# Poynting vector in static electromagnetic field

1. ### zql

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=E·D/2+B·H/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?

2. ### DrDu

4,407
Yes, correct. Look also for "hidden momentum".

3. ### zql

5
can you give me more details？thanks.

4. ### clem

1,276
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.

5. ### Andy Resnick

5,943
The relevant expression, from the conservation of energy, is:

$$\frac{\partial u}{\partial t} + \nabla \bullet S = -J \bullet E$$.

Does this help?

6. ### zql

5
I think I got it, thanks.

5