# Talk:Poynting's theorem

WikiProject Physics (Rated C-class, Low-importance)
This article is within the scope of WikiProject Physics, a collaborative effort to improve the coverage of Physics on Wikipedia. If you would like to participate, please visit the project page, where you can join the discussion and see a list of open tasks.
C  This article has been rated as C-Class on the project's quality scale.
Low  This article has been rated as Low-importance on the project's importance scale.

Isn't the right side the negative of the total power done by the electric field? Not work... --Bmk 15:01, 25 July 2006 (UTC)

Oh, and isn't it actually power per unit volume? --Bmk 15:02, 25 July 2006 (UTC)

Yes. Fixed now. JRSpriggs 05:30, 26 July 2006 (UTC)

I think phasor poynting's theorem should also be mentioned in this page. --139.179.12.105 (talk) 23:20, 1 January 2008 (UTC)Özgür

## wrong

Hello, this statement is totally wrong : "Poynting's theorem takes into account the case when the electric and magnetic fields are coupled – static or stationary electric and magnetic fields are not coupled. In other words, Poynting theorem is valid only in electrodynamics." take a look at the feynman course. Wwe can define a poynting vector for a single particle with constant velocity, all the discussiàon bout electrodynamic momentum talks about that. — Preceding unsigned comment added by 134.158.23.246 (talk) 09:34, 24 June 2011 (UTC)

## Where is E × B ?

Near the bottom the article says "Instead of the flux vector E × B as above ...". Where does E × B appear above? (I was hoping to be able to refer to it.) Vaughan Pratt (talk) 02:41, 10 January 2014 (UTC)

## Cumbersome derivation in "Derivation" section

The derivation in the section entitled "Derivation" could be done simply by differentiation. Going back and forth between the integral statements and the pointwise statements (several times!) is a red herring. It subjects the reader to a pedagogical rehash of the geometric justification for the divergence theorem, whether he wants it or not.

## Inconsistencies throughout article

(1) The article defines the Poynting vector S to be "the energy flux vector", rather than giving a specific formula. That is fine. But the definition is not made "out loud", but only implicitly -- one is forced to infer it from the proof of Poynting's theorem. That is not so good.

(2) The main problem I have with this article: it goes back and forth several times between S = E x B and S = E x H, between J and J_f, and between a vacuum and a medium.

For example, early in the article we hear about "free" current, but then we switch to just E and B, then suddenly we have constitutive relations, but they are merely the constitutive relations of empty space. Then we proceed to derive (to prove) that S = E x H. Then we're told that we really did S = E x B (!!!). This does not match the S = E x H definition given under Poynting vector.

What is the correct setting for proving the theorem? Do I need to consult a medium, or decide in a vacuum?

(3) At the end we are told that we could have gotten S = E x H, S = D x B, or even S = D x H, and that the content of Poynting's theorem is different depending on what we got. In the words of the text:

It is possible to derive alternative versions of Poynting's theorem. Instead of the flux vector E × B as above, it is possible to follow the same style of derivation, but instead choose the Abraham form E × H, the Minkowski form D × B, or perhaps D × H.

Choose?

The Poynting theorem we proved here was established independently of these conclusions. It made no commitment as to whether S = E x B or S = E x H. How can the theorem have different versions depending on the formula we later discover for S? Is Poynting's theorem the "continuity"-type statement

 ${\displaystyle -{\frac {\partial u}{\partial t}}=\nabla \cdot \mathbf {S} +\mathbf {J} _{f}\cdot \mathbf {E} }$

where S is defined to be the energy flux, or is it the conclusion S = E x B, where S is defined to be the energy flux? Or is it the formula that's obtained by substituting E x B for S in the boxed formula?

Are the hypotheses or definitions different, to get different theorems? In that case, what are these variable hypotheses?

Why don't we get S = E x H, like in the article Poynting vector? Or do we?

It seems that the definitions and background assumptions are constantly oscillating back and forth in this article.

178.38.181.216 (talk) 00:44, 24 May 2015 (UTC)

Pretty devastating takedown, if I say so myself. Is there such a thing as a C-class article? 84.226.185.221 (talk) 20:47, 15 October 2015 (UTC)
There is. I'm going to try to wade through the subject to figure out what the correct form of the equations should be. --Ipatrol (talk) 19:58, 18 October 2017 (UTC)