Inserting a dielectric with dielectric constant κ between the plates of a capacitor affects its capacitance by:

• A. C' = κ C0
• B. C' = C0 / κ
• C. C' = κ^2 C0
• D. C' = C0

Answer: (A)

Introducing a dielectric with dielectric constant κ between the plates increases the capacitor’s ability to store charge for a given voltage. For a parallel-plate capacitor, the capacitance becomes C = ε0 κ A / d, while the original (vacuum) capacitance is C0 = ε0 A / d. Therefore the new capacitance is κ times the original: C' = κ C0. This happens because the dielectric reduces the electric field between the plates for the same charge, thanks to polarization, so a larger charge is needed to reach the same voltage, i.e., more charge is stored per unit voltage.