I am going through the following tutorial: Time-Response Identification of a Resistor–Capacitor (RC) Circuit
Why do we call the RC circuit a first order system?
I understand the order of a system is based on the highest derivative involved in its governing equation. The input governing equation of an RC circuit is described by equation 1 in the tutorial:
$$e_i-iR-\frac{1}{C}\int i \;dt =0$$
I understand the equation. But, I'm confused because there is no derivative term in it and yet it is called a first order system.