Is the apparent power calculation in this text WRONG? - Electronic Circuits Projects Diagrams Free

powersys · 10th January 2006, 06:38 AM

The attached example was scanned from a textbook. I tried to calculate S1 but the answer I got was different from the one shown in text. Could someone please help me to verify whether the solution provided in the text is CORRECT or WRONG?

My calculation:

I
= (V1 - V2) / Z
= (240 - 208 - j120) / j10
= -12-j3.2

S1
= V1 x (-I*)
= 240 x (12-j3.2)
= 2880-j768

S2
= V2 x (I*)
= (208+j120) X (-12+j3.2)
= -2880-j774.4

Russlk · 11th January 2006, 03:19 AM

The only error I see in your calculation is: j squared = -1 so the current is +12-j3.2.

powersys · 11th January 2006, 06:01 AM

Quote:

Originally Posted by Russlk

The only error I see in your calculation is: j squared = -1 so the current is +12-j3.2.

I calculated the i using CASIO fx-570W. Pls advise where is "j squared" in your last message from? Thanks.

Russlk · 11th January 2006, 03:25 PM

j is the representation of (-1)^.5, an impossible math function, therefore called "imaginary". It is used to represent a 90 degree phase relation.

powersys · 11th January 2006, 09:21 PM

Quote:

Originally Posted by Russlk

j is the representation of (-1)^.5, an impossible math function, therefore called "imaginary". It is used to represent a 90 degree phase relation.

Do you think the steps and calculation shown in the text (attached image in my first post) are CORRECT? Pls advise. Thanks.

Russlk · 12th January 2006, 01:05 AM

In the text, the current is wrong; it should be +12-j3.2. Because of the current error, the value for S2 has the wrong sign but the numbers are correct. The value for S1 is wrong, I don't know how it was calculated.

Your calculation for S1 is correct. In the calculation for S2, you used (-12+j3.2) for the current which is not correct, otherwise you would have got the correct result.

powersys · 12th January 2006, 07:28 AM

In the text, the author add a '-' sign in front of I for the calculation of S1. Why did the author do so? Thanks.

Quote:

Originally Posted by Russlk

In the text, the current is wrong; it should be +12-j3.2. Because of the current error, the value for S2 has the wrong sign but the numbers are correct. The value for S1 is wrong, I don't know how it was calculated.

Your calculation for S1 is correct. In the calculation for S2, you used (-12+j3.2) for the current which is not correct, otherwise you would have got the correct result.

_3iMaJ · 12th January 2006, 09:16 AM

Quote:

Originally Posted by Russlk

j is the representation of (-1)^.5, an impossible math function, therefore called "imaginary". It is used to represent a 90 degree phase relation.

Actually j is not an impossible math function. It is possible to calculate the nth roots of unity (and nth roots of -1 if you wish). Its actually very interesting how sqrt(-1) falls out of the math.

**Nigel Goodwin** · 12th January 2006, 09:50 AM

Quote:

Originally Posted by _3iMaJ

Quote:

Originally Posted by Russlk

j is the representation of (-1)^.5, an impossible math function, therefore called "imaginary". It is used to represent a 90 degree phase relation.

Actually j is not an impossible math function. It is possible to calculate the nth roots of unity (and nth roots of -1 if you wish). Its actually very interesting how sqrt(-1) falls out of the math.

In that case perhaps you would care to show us the answer?.

I was certainly always taught that it's an impossible function, and the technique in using it was to have two of them that cancelled out (mind you, I'm going back a LONG time).

Still, I'll be interested to see how you can multiply two identical numbers together and get a negative result!.

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10th January 2006, 06:38 AM	(permalink)
powersys	Is the apparent power calculation in this text WRONG? The attached example was scanned from a textbook. I tried to calculate S1 but the answer I got was different from the one shown in text. Could someone please help me to verify whether the solution provided in the text is CORRECT or WRONG? My calculation: I = (V1 - V2) / Z = (240 - 208 - j120) / j10 = -12-j3.2 S1 = V1 x (-I) = 240 x (12-j3.2) = 2880-j768 S2 = V2 x (I) = (208+j120) X (-12+j3.2) = -2880-j774.4

11th January 2006, 03:19 AM	(permalink)
Russlk	The only error I see in your calculation is: j squared = -1 so the current is +12-j3.2. __________________ see my website: www.geocities.com/russlk

11th January 2006, 03:25 PM	(permalink)
Russlk	j is the representation of (-1)^.5, an impossible math function, therefore called "imaginary". It is used to represent a 90 degree phase relation. __________________ see my website: www.geocities.com/russlk

12th January 2006, 01:05 AM	(permalink)
Russlk	In the text, the current is wrong; it should be +12-j3.2. Because of the current error, the value for S2 has the wrong sign but the numbers are correct. The value for S1 is wrong, I don't know how it was calculated. Your calculation for S1 is correct. In the calculation for S2, you used (-12+j3.2) for the current which is not correct, otherwise you would have got the correct result. __________________ see my website: www.geocities.com/russlk