A spherical capacitor consists of an inner solid conducting sphere of radius \(a\) carrying charge \(+Q\) and an outer concentric thin conducting spherical shell of radius \(b\) carrying charge \(-Q\). The space between the conductors is evacuated. Which of the following expressions gives the total electric energy stored in the electric field within the concentric region from \(r' = a\) to \(r' = r\), where \(a < r < b\)?