I Poynting's theorem in Griffith's

[Zubair Ahmad]

I am in trouble with this theorem. I did it from Griffith's electrodynamics but I am not getting the physics of it. So can someone explain it lucidly.

 

[Lord Jestocost]

See: https://www.physicsforums.com/threads/question-on-poynting-theorem.502074/#post-3324904

The Feynman lectures might be of help, too: http://www.feynmanlectures.caltech.edu/II_27.html

 

[Dale]

I am not getting the physics of it

@Lord Jestocost has given some good general material on the topic. If you want more specific responses then you will need to make a more specific question. What specifically are you not getting.

 

[Zubair Ahmad]

I am not getting the way Griffith's has done. He says some charges move around. So I consider these to be some external charges.
Now he uses rho and J of these external charges but when he uses Maxwell equations he relates this rho and J with E and B of older charge and current configuration whose force acted on these external charges?

 

[Dale]

Forget about external or internal charges for a moment. Do you understand how the math goes from Maxwell’s equations to Poynting’s theorem?

 

[Zubair Ahmad]

Yes that's clear to me.

 

[Dale]

Yes that's clear to me.

Ok, so that means that ANY system which obeys Maxwell’s equations will necessarily also obey Poynting’s theorem. It doesn’t matter if the charges are internal or external, if they move in response to the fields or in response to some other constraints, or whatever. As long as Maxwell’s equations are valid, so is Poynting’s theorem.