Where does j come from in the reactance of L and C components?
The use of imaginary numbers follows naturally when analyzing oscillating motion, and particularly when analyzing electrical circuits which incorporate reactive components, where voltage and current are not identical. As has been suggested, do a little research to find out why.

As an interesting minor issue, the question of why electrical engineers use j rather than the mathematician's i is a bit murky. On the one hand, when complex notation was introduced, i was already in use for current (almost certainly from the German Intensität). So what to do? j was chosen, probably for 2 reasons. First, of course, it was the next letter in the alphabet, and was not spoken for. A second possibility is discussed here http://www.johndcook.com/blog/2013/04/23/why-j-for-imaginary-unit/ which suggests that it relates to representing complex numbers in an ij plane, rather than xy.