# Homework Help: Maxwell Equations Problem

1. Aug 1, 2006

### gjfelix2006

Hi, i am solving a problem about Maxwell Equation that invoves a lot of Vector Calculus, i have a partial solution for it but i have a few terms missing , i appreciate any help in this. Thanks

The problem is the following

Starting with the expression of the force by volume unit over a free space region with charges and currents:
$F_{v}=\rho E+J\timesB$
and using Maxwell's Equations:
$\begin{array}{l} \nabla \cdot \mathop E\limits^ \to = \frac{\rho }{{ \in _0 }} \\ \nabla \times \mathop E\limits^ \to = - \frac{{\partial \mathop B\limits^ \to }}{{\partial t}} \\ \nabla \cdot \mathop B\limits^ \to = 0 \\ \nabla \times \mathop B\limits^ \to = \mu _0 \mathop J\limits^ \to + \in _0 \mu _0 \frac{{\partial \mathop E\limits^ \to }}{{\partial t}} \\ \end{array}$
and the following vectorial identity:
$\mathop B\limits^ \to \times \nabla \times \mathop B\limits^ \to = \nabla ({\textstyle{1 \over 2}}B^2 ) - (\mathop B\limits^ \to \cdot \nabla )\mathop B\limits^ \to$,

Show that:
$\begin{array}{l} \mathop {F_v }\limits^ \to = - \in _0 \frac{\partial }{{\partial t}}(\mathop E\limits^ \to \times \mathop B\limits^ \to ) + \in _0 \mathop E\limits^ \to \nabla \cdot \mathop E\limits^ \to - \frac{1}{2} \in _0 \nabla (E^2 ) + \in _0 (\mathop E\limits^ \to \cdot \nabla )\mathop E\limits^ \to \\ {\rm{ + }}\frac{1}{{\mu _0 }}\mathop B\limits^ \to \nabla \cdot \mathop B\limits^ \to - \frac{1}{{2\mu _0 }}\nabla \mathop {(B^2 ) + }\limits^{} \frac{1}{{\mu _0 }}(\mathop B\limits^ \to \cdot \nabla )\mathop B\limits^ \to \\ \end{array}$

Now, let me show you my partial solution:

First, by Maxwell Equations, i get J:
$\mathop J\limits^ \to = \frac{1}{{\mu _0 }}(\nabla \times \mathop B\limits^ \to ) - \in _0 \frac{{\partial \mathop E\limits^ \to }}{{\partial t}}$

And i replace it in the first equation for $F_{v}$ to get:
$$\begin{array}{l} \mathop F\limits^ \to _v = \rho \mathop E\limits^ \to + \left( {\frac{1}{{\mu _0 }}(\nabla \times \mathop B\limits^ \to ) - \in _0 \frac{{\partial \mathop E\limits^ \to }}{{\partial t}}} \right) \times \mathop B\limits^ \to \\ {\rm{ }} = \rho \mathop E\limits^ \to + \frac{1}{{\mu _0 }}(\nabla \times \mathop B\limits^ \to ) \times \mathop B\limits^ \to - \in _0 (\frac{{\partial \mathop E\limits^ \to }}{{\partial t}} \times \mathop B\limits^ \to ) \\ \end{array}$$
Changing the order of the cross product (the sign changes), then
$$\mathop F\limits^ \to _v = \rho \mathop E\limits^ \to - \frac{1}{{\mu _0 }}(\mathop B\limits^ \to \times \nabla \times \mathop B\limits^ \to ) - \in _0 (\frac{{\partial \mathop E\limits^ \to }}{{\partial t}} \times \mathop B\limits^ \to {\rm{)}}$$
Now i can use the vectorial identity, thus:
$$= \rho \mathop E\limits^ \to - \frac{1}{{\mu _0 }}(\nabla ({\textstyle{1 \over 2}}B^2 ) - (\mathop B\limits^ \to \cdot \nabla )\mathop B\limits^ \to ) - \in _0 (\frac{{\partial \mathop E\limits^ \to }}{{\partial t}} \times \mathop B\limits^ \to {\rm{) }}$$

Also, from Maxwell equations:
$\rho = (\nabla \cdot \mathop E\limits^ \to ) \in _0$
and replacing it into the last equation for $F_{v}$, i get:
$\mathop F\limits^ \to _v = \mathop E\limits^ \to (\nabla \cdot \mathop E\limits^ \to ) \in _0 - \frac{1}{{\mu _0 }}(\nabla ({\textstyle{1 \over 2}}B^2 ) - (\mathop B\limits^ \to \cdot \nabla )\mathop B\limits^ \to ) - \in _0 (\frac{{\partial \mathop E\limits^ \to }}{{\partial t}} \times \mathop B\limits^ \to {\rm{) }}$
Some terms of what i should get can be seen already, but i have another terms missing. My last step is the following, what you think i should do to get the missing terms?
$\mathop {F_v }\limits^ \to = - \in _0 (\frac{{\partial \mathop E\limits^ \to }}{{\partial t}} \times \mathop B\limits^ \to ) + \in _0 (\mathop E\limits^ \to \cdot \nabla )\mathop E\limits^ \to - \frac{1}{{2\mu _0 }}\nabla \mathop {(B^2 ) + }\limits^{} \frac{1}{{\mu _0 }}(\mathop B\limits^ \to \cdot \nabla )\mathop B\limits^ \to$

¿What should I do to get the missing terms?
I think I must develop the first term in the last equation, but I don’t know how, can you help me?

I appreciate any help. Thanks a lot.

Last edited: Aug 1, 2006
2. Aug 1, 2006

### siddharth

Hi gjfelix2006, welcome to PhysicsForums.

First of all, please don't double post. I see you've made a duplicate thread in the other subforum as well. Also, to make it easier for those who wish to help, you might want to read https://www.physicsforums.com/showthread.php?t=8997" thread which explains how to use LaTeX mathematical typesetting. That way you needn't wait till your attachment is approved.

Last edited by a moderator: Apr 22, 2017
3. Aug 1, 2006

### gjfelix2006

I am sorry for my double post, i am new here and i dont know how to erase one of them, i made a double post because i dont know if my problem is a basic problem or an advanced problem. So sorry, if you can tell me how to erase it, i'll appreciate. Bye

4. Aug 2, 2006

### J Hann

I haven't followed your math thru but it looks like that you are missing
3 terms. The one with del dot B is obviously zero from Maxwell's
equations. Try writing a one dimensional version of the other two terms
and I think that they will also cancel out to zero!!!

5. Aug 2, 2006

### gjfelix2006

Thanks a lot for your help. I have solved the problem already. Thanks J Hann.