# Creating a RL/RC/RLC Circuit model

Hello! Would someone here guide me in creating a model for a RL/RC/RLC circuit? I have started studying this topic in advanced but I don't quite understand the concept in DE.
I have watched videos from youtube.com and they solve problems using EE such as V=IR. And yes, I am able to answer problems too using theoretical formulas from my EE class. What I want to know is how to apply DE in this topic. I really want to learn this soon because maybe next week, we are going to start on this topic.
Thanks!:tongue:

Anyone?

HallsofIvy
Homework Helper
Your question is really too vague to give a complete answer. Exactly how you model such a circuit as a differential equation depends on exactly what the circuit is like.

The basic ideas are
1) The total voltage change around a circuit is 0.
2) The voltage drop over a resistor is Ri where i is the current and R the resistance.
3) The voltage drop through a coil is L di/dt where i is the current and L the inductance.
4) The voltatge drop through a capacitor is Q/C where Q is the charge and C is the capacitance (you will have to ask an electrical engineer why C is divided and not multiplied).

Since current is the rate of change of charge, i= dQ/dt and so di/dt= d^2Q/dt^2 and the voltage drop through a resistor is R(dQ/dt) and the voltage change through a coil is L(d^2Q/dt^2).

Since those are all "drops" (negative change in voltage), there must be voltage supplied to the circuit for the total change to be 0. Adding those for a circuit that has all three kinds of objects and setting it equal to the voltage supplied gives you a second degree differential equation for Q.

(It is not a good idea to "bump" a thread after only 8 minutes. That can get you banned!)

Thanks! I get it now! Thank you very much physicsforum and HallsofIvy!