Answer
$84.5\ pF$
Work Step by Step
Capacitance of spherical capacitor is given by,
$C = 4\pi \epsilon_o \frac{ab}{b-a}$
From the given data, we know that,
radii of capacitors $a = 38 mm = 0.038 m$ and $b= 40 mm = 0.04 m$
Substituting these values in the formula and solving:
$C = 4\pi \epsilon_o \frac{ab}{b-a}$
$C =4\pi(8.85\times10^{12}\ c^2/Nm^2)\ (\frac{(0.038 m)(0.04 m)}{(0.04m-0.038m)}) $
$C=84.5\times 10^{-12} F$
$C= 84.5\ pF$
