# NCEES #107



## ndekens (Aug 6, 2008)

Okay so why divide by SQRT 2?


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## Flyer_PE (Aug 6, 2008)

The amplitude of the current is given as a peak value. For AC power, RMS values are used.

Irms = Ipeak / sqrt2


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## ndekens (Aug 6, 2008)

Flyer_PE said:


> The amplitude of the current is given as a peak value. For AC power, RMS values are used.
> Irms = Ipeak / sqrt2



Okay so if the frequencies of the two components were the same then the equation:

(7/SQRT2)^2 * 3 would be true?

However Since the frequencies are different we must use the equation:

3*(5/SQRT2)^2 + 3*(2/SQRT2)^2


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## benbo (Aug 6, 2008)

ndekens said:


> Okay so if the frequencies of the two components were the same then the equation:
> (7/SQRT2)^2 * 3 would be true?
> 
> However Since the frequencies are different we must use the equation:
> ...


Yes. If the signals have the same frequency and are in phase, then you can add the amplitudes.


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## Flyer_PE (Aug 6, 2008)

^I'm not so sure. The problem I have is that if either method can be used, then:

(7/SQRT2)^2 * 3 is equall to 3*(5/SQRT2)^2 + 3*(2/SQRT2)^2?

49*3/2 = 25*3/2 + 4*3/2

73.5 = 43.5

This one puzzles me as my first response is the same as benbo's.


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## benbo (Aug 6, 2008)

Flyer_PE said:


> ^I'm not so sure. The problem I have is that if either method can be used, then:
> (7/SQRT2)^2 * 3 is equall to 3*(5/SQRT2)^2 + 3*(2/SQRT2)^2?
> 
> 49*3/2 = 25*3/2 + 4*3/2
> ...


Maybe I was unclear.

I agree with you that they aren't interchangeable.

One is true when the frequencies are the same.

The other is true when the frequencies are different.

If you have 5sin(wt) + 2sin(wt) obviously it is just 7sin(wt)

But 5sin(wt) + 2sin(2wt) you obviously can't add without a little more math.

I think we agree.


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## Flyer_PE (Aug 6, 2008)

benbo said:


> I think we agree.


I think we agree also. I'm coming down with a head cold and I think it's making my thinking a little muddled.


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## mudpuppy (Aug 6, 2008)

I concur.

(really I'm just disappointed I didn't get to answer any questions. Darn job.)


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## benbo (Aug 6, 2008)

mudpuppy said:


> I concur.(really I'm just disappointed I didn't get to answer any questions. Darn job.)


I'm going out of town for a couple days (although I may have internet access). So you won't have to beat me to it! Just Jim and any other EEs who get in on the fun. Besides, somebody is bound to ask something about power and leave me for dead.


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