farad

Meaning: Capacitance measures how much charge a capacitor holds for each volt across its plates. One farad is one coulomb per volt. The farad is very large, so real capacitors are usually rated in microfarads, nanofarads or picofarads.

Equation: C = Q/V, where C is capacitance in farads (F), Q is the magnitude of charge on one plate in coulombs (C), and V is the potential difference across the plates in volts (V).

On the MCAT: Passages usually use a parallel-plate capacitor, where C = κε₀A/d. Here κ is the dielectric constant (unitless), ε₀ is the permittivity of free space (F/m), A is plate area (m²), and d is plate separation (m). You predict how C changes when the plates are moved apart, enlarged or filled with a dielectric. Stored energy is U = ½CV² = ½QV, in joules. Expect questions on capacitors in parallel (C adds) or in series (1/C adds), and on whether the battery stays connected (V constant) or is removed (Q constant).

Common confusion: Students treat the farad as a quantity of charge, or think C changes when Q or V changes. For a given capacitor, Q and V rise in proportion and C stays fixed. Only area, separation or dielectric changes C. Do not mix it up with the Faraday constant, about 96,500 C/mol, which is charge per mole of electrons.