Generally speaking, there are no hard and fast rules governing those particular adjectival endings. Some pairs are considered to be interchangeable (e.g. egotistic and egotistical), while others take on different meanings depending on what�s being modified (e.g. an electric shaver compared with an electrical engineer).
Michael Swan (in "Practical English Usage") treats electric and electrical as follows:
Electric is used with the names of particular machines that work by electricity (electric motor, electric blanket etc.)
Electrical is used before more general words (electrical equipment, electrical appliances etc.)
He also mentions pairs such as comic and comical. economic and economical, magic and magical, politic and political � as well as historic and historical.
There are obviously more, but a half-decent dictionary should give you some idea of the different usages, if you�re not sure.
Alex
