# Write Spin Force as Curl?

1. Feb 26, 2013

### jtleafs33

1. The problem statement, all variables and given/known data
I put this in the math forum because although it's for my EM waves class, it's a math question.

Show that the spin force can be written as:

$F_{spin}=\frac{-1}{2}Im(\alpha)Im(E\cdot\nabla E^{*})=\nabla\times L_s$

Find $L_s$.

Where $\alpha$ is complex. I'm using $E^{*}$ to denote the complex conjugate of $E$. Also, since these are all vectors, I'm omitting the arrow notation atop the vector quantities.

2. Relevant equations

$Im(z)=\frac{1}{2i}(z-z^{*})$

3. The attempt at a solution

From the relevant equations:
$Im(\alpha)=\frac{1}{2i}[\alpha-\alpha^{*}]$
$Im(E\cdot\nabla E^{*})=\frac{1}{2i}[E\cdot\nabla E^{*}-(E\cdot\nabla E^{*})^{*}]$

Substituting in,
$F_{spin}=\frac{1}{8}[\alpha-\alpha^{*}][E\cdot\nabla E^{*}-(E\cdot\nabla E^{*})^{*}]=\nabla\times L_s$

Here, in order to make a curl appear, I'd like to apply the identity:
$\nabla\times(A\times B)=A(\nabla\cdot B)-B(\nabla\cdot A)+(B\cdot\nabla)A-(A\cdot\nabla)B$

However, I'm not sure what the quantity $[(E\cdot\nabla E^{*})^{*}]$ looks like... I don't know how to conjugate this and I'm stuck here for the moment.

Last edited: Feb 26, 2013
2. Feb 26, 2013

### haruspex

If E is a vector then I'm not sure what ∇E means. ∇.E would be a scalar, making E.(∇.E) problematic. Do you mean ∇×E?

3. Feb 26, 2013

### jtleafs33

E is the electric field vector. it is a function of position and time, so that's just its gradient vector, also a function of position and time.

4. Feb 26, 2013