5/K.

2. Relevant equations

delta L = L initial (delta T) (alpha)

R = density L/A

3. The attempt at a solution

I got the answer to the percentage changes of L and A using the equation
L = L initial (delta T) alpha.
I tried to plug those answers into the equation R = density L/A which gives the answer.5
Unfortunately that answer is wrong and and now I am currently lost.

The percent changes of L and A are:
delta L / L initial = .0085%
delta A / A initial = .017%

2. Apr 7, 2008

### Mapes

Hi dari09, welcome to PF. Unfortunately, $\rho$ is used to mean multiple things in engineering, including density and (in this case) resistivity, which is a material property with units $\Omega\cdot m$. You can see how that correctly makes ohms the unit of resistance.

You need to find the resistivity $\rho$ of copper at the two different temperatures.