Sparks will occur between the plates of an air-filled capacitor at lower voltage when the air is humid than when dry. Explain why, considering the polar character of water molecules.
Moist air contains water. Water is a polar molecule and gets polarized when it comes between the plates of capacitor; a dielectric is formed due to polarization.
The Coulomb force between charge on the plates and the water molecules is attractive and also very strong, as they are very near to each other, this accumulates more charge onto the plates hence creates spark.