I know this is an electrical engineering convention, but I'm still wondering why it was chosen for Python. I don't know other programming languages with complex-number literals, so I don't have anything to compare against, but does anyone know any that do use i?
j (not J) is used in Electrical Engineering as mentioned before. 
i for current: yes, both I (dc) and i (ac) are used for current.
Why are complex numbers in Python denoted with 'j' instead of 'i'?