Thermal Properties Of Matter Ques 1
1. The total radiant energy per unit area, normal to the direction of incidence, received at a distance $R$ from the centre of a star of radius $r$, whose outer surface radiates as a black body at a temperature $T K$ is given by:
[2010]
(a) $\frac{\sigma r^2 T^4}{R^2}$
(b) $\frac{\sigma r^2 T^4}{4 \pi r^2}$
(c) $\frac{\sigma r^4 T^4}{r^4}$
(d) $\frac{4 \pi \sigma r^2 T^4}{R^2}$
(where $\sigma$ is Stefan’s constant)
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Answer:
Correct Answer: 1.(a)
Solution: (a) $ E=\frac{S}{S_0} \sigma T^4=\frac{4 \pi r^2}{4 \pi R^2} \sigma T^4 $
$=\sigma \frac{r^2}{R^2} T^4$
BETA
