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Homework Help: Derivation of the Eqation of Motion from Fermi Lagrangian density

  1. Dec 26, 2017 #1
    1. The problem statement, all variables and given/known data
    Hello, I am trying to find the equations of motion that come from the fermi lagrangian density of the covariant formalism of Electeomagnetism.

    2. Relevant equations
    The form of the L. density is:
    $$L=-\frac{1}{2} (\partial_n A_m)(\partial^n A^m) - \frac{1}{c} J_m A^m$$

    where J is the electric current.
    The result has to be:

    $$\partial_n \partial^n A^m = \frac{1}{c} J^m$$

    3. The attempt at a solution
    Using the Euler- Lagrange equations that derive the eq. of motion I do not understant if I have to treat the fields Am and Am. In addition, the Euler Lagrange equations from a L.density of the form:
    $$L=-\frac{1}{2} (\partial_n A_m)(\partial_n A_m) - \frac{1}{c} J_m A^m$$
    giveaway the wanted result. But I could not relate the two L.densities.
  2. jcsd
  3. Dec 27, 2017 #2


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    How did you lower the indices to get the density in §3 ?
    I think the density in §3 is not covariant and it is then incorrect.
    There must be elements of the metric tensor missing in the density of §3 .
  4. Dec 27, 2017 #3


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    Indeed. OP, please show your work.
  5. Dec 30, 2017 #4
    I did not lower any indices. And yes it is not covariant at all so it could not be a lagrangian. It was something a digged up on the internet while I was working at the solution of my problem. However, I did not notice while posting the thread that it was not covariant.

    Now, Lets focus on the first L.density.

    I tried playing with the indexes and my metric.

    Is the following correct? :

    $$L=-1/2(\partial_n A_m)( \partial^n A^m) = -\frac{1}{2} (g_{ns} \partial^s A_m)(g^{nr} \partial_r A^m) = -\frac{1}{2} \delta_{s}^{r}(\partial^s A_m)( \partial_r A^m)= -\frac{1}{2} (\partial^s A_m)(\partial_s A^m) $$

    So the partial derivative $$\frac{ \partial L}{\partial (\partial_s A^m)}$$ equals:

    $$\partial_s (\frac{ \partial L}{\partial (\partial_s A^m)})= \partial_s \partial^s A^m$$
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