= Capacitor
{wiki}
A <capacitor> stores <electrostatic energy> by separating <electric charge> between two <electrical conductors>. In the ideal two-terminal model their <electric charges> are $+Q$ and $-Q$, with a vacuum or insulating gap preventing direct transfer between them. Write $V=\phi_+-\phi_-$ for the terminal <electric potential difference>, where $\phi_+$ and $\phi_-$ are the <electric potentials> of the two <electrical conductors>. A linear <capacitor> obeys
$$
Q=CV,
$$
where its <capacitance> $C$ depends on the geometry and <permittivity>, and is independent of the applied <electric potential difference> while these stay fixed. https://openstax.org/books/university-physics-volume-2/pages/8-1-capacitors-and-capacitance[OpenStax University Physics 2, §8.1: capacitors and capacitance]{ref}.
For example, the <parallel-plate capacitor> below has <capacitance> $C=88.54\,\mathrm{pF}$. At terminal <electric potential difference> $V=10\,\mathrm V$, the <electric charge> magnitude on either plate is
$$
Q=CV=(8.854\times10^{-11})(10)\,\mathrm C
=8.854\times10^{-10}\,\mathrm C=0.8854\,\mathrm{nC}.
$$
The total <electric charge> of the device is $Q+(-Q)=0$: <charge neutrality> does not imply zero <capacitor energy>. This <capacitor> stores $4.427\,\mathrm{nJ}$ in its <electric field>. The schematic in <parallel-plate capacitor> shows the separation of <electric charge>; the plots in <capacitor energy> show how storage grows with terminal <electric potential difference>.
The useful distinction is between storing net <electric charge> and storing separated <electric charge>. A <capacitor> can release <electrostatic energy> into an <electrical circuit>; in an ideal <RLC circuit>, <energy> moves between the <capacitor>'s <electric field> and the <magnetic field> associated with <inductance>. An ideal disconnected <capacitor> retains its separated <electric charges> and <capacitor energy> because there is no path for discharge.
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