dielectric

Meaning: A dielectric is an electrical insulator whose molecules polarize in an electric field. Induced or permanent dipoles line up partly against the applied field, so the net field between the plates drops to E = E₀/κ. For the same stored charge, the potential difference falls, which means the capacitance rises. No charge flows through the material.

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 and close to 1 for air), ε₀ is the permittivity of free space (C²/(N·m²)), A is plate area (m²), and d is plate separation (m). Stored energy is U = ½CV² = Q²/2C (J).

On the MCAT: A passage inserts a dielectric into a charged capacitor and asks what changes. If the battery stays connected, V is fixed: C increases, Q = CV increases, and U increases. If the capacitor is isolated, Q is fixed: C increases, V = Q/C decreases, E decreases, and U decreases.

Common confusion: Students apply one outcome to every case, such as "inserting a dielectric raises the charge." Charge changes only if a source can supply it. First decide whether V or Q is held constant, then apply C = Q/V. Another slip is treating the dielectric as a conductor, which would let charge cross the gap.