= Capacitance
{title2=$C$}
{wiki}
The <capacitance> of a linear two-terminal <capacitor> is the constant relating its separated <electric charge> to its terminal <electric potential difference>:
$$
Q=CV,\qquad C=\frac QV\quad(V\ne0).
$$
Here $Q$ is the <electric charge> on one <electrical conductor>, the other carries $-Q$, and $V$ is their <electric potential difference> with the same ordering. Reversing the terminal ordering reverses both $Q$ and $V$ and leaves <capacitance> unchanged. An ideal passive <capacitor> has $C\ge0$; an ordinary pair of separated <electrical conductors> with positive <permittivity> has $C>0$. At $V=0$, use $Q=CV$ rather than the undefined ratio $0/0$.
For example, <electric charge> magnitude $Q=0.8854\,\mathrm{nC}$ at terminal <electric potential difference> $V=10\,\mathrm V$ gives <capacitance> $C=88.54\,\mathrm{pF}$. At $20\,\mathrm V$, the same linear <capacitor> has $Q=1.7708\,\mathrm{nC}$ and the same <capacitance>. The plots in <capacitor energy> show that <capacitance> is the <slope> of the <electric charge> versus terminal <electric potential difference> graph.
For fixed geometry and linear material response, <capacitance> depends on dimensions and <permittivity>, rather than the source's terminal <electric potential difference>. This is why <capacitance> is a useful property of a component: once it is known, $Q=CV$ predicts the separated <electric charge> for each operating terminal <electric potential difference>. The <SI> unit is the <farad>, $1\,\mathrm F=1\,\mathrm{C/V}$. https://openstax.org/books/university-physics-volume-2/pages/8-1-capacitors-and-capacitance[OpenStax University Physics 2, §8.1: capacitance and its unit]{ref}.
For combinations of <capacitors>, the same terminal definition gives an equivalent <capacitance>. Two identical <capacitors> of <capacitance> $C$ connected in parallel share the same terminal <electric potential difference>, and their <electric charges> add, giving $C_{\rm eq}=2C$. Connected in series with an initially neutral isolated junction, they carry equal plate <electric charge> magnitudes and their terminal <electric potential differences> add, giving $C_{\rm eq}=C/2$. https://openstax.org/books/university-physics-volume-2/pages/8-2-capacitors-in-series-and-in-parallel[OpenStax University Physics 2, §8.2: capacitors in series and in parallel]{ref}.
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