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The filament in an incandescent light bulb is made from tungsten (resistivity 5.6 x 10-8 Ù·m). The light bulb is plugged into a 120-V outlet and draws a current of 2.36 A. If the radius of the tungsten wire is 0.00464 mm, how long must the wire be?

restivity of the wire = 5.6 x 10-8 Ù·m

the cross sectional area (in m^2) of the wire is

A=pi x (5.6 x 10-8)^2 =2304
so the resistamce per metre is

P/A (units ohm/m)

if the bulb draws 2.36A from 120 V then the resistance, R, is

R=120/2.36
= 50.84745766

so the length of wire is

R/(P/A)=RA/P

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2 answers
  1. am I on the right path, sorry 5 hrs of this is getting to me lol

    restivity of the wire = 5.6 x 10-8 Ù·m

    the cross sectional area (in m^2) of the wire is

    A=pi x (0.00464)^2 = 0.0000215296
    so the resistamce per metre is

    P/A (units ohm/m)

    if the bulb draws 2.36A from 120 V then the resistance, R, is

    R=120/2.36
    = 50.84745766

    so the length of wire is

    50.84745766/(0.000000056)=50.84745766*0.0000215296/P
    907990315.357=0.00109472542/P
    =

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  2. R=pL/A
    R is resistance
    P is resistivity
    L is length
    A is area

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