Calculating Resistance of a Light Bulb Filament

[chris85]

Homework Statement

the filament of a 230V light bulb is 0.72M long and has a radius of 6.0 x 10^-2mm. The resistivity of the filamen metal is 1.2 x 10^-5 ohm metres. Work out the resistance of the bulb

Homework Equations

I have tried working out the area using pi x radius square but am having trouble converting 6.0x10^-2mm into metre square.

i am using R=pL/A so i did 1.2x10^-5 x 0.72/1.1 x 10^-8m^2 which equals 785 ohms but the answer is 760 ohms can anyone please help me out?

The Attempt at a Solution

 

[chris85]

can anybody help?

 

[learningphysics]

I think the problem is with your rounding...

I get 763.94ohms... which when taken as 2 sig figs is 760ohms.

convert 6.0*10^-2mm to m first

6.0*10^-2mm * 1m/(1000mm) = 6.0*10^-5m

area is pi*(6.0*10^-5m)^2 = 1.13097*10^-8 m^2

 

[chris85]

hmmm thanks when i enter the pi x (6.0x10^-5)^2 i keep getting 1.1 x 10^-8 i don't get why

 

[chris85]

could it be a problem with my calculator??

 

[learningphysics]

could it be a problem with my calculator??

set your calculator to scientific notation. then you should see the other digits.