Hamiltonian for a free electron in electromagnetic field

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Discussion Overview

The discussion revolves around the derivation of the Hamiltonian for a free electron in an electromagnetic field, focusing on the initial step of determining the Lagrangian in classical mechanics. Participants express a desire for a mathematical derivation rather than relying on direct results from textbooks.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification

Main Points Raised

  • One participant requests a mathematical derivation of the Hamiltonian and seeks the Lagrangian for a free electron in an electromagnetic field.
  • Another participant provides a link to a resource that presumably contains relevant information.
  • A participant acknowledges understanding the process of deriving the Hamiltonian from the Lagrangian but notes that the provided resource gives the Lagrangian without justification.
  • One participant mentions that literature typically derives the Lagrangian from the Lorentz force law through inspection, rather than from energy considerations, and suggests checking the derived Lagrangian against the Lagrange-Euler equation.
  • A participant confirms that they successfully derived Newton's equations of motion using the Lagrangian but expresses admiration for the process of determining the Lagrangian itself.

Areas of Agreement / Disagreement

Participants do not appear to reach a consensus on the method of deriving the Lagrangian, with some expressing uncertainty about the typical approaches found in literature.

Contextual Notes

Participants note a lack of references that derive the Lagrangian from energy considerations, indicating a potential limitation in available literature on this topic.

athosanian
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hello, how to derive the hamiltonian for a free electron in electromagnetic field mathematically ?
for a first step what is the lagrangian for a free electron in the EM field in classical mechanics ?
the physics textbook always like to give the results directly.
 
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thanks, I understand how to derive the Hamiltonian from Lagrangian.,
however, the file gives the Lagrangian directly without any reason.
 
Up to now, all literatures I have read always deduce the Lagrangian from the equation of motion (Lorentz force law) by, apparently, inspection - kind of trying to seek for the right form of L and check it by plugging it into the equation of motion of L. I have never seen a reference doing the reverse way, that is deriving the Lagrangian from the energies like we usually do in systems involving gravitational potential only. Perhaps there are such references but I just never come across them. Anyway, just for a check you can plug in the final form of the Lagrangian you found there into the Lagrange-Euler equation (the equation of motion), I think you should end up with the expression of Lorenty force.
 
yes, I put the L into the E-L equation, could derive the Newton's equaiton of motion. But I think it is very smart to work out the lagrangian.
 

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