electromagnetic wave, what is the direction of E?

Question

For an electro-magnetic wave travelling in the y direction, when B points in the x-direction what should be the direction of E?

a) positive z
b) negative z
c) positive y
d) negative y
e) positive x
f) negative x

i'm thinking positive z, i need confirmation. i was using poynting.

Answer 1

You are correct, positive z. The magnetic and electrical fields are perpendicular to the direction of travel and to each other; that's how we know E will be on the z-axis. The cross product E x B must yield the direction of propogation of the wave. The cross product of positive z and positive x gives us positive y, which is what we wanted.

Answer 2

so you have the right idea. Energy propagates in the direction of propagation. You are told that energy is propagating in the +y direction. Energy propagation direction is the same direction as the Poynting vector =+ y = E x B (with apologies since I cant bold the vectors and put carets over the unit vector). If B is in the direction of +x then E can't be along the y axis since E x B would't be in the y direction. E cant be along x since E x B would be zero. Your only options are along plus or minus z. If you have a right hand coordinate system then E must be along -z to get the poynting vector in the direction y

Answer 3

The direction of motion is in the direction of E x B, vector product. If the direction of motion is +y, and B points in +x, then I think E must be in +z direction. Using the right hand rule, put thumb along direction of motion, +y, then E must be along +z so that fingers rotate from E into B. And since B is along +x, then E must be along +z.

Answer 4

a)positive z
because magnetic field,electric field and direction of propagation of wave ,all three are mutually perpendicular to each other.