Question

Which of the following is the value of characteristic length L which appears in Biot number for a sphere of radius R?

• A. $\frac{R}{3}$
• B. $\frac{R}{2}$
• C. $\frac{R}{\pi }$
• D. $\frac{2R}{\pi }$

Answer 1

The correct option is A $\frac{R}{3}$

Characteristic length,

$l=\frac{\text{Volume of solid}}{\text{Surface area}}$

$\Rightarrow l=\frac{4/3\pi R^3}{4\pi R^2}$

$\Rightarrow l=\frac{R}{3}$