The existence of a scalar current does not preclude the existence
of a vector current, nor vice versa.
2) I think it may be useful to draw an analogy to the vector acceleration.
There is *also* a perfectly well defined and widely used scalar
acceleration. People (including physicists) freely speak of driving
around a corner without accelerating or decelerating ... obviously
referring to the scalar acceleration.
Note that the scalar acceleration is *not* the magnitude of the vector
acceleration; the scalar acceleration can be positive or negative.
The analogy works like this:
Scalar acceleration == component of vector acceleration in the
forward direction.
Scalar current == component of vector current as projected onto a
basis vector pointing along the wire in one direction
or the other.
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I think a lot of physics books (and physics teachers) get themselves into
trouble unnecessarily by pretending scalar acceleration doesn't exist.
They tell the class it doesn't exist, then they get caught using it, and
thereby lose credibility.
It is a pain in the neck to have two concepts be so similar yet not quite
identical; there is virtually guaranteed to be confusion and negative
transference. We should recognize these situations for what they are,
and recognize that they cannot be made to go away simply by pretending
that the other half of the concept does not exist.