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RMS Voltage
Consider the "120 VAC" power line coming into your home. The average voltage in this signal is zero. (So, why pay the light bill?) In fact, electricity is billed in "kilowatt hours", or the time integral of power. The power is what is important to run electrical equipment. In general, electric power varies as voltage squared, (P=V2/R) which is strictly non-negative. The average (mean) power can be computed using the "mean squared voltage". Specifically, power is found using the square of the "root mean squared voltage" (VRMS or VRMS). This is the quantity which is important when shopping for AC signals. In fact, when talking about these signals, "120 VAC" refers to the RMS voltage of the cosine waveform delivered to your doorstep.