- #1
billybomb87
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Homework Statement
The angular momentum density in the electromagnetic field is defined in terms of the momentum density (3.6, BELOW) by
[tex]\textbf{L}_{EM} = \textbf{x}\times\textbf{P}_{EM} = \textbf{x}\times(\textbf{E}\times\textbf{B})/\mu_{0}c^2[/tex]
Show that if the continuity equation for angular momentum is written in the form
[tex]\frac{\partial}{\partial t}(\textbf{L}_{EM})_{i} + \frac{\partial}{\partial x_{j}}(\textbf{M}_{EM})_{ij} = (\textbf{S}_{EM})_{i}[/tex]
then (3.5, BELOW) implies
[tex](\textbf{M}_{EM})_{ij} = \epsilon_{irs}(\textbf{T}_{EM})_{jr}x_{s},[/tex]
[tex]\textbf({S}_{EM})_{i} = -\rho\epsilon_{ijk}x_{j}E_{k}-J_{i}\textbf{x}\cdot\textbf{B}+B_{i}\textbf{x}\cdot\textbf{J}.[/tex]
Homework Equations
[tex]\frac{\partial}{\partial x_{j}}(\textbf{T}_{EM})_{ij} = \rho E_{i} + (\textbf{J}\times\textbf{B})_{i} + \frac{\partial}{\partial t}(\textbf{P}_{EM})_{i}[/tex] (3.5)
[tex]\textbf{P}_{EM} = \textbf{E}\times\textbf{B}/\mu_{0}c^2[/tex] (3.6)
[tex](\textbf{T}_{EM})_{ij} = -(\frac{1}{2}\varepsilon_{0}|\textbf{E}|^2+\frac{1}{2}|\textbf{B}|^2/\mu_{0})\delta_{ij} + \varepsilon_{0}E_{i}E_{j} + B_{i}B_{j}/\mu_{0}[/tex]
The Attempt at a Solution
The biggest problem I have is with the tensor algebra. Havent have much practice in it and now all of a sudden I was thrown in a class where tensor algebra is something one should be common with.
This is what I have so far:
I expand Lem and get
[tex](x_{m}E_{i}B_{m} - x_{j}E_{j}B_{i})/\mu_{0}c^2[/tex]
then take the time derivative and using some of Maxwells equations in tensor form i get
[tex]\frac{\partial}{\partial t}(\textbf{L}_{EM})_{i} = (x_{m}E_{i}\epsilon_{mno}\frac{\partial E_{o}}{\partial x_{n}}+x_{j}E_{j}\epsilon_{pqr}\frac{\partial E_{r}}{\partial x_{q}})/\mu_{0}c^2+(x_{m}B_{m}(\epsilon_{mno}\frac{\partial B_{i}}{\partial x_{n}}-\mu_{0}J_{m})-x_{j}B_{i}(\epsilon_{pqr}\frac{\partial B_{r}}{\partial x_{q}}-\mu_{0}J_{j}))/\mu_{0}[/tex]
From here I have no idea how to continue. It is probably something wrong with how i introduce new indices.
The second term d/dxj (Mem)ij I expand and get
[tex]\frac{\partial}{\partial x_{j}}(\textbf{M}_{EM})_{ij} = \epsilon_{irs}(-\frac{\partial}{\partial x_{j}}(\frac{1}{2}\varepsilon_{0}|\textbf{E}|^2+\frac{1}{2}|\textbf{B}|^2/\mu_{0})\delta_{ij} + \frac{\partial}{\partial x_{j}}(\varepsilon_{0}E_{i}E_{j}) + \frac{\partial}{\partial x_{j}}(B_{i}B_{j}/\mu_{0}))x_{s}[/tex]
Will the first term be equal to zero? I don't think there are any indice problems here. The rest is probably taking derivates of products and then use Maxwells equation and many terms will probably then cancel out each other.