Electrostatic Potential and Capacitance

Six capacitors each of capacitance of $$2\mu F$$ are connected as shown in the figure. The effective capacitance between $$A$$ and $$B$$ is:
Can a metal be used as an medium for dielectric?
Some cell walls in the human body have a layer of negative charge on the inside surface and a layer of positive charge of equal magnitude on the outside surface. Suppose that the surface charge densities are $$\pm 0.50 \times 10^{-3} Cm^{-2}$$, the cell wall is $$0.50 \times 10^{-9} m$$ thick and the cell wall material has a dielectric constant of $$k = 5.4$$. A typical cell in human body has volume $$10^{-16} m^3$$. Assume the cell to be spherical.
The parallel-plate air capacitor shown in fig consists of two horizontal conducting plates of equal area A. The bottom plate rests on a fixed support and the top plate is suspended by four springs with spring constant k, positioned at each of the four corners of the plate as shown in Fig. When uncharged, the plates are separated by a distance $$z_0$$. A battery is connected to the plates and produces a potential difference V between them. This causes the plate separation to decrease to z. Neglect any fringing effects.
A $$1\mu F$$ and a $$2\mu F$$ capacitor are connected in series across a $$1200V$$ supply.