insulator

Meaning: An insulator is a material whose electrons are tightly bound to atoms, so it has very few free charge carriers and passes almost no current. Its resistivity is many orders of magnitude above that of a metal. Excess charge on an insulator stays roughly where it was placed, while on a conductor it spreads over the surface.

Equation: R = ρL/A, where R is resistance (Ω), ρ is resistivity (Ω·m), L is length (m), and A is cross-sectional area (m²). A very large ρ gives a very large R, so the current I = V/R (A, with V in volts) is tiny.

On the MCAT: Questions ask which material makes a safe handle or wire coating, or ask you to predict negligible current. A dielectric between capacitor plates raises capacitance, C = κC₀, where κ is the dimensionless dielectric constant. In biology passages, myelin insulates axons, raising membrane resistance and speeding conduction.

Common confusion: An insulator blocks current, not electric field. A field can exist inside an insulator, whose molecules polarize, whereas inside a conductor at electrostatic equilibrium the field is zero. Insulators are also not perfect: a strong enough field causes breakdown.