Highest dielectric strength among common insulating materials Among the following materials, which typically exhibits the highest dielectric strength (breakdown field): mica, air, cotton, or rubber?

Introduction / Context:
Dielectric strength indicates how much electric field a material can withstand before electrical breakdown. It is a key selection parameter for capacitors, slot liners, and high-voltage insulation. Different materials span orders of magnitude in breakdown field, influenced by purity, microstructure, and moisture content.

Given Data / Assumptions:

• Representative values at room temperature, dry conditions, and standard test thicknesses.
• Comparative, order-of-magnitude reasoning; exact values vary by grade and test method.

Concept / Approach:

Mica is a laminated, layered silicate with very high dielectric strength, typically on the order of 100–200 kV/mm (or higher for special grades). Rubber (and many polymers) are strong insulators but usually in the 20–40 kV/mm range. Dry air breaks down near 3 kV/mm under uniform fields. Cotton, being a fibrous organic material that readily absorbs moisture, has low and inconsistent dielectric strength. Therefore, among the listed choices, mica clearly provides the highest breakdown field and is widely used as a premium insulation and capacitor dielectric.

Step-by-Step Solution:

Verification / Alternative check:

Capacitor industries employ mica sheets in high-voltage, high-reliability applications, confirming its superior breakdown properties compared to common polymers and air gaps.

Why Other Options Are Wrong:

Air and cotton are weak dielectrics; rubber, while decent, does not match mica’s breakdown field.

Common Pitfalls:

Overlooking moisture effects (cotton and even rubber can degrade substantially when wet). Test geometry and defects also lower apparent dielectric strength.

Final Answer:

Mica