Many branches of science and engineering have found uses for this number. For example. electrical engineers need "i" to understand how an electric circuit will work when they are designing it (electrical engineers use "j" instead of "i" to avoid confusion with the symbol for the current). As another example. certain branches of physics such as quantum physics and high energy physics use "i" as often as they use any other regular number. Many of the equations in the world simply cannot be solved without "i".