# Electromagnetic Momentum and Energy

1. Apr 16, 2009

### quantoshake11

I've been presented with these concepts, and the first time i saw them i thought they were just mathematical mambo-jumbo, but of course i didn't gave much importance to these thoughts. I've been constructing an idea of these after seeing how they behave in different circumstances, but really, i just seem to make a sense out of them from the examples and not from the 'derivation' they gave me. they're quite strange concepts, and i just wanted you guys to throw me some facts about them to chew in a bit and give a sense of them :)

2. Apr 16, 2009

### Bob S

3. Apr 18, 2009

### quantoshake11

well, as far as i know, any EM field will have an energy associated with it. the expression for it would be
(1/2)*$$\int E^2+B^2$$
This would be valid for any EM field, including light. the integral is taken over a certain volume, and you can get the energy density as the limit.

4. Apr 18, 2009

### Bob S

The equation you are writing relates to the stored energy density in a volume. It is the physics version of such electrical engineering equations as (1/2) L I2 and (1/2) C V2. The electromagnetic energy flow is given by the Poynting vector P = integral[E x H] dA, which relates to the electromagnetic power flowing across an area element dA.

5. Apr 18, 2009

### Staff: Mentor

These pages derive electromagnetic energy and momentum conservation from Maxwell's equations.

6. Apr 19, 2009

### quantoshake11

thanks. those links really helped :)