查看“︁學院物理/電容與介電質”︁的源代码

{{wikipedia|電容}}
==定義==
<math>C=\frac Q V\begin{matrix}\begin{smallmatrix}(\end{smallmatrix}\frac{Col}{Volt}\begin{smallmatrix}=\,Faraday\,)\end{smallmatrix}\end{matrix}=\frac{\sigma A}{V=\sigma d/\epsilon}\quad\because V=Ed=\frac{\sigma d}\epsilon</math>

==例題==
===一===
{{see also|學院物理/連續電荷分布與高斯定律#七{{!}}例題23.8}}
;圓柱電容
:<math>
\begin{align}&
V=-\int Edr=-\int{2k\lambda}rdr=-2k\lambda\int\frac1rdr=-2k\lambda\ln r
\\&
V=\int_a^b\frac{2k\lambda}rdr=2k\lambda\Big[\ln r\Big]_a^b=2k\lambda\ln\frac ba 
\\&
C=\frac QV=\frac l{2k\ln\frac ba}
\end{align}
</math>
;示意圖
<div style=left:26px;position:relative;width:200px><b><i>
<div style="border-left:4px solid;height:80px;margin-left:14px;"></div>
<div style="border-bottom:2px solid;left:16px;position:absolute;top:40px;width:60px;"></div>
<div style="border:dashed;border-width:0 1px;height:86px;left:26px;position:absolute;top:0;width:12px;"></div>
<div style=left:0;position:absolute;top:4px>V</div>
<div style=left:80px;position:absolute;top:30px>r</div>
<div style=left:74px;position:absolute;top:12px><span style=font:initial>ln</span> r</div>
<div style="border:solid 2px;border-radius:50%;border-color:transparent;border-left-color:initial;height:100px;left:22px;position:absolute;top:32px;transform:rotate(45deg);width:100px;"></div>
<div style=left:66px;position:absolute;top:50px>-<small> <span style=font-style:initial;font-weight:initial;font-family:serif>ln</span> r</small></div>
<div style="border:solid 1px;border-radius:50%;border-color:transparent;border-bottom-color:initial;height:100px;left:22px;position:absolute;top:-52px;transform:rotate(45deg);width:100px;"></div>
<span style=display:inline-block;width:22px></span><math>a\;b</math>
</i></b></div>

===二===
;圓球電容
<math>
\begin{align}&
V=\int_a^b\frac{kQ}{r^2}dr=-kQ\left[\frac1r\right]_a^b=kQ\left(\frac1a-\frac1b\right)\\&
C=\frac QV=\frac1k\frac{ab}{b-a}\end{align}
</math>