Effective or RMS Value

• We have a few different ways to specify the size of an ac current or voltage.
• We can give either
  • the peak value, or
  • the peak-to-peak value, or
  • something called the effective value (also called rms value).
• Example: As you can see, the sine wave shown below has a peak voltage of 6 V_p. Also, its peak-to-peak voltage is 12 V_pp. And, as we'll see below, it's effective voltage is 4.24 V_rms. So you can't just say something like "The sine wave had a voltage of 6 V." You've got to be careful to say whether you're talking about peak voltage, peak-to-peak voltage, or effective voltage.
   
• This may seem confusing, but you have to be able to deal with it. It's similar to the situation that we have with temperatures or distances: when you give the distance between two cities, you can give it either in miles or in kilometers. It's the same distance, but expressed with two different units.
• These distinctions apply only to ac, not to dc.

RMS Value (Effective Value)

• The root-mean-square (rms) value or effective value of an ac waveform is a measure of how effective the waveform is in producing heat in a resistance.
• Example: If you connect a 5 V_rms source across a resistor, it will produce the same amount of heat as you would get if you connected a 5 V dc source across that same resistor. On the other hand, if you connect a 5 V peak source or a 5 V peak-to-peak source across that resistor, it willnot produce the same amount of heat as a 5 V dc source.
• That's why rms (or effective) values are useful: they give us a way to compare ac voltages to dc voltages.
• To show that a voltage or current is an rms value, we write rms after the unit: for example, V_rms = 25 V rms.