A spherical capacitor is formed by a solid conducting sphere of radius \(a\) concentric with a thin conducting spherical shell of radius \(b\), where \(a < b\). The inner sphere carries charge \(+Q\), and the outer shell carries charge \(-Q\). The region between the conductors is filled with an inhomogeneous dielectric whose dielectric constant depends on the radial distance \(r\) according to \(\kappa(r) = \kappa_0 \dfrac{r}{a}\), where \(\kappa_0\) is a positive dimensionless constant. Which of the following expressions correctly represents the potential difference \(\Delta V = V(a) - V(b)\) between the two conductors?