Lag1989 problem number 14.32

The potential difference between the plates of a spherical air condenser U. The radii of the plates are R₁, R₂, and R₁ <R₂. Find the field energy of this capacitor and prove that it is equal to 0.5C * U², where C is the capacitance of the capacitor, if the expression for the current energy density is known. ₂