Register to reply

Poynting vector in static electromagnetic field

by zql
Tags: poynting vector
Share this thread:
zql
#1
Oct21-10, 07:06 AM
P: 5
There is a situation, we have an electric field and a magnetic field, both are static. And we know the density of energy is u=ED/2+BH/2, so dU/dt=0, but Poynting vector S=ExH is not zero, which means energy is flowing. This confused me. Static field also has energy flux?
Phys.Org News Partner Physics news on Phys.org
Symphony of nanoplasmonic and optical resonators produces laser-like light emission
Do we live in a 2-D hologram? New Fermilab experiment will test the nature of the universe
Duality principle is 'safe and sound': Researchers clear up apparent violation of wave-particle duality
DrDu
#2
Oct21-10, 07:23 AM
Sci Advisor
P: 3,593
Yes, correct. Look also for "hidden momentum".
zql
#3
Oct21-10, 08:51 AM
P: 5
can you give me more details?thanks.

clem
#4
Oct22-10, 12:04 PM
Sci Advisor
P: 1,261
Poynting vector in static electromagnetic field

Quote Quote by zql View Post
There is a situation, we have an electric field and a magnetic field, both are static. And we know the density of energy is u=ED/2+BH/2, so dU/dt=0, but Poynting vector S=ExH is not zero, which means energy is flowing. This confused me. Static field also has energy flux?
The static magnetic field is produced by a constant electric current. That means there is resistance, and energy is flowing into matter. "Hidden" momentum is not involved.
Andy Resnick
#5
Oct22-10, 01:16 PM
Sci Advisor
P: 5,523
Quote Quote by zql View Post
There is a situation, we have an electric field and a magnetic field, both are static. And we know the density of energy is u=ED/2+BH/2, so dU/dt=0, but Poynting vector S=ExH is not zero, which means energy is flowing. This confused me. Static field also has energy flux?
The relevant expression, from the conservation of energy, is:

[tex]\frac{\partial u}{\partial t} + \nabla \bullet S = -J \bullet E[/tex].

Does this help?
zql
#6
Oct22-10, 01:26 PM
P: 5
Quote Quote by Andy Resnick View Post
The relevant expression, from the conservation of energy, is:

[tex]\frac{\partial u}{\partial t} + \nabla \bullet S = -J \bullet E[/tex].

Does this help?
I think I got it, thanks.
zql
#7
Oct22-10, 01:28 PM
P: 5
And how about "hidden momentum"? I didn't know about this.
DrDu
#8
Oct22-10, 02:45 PM
Sci Advisor
P: 3,593
Probably the best source on these matters is still Jackson's book on Electrodynamics.


Register to reply

Related Discussions
Poynting vector: Why We-Wm Classical Physics 8
Poynting vector Advanced Physics Homework 3
Poynting vector General Physics 3
Poynting vector Introductory Physics Homework 5