How is the energy tranported at a certain time and point?

[hidemi]

Homework Statement

At a certain point and a certain time the electric field of an electromagnetic wave is in the negative z direction and the magnetic field is in the positive y direction. Which of the following statements is true?

A. Energy is being transported in the positive x direction but half a cycle later, when the electric field is in the opposite direction, it will be transported in the negative x direction

B. Energy is being transported in the positive x direction and half a cycle later, when the electric field is in the opposite direction, it will still be transported in the positive x direction

C. Energy is being transported in the negative x direction but half a cycle later, when the electric field is in the opposite direction, it will be transported in the positive x direction

D. Energy is being transported in the negative x direction and half a cycle later, when the electric field is in the opposite direction, it will still be transported in the negative x direction

E. None of the above are true

Answer: B

Relevant Equations

E = v*B

I get the first part of B, but why doesn't the transported direction not change as the electric field changes? Does it follow the right-hand rule?

 

[Steve4Physics]

I get the first part of B, but why doesn't the transported direction not change as the electric field changes? Does it follow the right-hand rule?

In the second half of the cycle, the magnetic field will have changed to the negative y direction. So applying the right-hand rule leaves the direction of propagation unchanged.

You can see the process in this animation ('play button' is on the bottom left): https://ophysics.com/em3.html)

Mathemtically speaking, the direction of propagation is the direction of the vector product $\vec E \times \vec B$.

 

[hidemi]

In the second half of the cycle, the magnetic field will have changed to the negative y direction. So applying the right-hand rule leaves the direction of propagation unchanged.

You can see the process in this animation ('play button' is on the bottom left): https://ophysics.com/em3.html)

Mathemtically speaking, the direction of propagation is the direction of the vector product $\vec E \times \vec B$.

Thanks for the amazing animation! I got it!