When the tungsten filament in a 60 W, 120 V light bulb reaches 1950° C, 60 W of

ID: 2002616 • Letter: W

Question

When the tungsten filament in a 60 W, 120 V light bulb reaches 1950° C, 60 W of electric power is used to produce light and heat.

(a) Find the current through the filament under these conditions, which are established very shortly after the light is turned on.

I got this one correct! i=p/v =0.5 A

(b) Find the filament current when the light is first turned on, assuming an initial filament temperature of 20° C.

(d) Estimate the time needed to heat the filament to 1950° C. The filament has a mass of 0.038 g and a specific heat of 0.032 cal/g-C°.

What you got for answer of a) is incorrect it’s answer for b).

Detainled solution as below,

b) I = P/V = 60/120 = 0.5A

a) Initial resistance at 20 C = R = V/I = 120/0.5 = 240 ohm

Final resistance at 1950 C = Rf = Ri ( 1+tungsten*T) = 240*(1+0.0045*(1950-20)) = 2324.4 ohm

If = V/Rf = 120/2324.4 = 0.0516 A

c) Q= Wt = mc T

t= mcT/W = [0.038*0.032*(1950-20)]/60 = 0.039s = 0.04s

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