Capacitance

Capacitance is the ratio of stored electric charge to voltage difference:

An ideal capacitor stores energy in an electric field. The stored energy is:

In circuits, capacitance resists sudden voltage change because changing capacitor voltage requires current. For an ideal capacitor:

Engineering role

Capacitance is used for filtering, timing, decoupling, energy storage, power-factor correction, coupling, sensing, and electromagnetic compatibility. It is also a frequent parasitic effect in high-speed circuits, power electronics, cables, transformers, switchgear, sensors, and printed circuit boards. A few picofarads can be irrelevant in a low-frequency power circuit but decisive in a radio-frequency or fast digital design.

Frequency behaviour

In sinusoidal steady state, ideal capacitive reactance decreases as frequency rises:

Equivalently, capacitive admittance increases with frequency:

This is why capacitors pass high-frequency components more easily than low-frequency components, and why they are central to low-pass, high-pass, and decoupling networks.

Real capacitors

Real capacitors have voltage rating, tolerance, equivalent series resistance, equivalent series inductance, leakage current, dielectric absorption, temperature coefficient, aging, ripple-current limit, and failure modes. Ceramic, film, electrolytic, tantalum, mica, and supercapacitor technologies behave differently. The nominal capacitance printed on a part may not be the capacitance available under DC bias, temperature, frequency, or aging.

Common mistakes

Common mistakes include treating capacitance as constant outside its specified voltage and temperature range, ignoring parasitic inductance at high frequency, and assuming a capacitor is an ideal open circuit for DC despite leakage. In safety-critical or high-energy systems, stored energy and discharge time must be checked explicitly because a charged capacitor can remain hazardous after power is removed.