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).
- These distinctions apply only to ac, not to dc.
In the previous lesson/experiment I learned how to use the multimeter to measure voltages and currents in dc circuits and how to use the oscilloscope to measure the peak voltage or peak-to-peak voltage of an AC waveform. When we used to measure AC voltages or currents, the multimeter gives us something called the effective value, or rms 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.
The rms value is am constant itself which depending on the shape of the function i(t).
The Effective value or rms value of an AC waveform is an equivalent DC value.
Example: If you connect a 5 Vrms 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 will
Not 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, Vrms = 25 V rms.
| P=1/2 VmIm cos(angle of voltage – angle of current)or= Vrms I rms cos(angle of voltage – angle of current) |
Resistive load only:
True power, reactive power, and apparent power for a purely resistive load.
In this topic, I've learned that RMS (Root Mean Square) is used to measure varying signals effective value and the mathematical relationship to peak voltage do varies depending on the type of waveform.
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