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Homework Help: LRC equation using Poynting theorem and conservation laws

  1. Feb 11, 2016 #1
    1. The problem statement, all variables and given/known data
    We have an ordinary LRC circuit with inductance L, capacitance C and resistance R with an oscillating voltage with low frequency (U^e). Using the energy conservation law and Poynting's theorem, find the differential equation:

    $$L \frac{\partial ^2}{\partial t^2}I + R \frac{\partial }{\partial t}I + L \frac{1}{C}I = \frac{\partial }{\partial t}U^e$$

    2. Relevant equations
    I'll need to take advantage from the fact that I is defined as
    $$I = \frac{\partial}{\partial t}q$$.

    Knowing q is the charge.

    3. The attempt at a solution
    Well, I've been looking around on the web to find something but I really can't find how to connect the Poynting theorem to that kind of circuit.

    If any of you have some insights of a head start to give me that'd be great!
  2. jcsd
  3. Feb 12, 2016 #2
    Hi emeriska!

    Your equation looks a lot like what I would get if I wrote down Kirchoff's 2nd law for the circuit and differentiated with respect to time. Except I think you have an extra factor of [itex]L[/itex] in the 3rd term?

    Poynting's theorem would give you a nice 3d snapshot of the energy flow going on in and around the circuit.
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