# Homework Help: Resistivity of a wire

1. Jul 7, 2009

### stardust289

A carbon wire and a Nichrome wire are connected one after the other. If the combination has total resistance of 10.0 k ohm at 20°C, what is the resistance of each wire at 20°C so that the resistance of the combination does not change with temperature?(resistivity at 20°C of: nichrome= 1.5x10^-6 ohms m, Carbon= 3.5x 10^-8 ohms m, Temp. coef of resistivity: nichrome= .4x 10^-3/°C, carbon:-.5x10^-3/°C)

that's the question and here is what i did...

[1.5x 10^-6ohms(1+.4x10^-3)0]+[3.5x10^-8ohms(1+-.5x10-3)0]= 10 ohms
i was given 4 points out of 20. could someone explain the correct answer to me...

2. Jul 7, 2009

### turin

I have no idea what you were trying to do with your calculation.

You do not need resistivity; only resistance. The temperature coefficients tell you the percentage change in resistance for a change by one celcius degree. You can write an equation that equates the resistance of the combo at two different temperature values. The basic idea is that you want the resistance in one material to change in the exact opposite way as the other, so that the change is compensated (cancelled).

3. Jul 7, 2009

### stardust289

thanks for responding! so then

10ohms=N(1.5x10^-6) + -C(3.5x10^-8) right?

how do i find the resistance if i dont know the lenghth?

4. Jul 7, 2009

### turin

I don't know what your equation means. Try to explain your logic in words.

5. Aug 15, 2011

### solas99

Given the following data on copper, how do i calculate the resistivity?

Relaxation time: 2.50e10-14s
Density: 8940Kgm-3
molar mass: 63.5g

is there an equation for it.