Physics question

[Mo0o]

The filament of a light bulb has a resistance of 12ohms at 20C and 140ohms when hot. Assume the filament is made of tungston and under goes thermal expansion.
Coefficient for resistivity change is .0045 , thermal expansion coefficient is 5e-6

b) In this temperature range what is the percentage change in resistance due to thermal expansion and what is the percentage change in resistance due soley to change in rho.


I've already determined the change in temperature (part a that I didn't write) to be 2401 degrees which means the final temperature is 2.42e3 C. So my question is, what do they mean by percentage change. What I did was simply calculate the final resistance if I was only factoring in thermal expansion or change in rho and divided those values by 12, my orignal resistasnce, to find the percentage change. But then there's also another way in which i divide the contributions of thermal and resistivity change by the total resistance change, 120. WHich method is correct?

 

[drinkmorejava]

calc res resistance due to thermal expansion, calc resistance due to change in rho, if you add them together, you should get 128ohms. then just take res change from thermal expansion and divide it by 128ohms. I only can see one way of doing it.

 

[Mo0o]

Originally posted by: drinkmorejava
calc res resistance due to thermal expansion, calc resistance due to change in rho, if you add them together, you should get 128ohms. then just take res change from thermal expansion and divide it by 128ohms. I only can see one way of doing it.

ah ok.

But so do u take the abs of the values? Since the thermal expansion lowers the resistance

 

[drinkmorejava]

in that case, yes use abs. Haven't figured it out, but if therm expansion is like -10ohms and rho is 138, you would do 10/(10+138).

 

[Mo0o]

Originally posted by: drinkmorejava
in that case, yes use abs. Haven't figured it out, but if therm expansion is like -10ohms and rho is 138, you would do 10/(10+138).

ahhh, got it, thanks