Though in a phasor diagram, voltage and current are represented by
vectors, these quantities are not really vectors themselves They are
scalar quantities It so happens that the amplitudes and phases of
harmonically varying scalars combine mathematically in the same
way as do the projections of rotating vectors of corresponding
magnitudes and directions The ‘rotating vectors’ that represent
harmonically varying scalar quantities are introduced only to provide
us with a simple way of adding these quantities using a rule that
we already know as the law of vector addition