dielectrics

Meaning: A dielectric is an insulator placed between capacitor plates. Its molecules polarize in the field, and the induced surface charge sets up a field opposing the plates' field. The net field falls by a factor κ, so for the same charge the voltage drops and capacitance rises.

Equation: C = κC₀ = κε₀A/d, where C is capacitance with the dielectric (F), C₀ is capacitance without it (F), κ is the dielectric constant (unitless, 1 for vacuum), ε₀ is the permittivity of free space (8.85 × 10⁻¹² F/m), A is plate area (m²), and d is plate separation (m).

On the MCAT: A slab is inserted into a charged capacitor and you predict Q, V, E and stored energy. If the battery stays connected, V is fixed, so Q = CV and U = ½CV² increase. If it is disconnected first, Q is fixed, so V = Q/C, E and U = Q²/2C decrease.

Common confusion: Students think charge crosses the dielectric, or that higher C means more stored energy. The slab is an insulator, and C rises either way; whether Q or V is held constant decides the rest.