Comparative dielectric strength of common insulating media Among the following, which typically exhibits the lowest dielectric strength under comparable conditions of thickness and environment?

Introduction / Context:
Dielectric strength indicates the maximum electric field a material can withstand without breakdown. Environmental factors like moisture dramatically affect gaseous and fibrous insulators. This question emphasizes how humidity changes breakdown behavior.

Given Data / Assumptions:

• Comparison is qualitative but based on typical published ranges.
• Materials considered: air (dry vs. humid), paper, cotton, and mica.
• Similar geometry and test conditions are assumed for a fair comparison.

Concept / Approach:
Moisture reduces dielectric strength by providing ionization paths and increasing conductivity. Wet air breaks down at lower fields than dry air. Solid dielectrics like paper and mica generally exhibit higher breakdown strengths when dry, with mica among the best common dielectrics.

Step-by-Step Solution:
Consider gaseous media: wet air < dry air in dielectric strength due to increased ionization and conductive pathways.Consider solids: paper and cotton (dry) have higher strengths than wet air under standard conditions; mica is higher still.Hence, the lowest among the options is wet air.

Verification / Alternative check:
Reference tables show dry air breakdown around 3 kV/mm (at STP), decreasing noticeably with humidity. Paper varies roughly 6–10 kV/mm when dry, and mica can exceed 100 kV/mm depending on grade.

Why Other Options Are Wrong:

• Dry air: higher than wet air.
• Cotton (dry) and paper (dry): fibrous insulators with greater strength than moist air under similar conditions.
• Mica: among the highest strengths, not the lowest.

Common Pitfalls:

• Comparing per-thickness values without noting moisture and environmental conditions.
• Assuming all gases behave similarly regardless of humidity.

Final Answer:
Wet air