Hi, I've not posted on here before but I'm trying to keep on top of work over the summer and I'm having some real problems with this question(adsbygoogle = window.adsbygoogle || []).push({});

1. The problem statement, all variables and given/known data

Consider the plane polarised EM wave in a source free vacuum with magnetic field B = (1,1,0)B_{0}cos(kz-wt) where B_{0}= 0.001T. Find the direction of propagation of the wave, the average energy density of the wave and the fraction of this energy which would pass through a perfect polaroid sheet at normal incidence if the angle between B and the transmission axis of the polariser is 30^{o}.

2. Relevant equations

For the second part, I'm pretty sure you have to use U=B^{2}/u_{0}where U is the energy density and u_{0}is the permeability of free space.

3. The attempt at a solution

The first part seems relatively simple, the wave travels perpendicularly to the magnetic field, so it moves in the (0,0,1) direction.

The second part asks for the average energy density but since B varies with respect to both time and position I'm not sure how to find it, would I have to integrate the expression for B with respect to t and then use the equation for U?

Obviously without an answer to part two I can't complete the third section. However, I'm pretty sure I have to find the fraction of the wave that passes through the polariser and hence find its energy density and then express that as a fraction of the incident energy density. I'm quite new to polarisation so I don't really know where to start and looking online has just confused me so far. Suggestions or links to relevant material would be really appreciated.

Thanks a lot.

**Physics Forums | Science Articles, Homework Help, Discussion**

Dismiss Notice

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Homework Help: Finding the average energy density of an EM wave in a magnetic field

**Physics Forums | Science Articles, Homework Help, Discussion**