An air spaced parallel-plate capacitor has square plates of side l separated by a distance t. Write down an expression for its capacitance C. A square block of dielectric of side l, thickness t and relative permittivity εr is now inserted so as to completely fill the space between the plates. Calculate the change in the stored energy of the system:(i) If the plates have a constant charge Q,(ii) If a constant potential difference V is maintained between the plates by means of a battery. With the battery still connected between the plates, the block of dielectric is withdrawn in a direction parallel to one side of the plates until only a length x remains between the plates (see figure). Find the magnitude and direction of the force which acts on the block when it is in this position. (Ignore edge effects throughout this problem.)