# Homework Help: Find total momentum as function of energy density

1. Apr 10, 2013

### hammel54

1. The problem statement, all variables and given/known data
So here's the question:
The momentum flux dP from an infinitesimal monochromatic energy field dE is given by:
dP = I/c cos^2θdΩ, where dΩ = sinθdθd∅

Assuming the field is isotropic, find the total momentum P as a function of the energy density u=4πI/c

2. Relevant equations
Potentially the poynting vector
u=εoE2

3. The attempt at a solution

Honestly, have no idea. I do not believe the answer for the question is that difficult. This was meant to be a "quicker" assignment.

Stating the obvious from the question itself, we know that the field is uniform (isotropic) and small (infinitesimal).

We have to find the total momentum and are given the momentum flux, which is essentially the rate of transfer of momentum across an area. A guess would be that integration is going to come into play somewhere?

Any help or tips is much appreciated. Thank you in advance.