The quantity $\frac{1}{2}\varepsilon_0 E^2$ represents the energy density of an electric field, which is energy stored per unit volume.
Dimension of Energy $[E] = ML^2T^{-2}$. Dimension of Volume $[V] = L^3$.
Thus, the dimension of energy density is $\frac{[E]}{[V]} = \frac{ML^2T^{-2}}{L^3} = ML^{-1}T^{-2}$.
Therefore, the dimension of $\frac{1}{2}\varepsilon_0 E^2$ is $ML^{-1}T^{-2}$.

Two resistors $2\,\Omega$ and $3\,\Omega$ are connected in the gaps of a bridge as shown in the figure. The null point is obtained with the contact of jockey at some point on wire $XY$. When an unknown resistor is connected in parallel with $3\,\Omega$ resistor, the null point is shifted by $22.5\,\text{cm}$ towards $Y$. The resistance of unknown resistor is ___ $\Omega$. 

