calculo2718
- 25
- 1
Homework Statement
The momentum carried by an electromagnetic field is [;\vec{P}(\vec{x}, t) = \frac{1}{4\pi c} \int d\vec{x}\vec{E}(\vec{x},t) \times \vec{B}(\vec{x},t);]
show that for a finite field extension
[;\vec{J}(\vec{x}, t) = \frac{1}{4\pi c}\int -i A_i(\vec{x},t)\vec{L}E_i(\vec{x},t) + \vec{E}(\vec{x},t) \times \vec{A}(\vec{x},t) d\vec{x};] where [;\vec{J};] is the angular momentum, [;L = -ir \times \nabla;], and [;\vec{A};] is the vector potential in radiation gauge, i.e. [;A^0 = 0;] and [;\nabla \cdot \vec{A} = 0;] (hint: using partial integration)
Homework Equations
coulomb gauge [;\vec{B} = \nabla \times \vec{A};]
The Attempt at a Solution
There are some things in the wording in the problem that I do not understand so I am making some assumptions that may not even be true.
1) Finite field extension means that the field does NOT go to infinite so [;\nabla \cdot \vec{E} = 0;]
2) The subscripts in the [;-i A_i(\vec{x},t)\vec{L}E_i(\vec{x},t);] represents a sum.
3) I have no clue what partial integration is.
[;\int \vec{x} \times (\vec{E} \times \vec{B}) d\vec{x};]
[;\int \vec{x} \times (\vec{E} \times (\nabla \times \vec{A})) d\vec{x};]
using triple product expansion
[;\int \vec{x} \times (\nabla(\vec{E} \cdot \vec{A}) - \vec{A}(\vec{E}\cdot \nabla)) d\vec{x};]
distributing the [;\vec{x} \times;]
[;\int \vec{x} \times \nabla(\vec{E} \cdot \vec{A}) - \vec{x} \times \vec{A}(\vec{E}\cdot \nabla) d\vec{x};]
This is where I get stuck. If I assume that [;\nabla \cdot \vec{E} = 0;], I have no idea where to get the [;\vec{E} \times \vec{A};] term in the answer I am supposed to get. If I don't assume that, I try to expand my [;\nabla(\vec{E} \cdot \vec{A});] term using some vector identity the [;\nabla \cdot \vec{E};] cancels anyway and I simply get the equation in my second line of work.
My gut tells me that I missing something having to do with this "partial integration" business. I have googled this and I get it as an alternative term to integration by parts or as integrating a partial derivative. If I am to integrate by parts, what am I integrating by parts? I don't have a product of two functions I have different vector products of different vector fields. If I am supposed to integrate some partial derivative, which one? I have so many!
please help, thanks!