To toss another 2-cents in here .... and boil down the maths:
-
the basic formula is V = I * R for simple resistors, and V = I * Z for impedances
[circuits with inductors and capacitors]. -
with resistors, voltage and current are always in phase with each other.
-
with inductors and capacitors, voltage and current are out of phase with
each other, so it takes more complicated maths [involving both magnitude
and phase] to solve the equations [as the other tutorials indicated].