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Momentum of electromagnetic field

  1. Jul 29, 2013 #1
    Hello, I am trying to prove that the momentum of an electromagnetic field is [tex]E \times B[/tex] by considering the conserved quantity due to the spatial translation of the Lagrangian.

    [tex]L = - \frac{1}{4}\int {{F^{\mu v}}{F_{\mu v}}} {d^3}x[/tex]

    So far, I have calculated the canonical momentum.

    [tex]{\Pi _{{A_\mu }}} = \frac{{\partial {A_\mu }}}{{\partial t}} = \left( {\begin{array}{*{20}{c}}
    0\\
    {{E_x}}\\
    {{E_y}}\\
    {{E_z}}
    \end{array}} \right)[/tex]

    I know that the conserved quantity is

    [tex]Q = \int {{\Pi _{{A_\mu }}}} \delta {A_\mu }{d^3}x[/tex]

    But I am not sure how [tex]\delta {A_\mu }[/tex] is going to give me components of the curl.
    Thank you very much.
     
  2. jcsd
  3. Jul 30, 2013 #2

    samalkhaiat

    User Avatar
    Science Advisor

    The 3-momentum of the field is given by
    [tex]P^{ j } = \int d^{ 3 }x \ T^{ 0 j } ,[/tex]
    where
    [tex]T^{ \mu \nu } = \frac{ \partial \mathcal{ L } }{ \partial ( \partial_{ \mu } A_{ \sigma } ) } \partial^{ \nu } A_{ \sigma } - \eta^{ \mu \nu } \mathcal{ L } .[/tex]
    If you use the relation
    [tex]\frac{ \partial \mathcal{ L } }{ \partial ( \partial_{ \mu } A_{ \sigma } ) } = 4 F^{ \mu \sigma } ,[/tex]
    you find
    [tex]P^{ j } = - \int d^{ 3 }x \ \eta_{ \mu \sigma }\ F^{ 0 \sigma }\ \partial^{ j } A^{ \mu } .[/tex]
    Now, add and subtract [itex]\partial^{ \mu } A^{ j }[/itex], integrate by parts, neglect surface term, and use the equation of motion [itex]\partial_{ \sigma } F^{ 0 \sigma } = 0[/itex]. You will find
    [tex]P^{ j } = \int d^{ 3 }x \ \eta_{ \mu \nu } F^{ 0 \mu } F^{ \nu j } = \int d^{ 3 }x \ \eta_{ i k } F^{ 0 i } F^{ k j } = \int d^{ 3 }x \left( \vec{ E } \times \vec{ H } \right)^{ j } .[/tex]

    Sam
     
  4. Jul 30, 2013 #3

    dextercioby

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    Science Advisor
    Homework Helper

    It's important to know the difference between momentum and volumic momentum density. E × H is the momentum density of the free field.
     
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