Question

A parallel plate air capacitor has a capacitance 18μF. If the distance between the plates is tripled and a dielectric medium is introduced, the capacitance becomes 72μF. The dielectric constant of the medium is 

(A) 4 

(B) 12 

(C) 9 

(D) 2

Answer 1

The correct option (B) 12

Explanation:

 c = (A∈₀/d) = 18

c' = [(A∈₀k)/d'] = 72

∴ (c¹/c) = (kd/d')

∴ (72/18) = [(k ∙ d)/(3d)] ------ (d' = 3d)

 ∴ 4 = (k/3)

 k = 12