Air is often considered a good insulator, and indeed, dry air has a very high resistivity of approximately 3 x 1013 Ω·m. However, it’s important to understand that ‘perfect’ electrical insulators do not exist. While air can resist electric current to a large extent, it can still conduct electricity under certain conditions.
When voltage between two conductors in air becomes too high, the air can become ionized, allowing it to conduct electricity. This process is known as dielectric breakdown and occurs when the electric field strength exceeds a certain threshold (approximately 3 x 106 V/m for air). When this happens, air loses its insulating properties temporarily.
Your question also mentions a capacitor with square plates that are 14 cm on each side and separated by 2.0 mm. While this specific capacitor might be designed to take advantage of air’s high resistivity, the physical separation between the plates and the amount of voltage applied will ultimately determine how effectively it can act as an insulator. If the voltage reaches the breakdown strength of air, the capacitor can fail as an insulator, leading to current flow between the plates.
In summary, while air has a high resistivity and serves as an effective insulator in many applications, it is not perfect due to its potential to break down under high electric fields, allowing for the possibility of electrical conduction.