34: Electric and Magnetic Field Vectors Conceptual Question

INTRO:

Figure 1

Figure 2

Figure 4

Figure 3

all of the questions will refer to the following answer choices:

a) +x
b) –x
c) +y
d) –y
e) +z
f) –z
g) at a +45∘ angle in the xy plane

Part C:
The electric and magnetic field vectors at a specific point in space and time are illustrated. (Figure 1) Based on this information, in what direction does the electromagnetic wave propagate?

SOLUTION:
the Poynting Vector S points in the direction of E×B
which in this instance would point in the positive z direction, or option e)
~ thumb is E, pointer is B → middle finger points +z

Part B:
The electric and magnetic field vectors at a specific point in space and time are illustrated. (Figure 2) (E and B are in the xy plane. Both vectors make45∘ angles with the +y axis.) Based on this information, in what direction does the electromagnetic wave propagate?

SOLUTION:
Since E & B are both in the xy plane, their cross product will be along the z-axis
In this case, E×B points in a negative z direction, or option f)
~thumb is E, pointer is B, middle points -z

Part C:
The magnetic field vector and the direction of propagation of an electromagnetic wave are illustrated. (Figure 3) Based on this information, in what direction does the electric field vector point?

SOLUTION:
Basically just work backwards
~middle is v, pointer is B, thumb points +y → option c)

Part D: 
The electric field vector and the direction of propagation of an electromagnetic wave are illustrated. (Figure 4) (E is in xz plane and makes a 45° angle with the +x axis.) Based on this information, in what direction does the magnetic field vector point?

SOLUTION:
work backwards again but be mindful of angles
~thumb is E, middle is v, pointer points at 45° between +x & -z, or option g)