Aparallel-plate capacitor having circular plates of radius r and separation d is held at a fixed potential difference by a battery. if the plates are moved closer together while they are held at the same potential difference
a. the amount of charge on each of them will increase.
b. the amount of charge on each of them will decrease.
c. the amount of charge on each of them will stay the same.
d. the energy stored in the capacitor increases.

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explanation:

speed of light in vacuum is 300,000 km/s

speed of light in other mediums is given by the expression v = c/n, where c is the speed of light in vacuum and n is the refractive index of medium.

for the first medium v = 200,000 km/s

  so, refractive index of medium = 300000/200000 = 1.5

    1.5 refractive index is usually for glass. so first medium is glass.

for the second medium v = 250,00 km/s

so, refractive index of medium = 300000/250000 = 1.2

    1.2 refractive index is usually for water. so second medium is water.

for the third medium v = 0 km/s

so, refractive index of medium = 300000/0 = ∞

    ∞ refractive index is usually for opaque mediums. so third medium is opaque medium.