Suppose water flows through a square surface of area A with a
uniform velocity
.

We want to define flux as the amount of water that flows through
the surface per second. First, notice that the flux depends on the
direction of the velocity vector
relative to the surface.
Compare the water flux in the following three situations. Rank the
flux in cases (a), (b), and
(c), from largest to smallest.
(a)

is perpendicular to the surface
(b)

is at a 45 degree angle to the surface
(c)

is parallel to the surface
Once you're ready, move on to the next section to learn more about flux.