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What is rms power?by songoku
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#1
Oct2611, 08:54 PM

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1. The problem statement, all variables and given/known data
What is the meaning of rms power? Is it useful? 2. Relevant equations P_{rms}=P_{max} / √2 3. The attempt at a solution I' m not very sure about the equation I write above. Is that correct? I am learning about AC. I find that the average current is the same as I_{rms} and average voltage is the same as V_{rms}, but average power is not the same as P_{rms}. Average power is the value of electrical energy received by a load every second. And what about P_{rms}? Thanks EDIT: Oh my god, I posted in the wrong place. Please move my post to the correct place. I am sorry 


#2
Oct2611, 11:46 PM

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For a sinusoidal voltage and current
V_{rms} is defined to be the constant voltage that supplies the same power over a single period to a resistance R as the sunusoidal voltage: [tex] P = \frac{V_{rms}^2}{R}[/tex] and you have the same type of definition for the current: [tex]P = I_{rms}^2R[/tex] If you multiply these two equations by each other you get: [tex]P^2=\frac{V_{rms}^2}{R}\cdot I_{rms}^2R = V_{rms}^2I_{rms}^2[/tex] [tex]P = \sqrt{V_{rms}^2I_{rms}^2}=V_{rms}I_{rms}[/tex] Does that answer your question? 


#3
Oct2711, 07:16 AM

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Vrms gives an idea of a typical value you might get for the absolute value of the voltage if you measured it. (Absolute meaning unsigned). Vrms=Vmax/√2 For a sinusoidal wave. So for a sinusoidal wave, a typical value for the unsigned voltage is roughly 0.7 times the max voltage. The power depends on the voltage, so it changes all the time, but it is always positive. The average power is equal to the max power divided by 2. This is also equal to the power when the voltage is equal to Vrms. In other words, if we replaced the voltage at all times with Vrms, the average power would remain unchanged. I don't know what you mean by Prms. Do you mean the power given by a voltage equal to Vrms? In this case it is just the average power. Or do you mean the squared variance of the power output? 


#4
Oct2711, 11:04 AM

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What is rms power?
"RMS", "rootmeansquare" is (almost) exactly what it says. Given a list of numbers, Find the mean (arithmetic average) of the squares of the numbers, then take the square root of that. It is one of many different kinds of "averaging".



#5
Oct2711, 12:23 PM

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That's not quite right. Its defined as:
[tex]\sqrt{\bar{x^2}  {\bar{x}}^2}[/tex] (Where [itex]\bar{z}[/itex] says 'take the mean of this set of numbers denoted by z'). (As long as the list of numbers is much greater than one). So I guess you're right as long as the mean of the numbers is equal to zero. 


#6
Oct2711, 12:40 PM

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I think that the term RMS is required only for quantities which can take both positive and negative values. Since power is always positive, what is the use of RMS power?



#7
Oct2711, 01:14 PM

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Not necessarily. The RMS gives an idea of how much something varies. So the RMS of the power would tell us how much the power varies by. But I don't see why that would be important in circuits. I would have thought only the average power was important...



#8
Oct2811, 12:13 AM

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I see the thread hasn't been moved to physics so I assume it's okay to continue the discussion here
thanks 


#9
Oct2811, 02:29 AM

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RMS means rootmeansquare.
The AC current is I=I_{0}cos(wt). If it flows through a resistor, the instantaneous power is P(t) = RI^{2}(t). When using an AC device you are interested in the average power. If the power of a heater is said to be 2000 W, it means the average power. The average power of a time dependent current flowing through a resistor R is [tex]P_{av}=1/T \int_0^T{R I^2(t) dt}=R(1/T \int_0^T{ I^2(t) dt})[/tex] that is, the average power is R times the mean of the squared current. We call the square root of the integral "rootmeansquare current", I_{rms}. [tex]I_{rms}=\sqrt{1/T \int_0^T{I^2(t) dt}}[/tex] For the AC current, [tex]P_{av}=R/T\int_0^T{(I_0cos(\omega t))^2 dt}=RI_0^2/T \int_0^T{\frac{(1+cos(2 \omega t))}{2} dt}\rightarrow P_{av}=R\frac{I_0^2}{2}=RI_{rms}^2[/tex]. So the average power is the same as that of a DC current I_{rms}=I_{0}/√2 The average power is [tex]P_{av}=RI_{rms}^2=\frac{V_{rms}^2}{R}[/tex] There is no use of rms power: The average power is connected to rms current and voltage. ehild 


#10
Oct2811, 09:09 PM

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Thanks for your explanation :)



#11
Oct2811, 09:42 PM

P: 599

If you live in the real world of power electronics, RMS power is very important because AC power is not always sinusoidal. The average values of voltage and current may have little relationship with the true (VA/RMS) levels.
http://www.google.com/url?sa=t&sourc...AkFge8WDsV4DlA 


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