Q 12-03-254JEE MainJEE Main 2019 (10 Apr, Shift 2)Medium
Space between two concentric conducting spheres of radii $a$ and $b$ $(b > a)$ is filled with a medium of resistivity $\rho$. The resistance between the two spheres will be:
Answer: (C) $\dfrac{\rho}{4\pi}\left(\dfrac1a - \dfrac1b\right)$
Current flows radially. A thin shell of radius $r$ and thickness $dr$ has resistance $\dfrac{\rho\,dr}{4\pi r^2}$:
$$R = \int_a^b\frac{\rho\,dr}{4\pi r^2} = \frac{\rho}{4\pi}\left(\frac1a - \frac1b\right)$$
Solution by Sreeraj P, M.Sc Physics