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

Registered User regular
edited October 2011

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

http://en.wikipedia.org/wiki/Luke'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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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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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