physics
posted by Anonymous .
The filament in an incandescent light bulb is made from tungsten. The light bulb is plugged into a 240 V outlet and draws a current of 1.16 A. If the radius of the tungsten wire is 0.0052 mm, how long must the wire be?

You need the restivity of the wire, lets say it is P ohm.m
the cross sectional area (in m^2) of the wire is
A=pi x (5.2x10^6 m)^2
so the resistamce per metre is
P/A (units ohm/m)
if the bulb draws 1.16 A from 240 V then the resistance, R, is
R=240/1.16
so the length of wire is
R/(P/A)=RA/P