Question

The velocity of the electromagnetic waves is parallel to : 

• A. \(\vec B\times \vec E\)
• B. \(\vec E\times \vec B\)
• C. \(\vec E\)
• D. \(\vec B\)

Answer 1

The Correct Option is B

To understand the direction of the velocity of electromagnetic waves, it's essential to grasp how electric and magnetic fields interact in electromagnetic waves.

Electromagnetic waves comprise oscillating electric field vectors (\(\vec{E}\)) and magnetic field vectors (\(\vec{B}\)) that are perpendicular to each other and travel through space. The velocity of the wave is in the direction of propagation of the wave.

The direction of the velocity of electromagnetic waves, denoted by the Poynting vector, is given by the cross-product of the electric field vector and the magnetic field vector: \(\vec{E} \times \vec{B}\).

1. Understanding \(\vec{E} \times \vec{B}\): The cross-product \(\vec{E} \times \vec{B}\) is a vector that is perpendicular to both \(\vec{E}\) (electric field) and \(\vec{B}\) (magnetic field). This vector indicates the direction of energy propagation and hence the direction of the velocity of the wave.
2. Other Options:
   • \(\vec{B} \times \vec{E}\): This is the reverse of the required cross-product, leading to a vector in the opposite direction of energy propagation.
   • \(\vec{E}\): Electric field by itself only indicates the direction of the electric component, but not the velocity of the wave.
   • \(\vec{B}\): Magnetic field by itself only indicates the direction of the magnetic component, but not the velocity of the wave.

Therefore, the direction of the velocity of the electromagnetic wave is parallel to \(\vec{E} \times \vec{B}\).

The correct answer is \(\vec{E} \times \vec{B}\).