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Physics Question: Lagrangian Density and Fields. [Please Lock]

dexterdexter Registered User regular
edited October 2011 in Help / Advice Forum

I have this question for an advanced physics unit I'm taking, and I'm really not sure how to start it. We had a guest lecturer teach us about this stuff for 8 lectures, but I'm still really confused by it all. For a, do I find the energy current density (something I don't fully understand) by taking the derivative of the Hamiltonian with respect to time and then negating it? Since apparently (del H/del t) + (del S/del x) = 0. Thanks in advance, guys!

dexter on


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    Dunadan019Dunadan019 Registered User regular
    I doubt you are going to find a fluid dynamics expert in wave variance in H/A however's_variational_principle#Hamiltonian_formulation

    there is a wikipedia page for everything. About 3/4 of the way down it talks about relating the hamiltonian to lagrangian using the Leibniz integral rule. I don't have time right now to read through it but I hope this at least helps you out with a place to start from.

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    DemerdarDemerdar Registered User regular
    edited October 2011
    Hmm, don't you have a book to reference?

    However, it sounds like going from Lagrange to Hamiltonian, I'm sure there is a coordinate transformation in there somewhere, in which you will probably need to find a Jacobian. I'm not sure what a Hamiltonian reference frame is though.. has something to do with quantum???

    For the second part, there should be some kind of reference in a book or something that determines what the energy current density is from your lagrange density field.

    I am a fluid dynamics guy but unfortunately I have never worked in acoustics (or hamiltonian reference frames, I usually work in Eulerian).

    Demerdar on
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    ceresceres When the last moon is cast over the last star of morning And the future has past without even a last desperate warningRegistered User, Moderator mod
    Dunadan019 wrote:
    I doubt you are going to find a fluid dynamics expert in wave variance in H/A however

    Don't be so sure; there are a good few experts in various sciences wandering these parts.

    And it seems like all is dying, and would leave the world to mourn
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    dexterdexter Registered User regular
    Hey guys, thanks for all of the advice. I apologize for the vagueness of my post, I was in a bit of a rush and I was getting pretty desperate... There isn't any set book, there were lecture notes accompanying the two weeks we had this lecturer for this specific field, and I had my own notes I took down in the class. Unfortunately I couldn't sort it out for myself! I handed in what I could today. Thanks a lot guys, appreciate the helpful and friendly advice!

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