How to determine where the B feild starts

[aglo6509]

Hello,
This is kind of a simple question but if an EM wave is propagating in the positive x direction and the E field starts in the positive z direction, how do you figure out if the B field starts in the - or + y direction?

Thanks

 

[Simon Bridge]

Have a look:
http://en.wikipedia.org/wiki/Electromagnetic_radiation#Wave_model

in the article, v is in the +y dir, +E is in the +z dir, so B is in the +x direction.
to get your situation, rotate the axis anti-clockwise about +z ... now v is in the +x direction and B starts out in the -y direction.

The relationship depends on the cross product in the wave equation.
(Scroll down the link for derivation.)

note: the end-on animation shows the view looking down the direction of v.