Properties of good electrical insulators: Select the characteristic that best defines a good insulating material used between conductors.

Introduction / Context:
Insulating materials prevent unintended current flow and arcing in cables, capacitors, and high-voltage equipment. Their selection affects safety, reliability, and size. Knowing what makes a good insulator helps in material choice and failure analysis (e.g., corona, tracking, puncture).

Given Data / Assumptions:

• We compare generic properties of dielectrics (glass, ceramics, polymers) versus conductors.
• Room-temperature, dry conditions unless otherwise specified.
• Focus on intrinsic material properties, not just geometry.

Concept / Approach:

Two key traits of insulators are low free-carrier density (high resistivity) and large dielectric strength (ability to withstand strong electric fields without breakdown). Among the options, “large dielectric strength” is the most direct and defining quality for insulation performance under high fields.

Step-by-Step Solution:

Verification / Alternative check:

Datasheets for insulators list breakdown field (e.g., PTFE ~ 60–170 kV/mm). High values correspond to excellent insulating capability, validating the chosen characteristic.

Why Other Options Are Wrong:

• Few free charge carriers: generally true, but dielectric strength more directly defines insulation performance under stress; we select the most specific rating attribute.
• Small breakdown voltage: the opposite of desired.
• Many free electrons / high DC conductivity: properties of conductors, not insulators.

Common Pitfalls:

• Equating only high resistivity with insulation; high dielectric strength is also essential for high-voltage applications.
• Ignoring environmental effects (humidity, temperature) that reduce dielectric strength.

Final Answer:

large dielectric strength (high breakdown field)