voltage spike
hmm... if you will read some books on inductors, this "sudden voltage/current increase across capacitors/inductors " that you're mentioning is used on lighting cold cathode lamps. the "flyback" voltage produced by inductors when they are suddenly switch off is used to generate voltages larger than the original voltage source. this is useful for lighting several LEDs using only a 1.5 volts battery! considering that typical red LEDs require around 1.7 V to light up. and also do you see those flourescent lamp ballast that is used on emergency lamps? their energy source is only a 6 or 12 V battery but they are able to light up flourescent tubes that requires much larger voltages than that!... and oh, did you know that it is also in AC form! just try to read more about inductors and i'm sure you'll find more applications. have fun!