A material can be defined as an insulator if
1. This is a poor conductor of electricity,
2. This has high resistance of the order of mega ohms. Insulator is also called as dielectric material,
3. width of forbidden band is extremely large in energy band structure,
4. its σ /ωε is very small where σ is conductivity, ε is permittivity, ω is angular frequency (ω = 2π f).
Several types of insulator are: air, wax, vacuum, paper, sandy earth, pure water, bakelite wood, plastics, , glass, mica, oil, rubber ,porcelain, etc.
Figure: Energy Band Structure for Insulator
Some of the significant applications of insulators are following:
1. They are utilized in capacitors; they are utilized to store electric charge and to store electrostatic energy.
2. Insulators protect from electric shock, and they are also utilized for good propagation of electromagnetic (EM) waves.