# Change in Resistance due to Thermal Expansion

1. Apr 14, 2010

### munchy35

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

The filament of a lightbulb has a resistance of R0=12 Omegas at 20 degrees Celsius and 140 Omegas when hot.

a) Calculate the temperature of the filament when it is hot, and take into account the change in length and area of the filament due to thermal expansion (assume tungsten for which the thermal expansion coefficient is approx 5.5 * 10^ - 6 degrees Celsius^-1.

Assuming an average temperature coefficient of resistivity alpha = 0.0045 degrees celsius^-1.

b.) In this temperature range, what is the percentage change in resistance due to thermal expansion?

c.) What is the percentage change in resistance due solely to the change in rho(p)?

2. Relevant equations

rho(T)=rho(0)(1+alpha(T-T(0))

part b. ) R(expansion) - R(0)/R(0) = %

part c.) R(change in rho) - R(0)/R(0) = %

3. The attempt at a solution

Well part a, I figured out...it's 2400 degrees Celsius.

I just don't know how to do the other parts. Any direction.equations would be helpful. =/

2. Apr 14, 2010

### munchy35

nevermind. figured it out!

3. Apr 14, 2010

### knobles8179

i have the same problem, how did you do parts b and c?