Question

A long solid cylinder (surface 1) of diameter $d_1$ is enclosed by a another long concentric cylinder of inner diameter $d_2$. If $d_2=2d_1$. The percentage radiation from surface 2 to itself is_____%.

• A. 75%
• B. 25%
• C. 0%
• D. 50%

Answer 1

The correct option is D 50%


$F_{11}+F_{12}=1,$

$F_{11}=0\Rightarrow F_{12}=1$

From reciprocity theorem,

$A_1{F}_{12}=A_2{F}_{21}$

$F_{21}=\frac{A_1}{A_2}{F}_{12}$

$=\frac{\pi d_{1}L}{\pi d_{2}L}\left(1\right)=\frac{d_1}{d_2}$

From summation rule,

$F_{21}+F_{22}=1$

$F_{22}=1-F_{21}$

$=1-\frac{d_1}{d_2}=1-\frac{1}{2}=0.5or50\%$