equivalent capacitance

Meaning: Equivalent capacitance (Ceq) is the capacitance of one capacitor that could replace a combination of capacitors and draw the same total charge from the source at the same applied voltage. In parallel, every capacitor has the same voltage and the charges add, like a larger plate area. In series, every capacitor holds the same charge and the voltages add, like a larger plate separation, so Ceq is smaller than the smallest individual capacitance.

Equation: Parallel: Ceq = C₁ + C₂ + C₃. Series: 1/Ceq = 1/C₁ + 1/C₂ + 1/C₃. Here Ceq and each C are capacitances in farads (F). Also Ceq = Q/V, where Q is the charge in coulombs (C) and V is the voltage in volts (V).

On the MCAT: A passage shows a battery with several capacitors in a mixed network. Reduce it step by step to Ceq, then find the total charge from Q = CeqV and the voltage across each part. Energy stored is U = ½CeqV², in joules (J). Questions also ask for a direction: adding a capacitor in parallel raises Ceq, and adding one in series lowers it.

Common confusion: The rules are reversed from resistors. Capacitors in parallel add directly, whereas resistors in series add directly. Check your answer: a series Ceq must be smaller than the smallest capacitor, and a parallel Ceq must be larger than the largest. Also, series capacitors share the same charge, not the same voltage.