# Foundations

> Before we go any further, we need to know something fundamental first.

**RC circuit**: a circuit made by `resistor` and `capacitor`.

**RL circuit**: a circuit made by `resistor` and `inductor`.

**LC circuit**: a circuit made by `inductor` and `capacitor`.

**RLC circuit**: a circuit made by `resistor`, `inductor` and `capacitor`.

Why use `L` instead of `I` to represente `Inductor`?

Well, maybe simplly because we already represented `current` using `I`.

Current:

$$
\begin{align*}
&1kA = 10^3A    & 1mA = 10^{-3}A
\\ \\
&1\mu A = 10^{-6}
\end{align*}
$$

Capacitance:

$$
\begin{align*}
&1\mu F = 10^{-6}F    & 1pF = 10^{-12}F
\end{align*}
$$

Inductance:

$$
\begin{align*}
&1mH = 10^{-3}H    & 1\mu H = 10^{-6}H
\end{align*}
$$

我会在看完可汗学院的关于本章内容的视频之后详细地记录本章知识，但现在, 先跟中文书和老师走。

