How to tell which direction a magnetic field is oreinted in?

Question

Q: An electromagnetic wave propagates in the -y direction. The electric field at a point in space is momentarily orientated in the +x direction. What is the direction of the magnetic field momentarily orientated at that point?
A: +z direction
how do you know this?

Answer 1

Well, the propagation of an EM wave should be perpendicular with its vibrations (electric and magnetic fields). Since the direction of its propagation would be given by E x B (direction of electric field crossed product with the direction of magnetic field), we have X x ___ = -Y. Knowlege of cross products tell us that the blank is Z.

Read up on cross products if you are not familiar with how they are obtained. Or, you can use right-hand rule to forego cross products. By holding your right thumb, index and middle finger at right angles (starting position is like thumb points up, index points forward and middle points left), the thumb represents propagation direction, index for electric field vibration, and middle finger for magnetic field vibration. Just orient your fingers along the directions mentioned above, and find out where your middle finger points to. By the way, convention has it that up is positive z, forward is positive x and left is positive y.

Hope this helps.

Answer 2

In an electro magnetic wave the magnetic and electric components are perpendicular to one another which are both perpendicular to the direction of propagation. You know the wave is moving in the negative y direction and the electric field is orientated in the positive x direction so it follows that the magnetic field is orientated in the positive z direction.

And the orientation of the magnetic field will follow te same sign as the electric field.

Answer 3

Well, as I understand it, you have to use this :

Poynting vector :

The poynting vector gives you the direction where the wave propagates, and this vector (S), is equal to = E X B

In other words, the propagation is perpendicular to magnetic field and electric field.

Then, if the wave propagates in the y direction, and the electric field is in the x direction :

y = (x) X (?)

you have the vectorial producto, so y is perpendicular to x and +z

why +z ?

because, you have "-y"

and x*z = -y

in vectorial product of course

Answer 4

To find the direction of a magnetic field, sprinkle fine bits of iron around the magnet and they will take the shape of the magnetic field.

Answer 5

EM waves are transverse waves. They are perpendicular to the direction they are traveling.