Posted by Anonymous on Wednesday, January 19, 2011 at 11:01am.
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?

physics  Dr Russ, Wednesday, January 19, 2011 at 11:17am
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