Question

what makes an object an insulator

Answer 1

Short answer
- An object is an electrical insulator when it strongly resists the flow of electric charge (very few free charge carriers).
- An object is a thermal insulator when it poorly conducts heat (low thermal conductivity, often by blocking phonon/electron transport or trapping air).

Why electrically insulating materials behave that way
- Electron structure: in solids an insulator has a large energy band gap (typically several eV) between the valence band and conduction band, so electrons cannot move freely under ordinary electric fields.
- Lack of charge carriers: there are very few mobile electrons or ions to carry current, so the material’s electrical resistivity is very high.
- Dielectric behavior: under an AC field the material polarizes (bound charges shift slightly) but does not conduct; its dielectric constant and loss tangent describe that response.
- Practical limits: extremely strong fields, defects, moisture, impurities or high temperature can produce leakage or dielectric breakdown and make the material conduct.

Why thermal insulators behave that way
- Low thermal conductivity: heat is transported mainly by phonons (lattice vibrations) and, in metals, free electrons. Materials that block phonon propagation (disordered structure, pores, many interfaces) and lack free electrons have low thermal conductivity.
- Trapped gas/air: many good thermal insulators (foams, fibreglass, aerogel) work by trapping air in small pockets — air is a poor conductor, and the pockets reduce convective transfer.
- Structure and scattering: microstructure that scatters phonons (porous, fibrous, layered) lowers heat transport.

Examples
- Electrical insulators: rubber, glass, most plastics, ceramics.
- Thermal insulators: expanded polystyrene (Styrofoam), mineral wool, aerogel, air, vacuum (vacuum is an excellent thermal insulator for conduction and convection; radiation can still transfer heat).

Measurable properties and practical notes
- Electrical: high resistivity (very low conductivity), large band gap, specified dielectric strength (voltage per thickness before breakdown), dielectric constant and loss.
- Thermal: low thermal conductivity (W/m·K), often low density and porous.
- No perfect insulator: every real material leaks a little and can break down under high fields or extreme conditions.

If you want, I can explain just electrical insulation (band diagrams, conduction mechanisms) or thermal insulation (heat-transfer mechanisms and examples) in more detail. Which do you mean?