✔ 最佳答案
THe "constant volgae or constant current" concept of rms values are only applicable to power consumption. It is by definition that the rms current (or voltage) produces the same heating effect produced by the same value of dc current (or voltage).
In a RC series circuit, it is correct that the supplied rms voltage is not equal to the direct sum of rms voltages across the capacitor and resistor.The peak-to-peak voltages of the RC series circuit can be written as,
(Vs)^2 = (Vc)^2 + (Vr)^2
where Vs, Vv and Vr are the peak-to-peak voltages of the power source, capacitor and resistor respectively.
Multiply both sides of the equation by(1/2), we have
(Vs)^2/2 = (Vc)^2/2 + (Vr)^2/2
but [Vs/square-root(2) equals to Vs(rms), thus (Vs)^2/2 = [Vs(rms)]^2. Similarly for Vc and Vr,
hence, we have: [Vs(rms)]^2 = [Vc(rms)]^2 + [Vr(rms)]^2
Clearly, the rms voltages could not be added up directly. The reason is because of there is a phase difference of 90 degrees (pi/2 radians) between Vc and Vr.
2011-01-17 09:06:42 補充：
First, I have to correct a typo. Vs, Vc and Vr are the peak values, NOT the peak-to-peak values.
The rms values is just a physical concept. It denotes the value of ac current as if it were constant such that it could produce the same heating effect as the same value of dc current....(cont'd)
2011-01-17 09:07:44 補充：
... Hence, be aware that in reality, the ac current does vary with time. In the actual fact, phase and current value are two different physical quantities. A constant value doesn't implies that there is no phase diffence....(cont'd)
2011-01-17 09:07:59 補充：
...There is no theory whatsoever that there is no phase difference between rms current of different circuit components.
