Question

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.

Short answer

Water molecules have a net dipole moment, thus they are easily polarized and get separated at lower voltage as compared to air. Because air is a better insulator, a higher voltage is required to conduct the charge.

Step-by-step solution

Definition of Air-filled capacitor

The capacitance of a capacitor will increase proportionally with the increase in the area of the plates and decreases proportionally with the increase in distance between the plates. Capacitance also depends on the material filled between the plates. If air is filled between the plates, it is called an Air-filled capacitor.

Cause of considering the polar character of water molecules

In the air, if the humidity level increase, then the concentration of water vapour will increase also.

Water molecules are easily polarized when an external field is applied to them because they act as dipoles.

If the field is sufficiently powerful, the dipoles will break loose, allowing the charge from one plate to move to another, resulting in a spark.

Air is a better insulator than water, thus breaking its insulation would require a larger voltage.