I did an exercise which probably is quite popular,
in which you draw an electromagnetic wave and prove that it should
propagate at the speed of light $1 \over \sqrt {\mu_0\epsilon_0}$ using Farday's law and Ampere's law.

Basically if this is the wave: 




Let's say the E-field (red) is in the X direction, the B-Field (blue) is in the Y direction,
and the velocity of the wave is in the Z direction.

You take for example for ampere's law a surface in the ZY plane with a length L
equal to the amplitude of the wave,
and a width equal to $\lambda\over 4$
You do a similar thing with Faraday's law and you get the speed of light,
assuming you know that the E-field and B-field propagate in this manner.

I got the right answer but I wondered about this:
Let's say I only had the E-field and I know the wave propagates at the speed of light, I assume this is enough information to draw the B-field at each point.

But how will I know the direction? Both Faraday's law and Ampere's law say you need a closed loop integral and the rules I've been taught say
you go over the loop in a clockwise direction for example and take the 
normal to the surface according to the right hand rule etc.

But clockwise and counter-clockwise direction don't really give me much information in this case, so how can I determine the direction of the B-field
if I only have the E-field?


Question: Bake particle system animation and delete few particles
Answer: The only way I know to edit already emitted particle objects would be to Make Duplicates Real Shift-Ctrl-A accessible in Object Menu after that you could select the mesh and invoke Separate by loose parts in Edit Mode P. This wouldn't help on animations.