Question

An electromagnetic wave is propagating towards the west. At a certain moment the direction of the magnetic field vector associated with this wave points vertically up. The direction of the electric field vector of this wave options:
vertical and pointing down.
vertical and pointing up.
horizontal and pointing north.
horizontal and pointing south.
horizontal and pointing east.

Answer 1

Answer:

Horizontal and pointing south.

Explanation:

The Poynting vector points in the direction of propagation (if the wave is a sinusoidal linearly polarized plane wave of fixed frequency, as it is in our case), and it's given by (all vectors):

$S=E\times H$

where $H$ points in the same direction as $B$, the magnetic field. This means that $E$ must point south, because the vectorial product of a vector pointing south by a vector pointing up gives a vector pointing west. This can be deduced by the right-hand rule: if our index finger is $E$ and our middle finger is $H$, then the thumb is $S$.