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Q.

The operating temperature of a tungsten filament in an incandescent lamp is 2450 K and its emissivity is 0.30. The surface area of the filament of a 25 watt lamp is : (Take : σ=5.67×10−8 S.I. unit)

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a

0.3  cm2

b

0.4 cm2

c

0.2  cm2

d

0.1  cm2

answer is D.

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Detailed Solution

p=25wp=∈σAT4250.3×(2450)4×σ=AA=250.3×(2450)4×5.67×10−8=0.41cm2

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