A capacitor has a voltage of applied across its plates, and then the voltage source is removed. What is the potential difference across its plates if the space between them is then filled with mica, having a dielectric constant of 5?

V₁/5

Explanation:

The charge stored in the capacitor of capacitance, C and voltage V is Q = C₁V₁. If the voltage is removed and mica with dielectric constant k = 5 is inserted, the charge stored Q = C₂V₂. Since the charge stored is the same,

C₁V₁ = C₂V₂ ⇒ V₂ = C₁V₁/C₂. Now C₂ = kC₁ ⇒ C₁/C₂ = 1/k = 1/5

So, V₂ = C₁V₁/C₂ = V₁/5