When the tungsten filament in a 100 W, 120 V light bulb reaches 2050° C, 100 W o

Question

When the tungsten filament in a 100 W, 120 V light bulb reaches 2050° C, 100 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.




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




Find the power used when the light is first turned on, assuming an initial filament temperature of 20° C.


(c) Estimate the time needed to heat the filament to 2050° C. The filament has a mass of 0.039 g and a specific heat of 0.032 cal/g-C°.

Explanation / Answer

(a) power = v*i
100 = 120*i
i = 5/6 = 0.833 A

(b) i = P/V = 0.833 A

(c) P*t = mst

100*t/4.18 cal = 0.039*0.032*(2050-20)

t = 0.106 seconds = 106 ms

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