A variable capacitor is made of two parallel semicircular plates with air between them. One plate is fixed in place and the other can be rotated. The electric field is zero everywhere except in the region where the plates overlap.
When the plates are directly across from one another, the capacitance is 0.694 pF.
(a) What is the capacitance when the movable plate is rotated so that only one half its area is across from the stationary plate?
(b) What is the capacitance when the movable plate is rotated so that two thirds of its area is across from the stationary plate?

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(a) (b)