A spherical condenser has 10 cm and 12 cm as the radii of inner and outer spheres. The space between the two is filled with a dielectric of dielectric constant 3. Find the capacity when the outer sphere is earthed.

4 × 10−10 F 1 × 10−10 F 2 × 10−10 F 6 × 10−10 F

2 × 10−10 F

C = $4 \pi k \varepsilon_0\left(\frac{ab}{b-a}\right)=\frac{3}{9 \times 10^9} \times \frac{0.1 \times 0.12}{0.12-0.1}$ = 2 × 10−10 F