Question

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?

Answer 1

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