Dear forum. I hope someone can tell why in the RMS we square the physical quantity (e.g. voltage or current) and then we take the square root? Why do we do this? What is the relevance of calculating the rms value for a certain wave/signal? What kind of information it gives to us? Does it helps us to differentiate between two very similar waves? Why we write in current/voltage as a cos function and not as a sin function? Is there any special reason? Examples: i(t) = 10 cos (wt -10) and not as i(t)= 10 sin (wt-10) ... Thanks for the answers.