An air capacitor is made by using two flat plates, each with area A, separated by a distance d. Then a metal slab has thickness a (less than d) and the same shape and size as the plates are inserted between them, parallel to the plates and not touching either plate (Fig).
(a) What is the capacitance of this arrangement?
(b) Express the capacitance as a multiple of the capacitance C0 when the metal slab is not present.
c) Discuss what happens to the capacitance in the limits a → 0 and a → d.

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