Question

An electromagnetic wave is transporting energy in the negative $z$ direction. At a certain point and certain time the direction of electric field of the wave is along positive $y$ direction. What will be the direction of the magnetic field of the wave at that point and instant?

• A. Positive direction of $z$
• B. Negative direction of $y$
• C. Positive direction of $x$
• D. Negative direction of $x$

Answer 1

Answer: (C)

As, poynting vector
$\vec{S}=\vec{E}\times \vec{H}$
Given energy transport $=$ negative $z$ direction
Electric field $=$ positive y direction
$(-\hat{k})=(+\hat{j})\times [\hat{i}]$
Hence according to vector cross product magnetic field should be positive $x$ direction.