Ac Circuit Problem
Thread Starter
Senior Member
Joined: Feb 2006
Posts: 442
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From: NJ
Hey guys,
Normally I would never result to posting a h/w problem on a forum but I can't get ahold of my professor, nor TA and my exam is in less then 90 mins. Also, my friends are in exams so I'm really SOL.
So here I am. The equation P=VI cosine theta, I'm a little bit confused about. I get the deal w/ DC P=VI but regarding AC circuits, how do I find cos theta if its not given. Obviously if its given in the problem I know to use it, but if it's not given, do just go about the problem using P=V (rms) x V(rms) or do I have to find the angle. If that's the case, would the phase shift be assumed to be 90 degrees or something else? In some problems I've gotten it's been 30, 20 etc. If someone could just explain to me real quick this ordeal, it'd be greatly appreciated.
Thanks!
Normally I would never result to posting a h/w problem on a forum but I can't get ahold of my professor, nor TA and my exam is in less then 90 mins. Also, my friends are in exams so I'm really SOL.
So here I am. The equation P=VI cosine theta, I'm a little bit confused about. I get the deal w/ DC P=VI but regarding AC circuits, how do I find cos theta if its not given. Obviously if its given in the problem I know to use it, but if it's not given, do just go about the problem using P=V (rms) x V(rms) or do I have to find the angle. If that's the case, would the phase shift be assumed to be 90 degrees or something else? In some problems I've gotten it's been 30, 20 etc. If someone could just explain to me real quick this ordeal, it'd be greatly appreciated.
Thanks!
Thread Starter
Senior Member
Joined: Feb 2006
Posts: 442
Likes: 0
From: NJ
lol, i figured out the answer on my way to the exam, good timing if you ask me tongue.gif ...odd though, i thought DTN would have been all over this. I don't think anyone cares for the answer so I won't bother posting it...kind of lengthy anyways Eyecrazy.gif
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From: Leesville, Louisiana
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Sorry, I didn't see the thread.
DC has no phase. E=IR. P=IE.
If AC has no phase specified, you can use RMS(P)
A/C with inductance and capacitance can be tricky because of phase shifting. You can assume the electric company has properly shifted the current to match the voltage, but once it hits a separate device it's game on.
I remember phase shifting from school 8 years ago. That was probly the most math intensive part of my schooling. I hated it intensly. They gave us a 110VAC circuit with 10 caps and 10 inductors a handful of resistors, then said figure the output. After I calculated all the math, finally it came up to a phase of 0 and I was pissed off.
I havn't ever had to use it since then. Last time I used a power meter to check out-of-phase power was an output from a waveguide on a radar and that was a go/no go check. Before that, I had to tweak a power output for a RF modulator to adjust to a specified dB. Usually I deal with in-phase A/C, or current is not a factor.
For out-of-phase A/C, check this document out. It's pretty good http://www.ep2000.com/Templates/white%20pa...estFactorEP.pdf
What are you studying?
DC has no phase. E=IR. P=IE.
If AC has no phase specified, you can use RMS(P)
A/C with inductance and capacitance can be tricky because of phase shifting. You can assume the electric company has properly shifted the current to match the voltage, but once it hits a separate device it's game on.
I remember phase shifting from school 8 years ago. That was probly the most math intensive part of my schooling. I hated it intensly. They gave us a 110VAC circuit with 10 caps and 10 inductors a handful of resistors, then said figure the output. After I calculated all the math, finally it came up to a phase of 0 and I was pissed off.
I havn't ever had to use it since then. Last time I used a power meter to check out-of-phase power was an output from a waveguide on a radar and that was a go/no go check. Before that, I had to tweak a power output for a RF modulator to adjust to a specified dB. Usually I deal with in-phase A/C, or current is not a factor.
For out-of-phase A/C, check this document out. It's pretty good http://www.ep2000.com/Templates/white%20pa...estFactorEP.pdf
What are you studying?
Thread Starter
Senior Member
Joined: Feb 2006
Posts: 442
Likes: 0
From: NJ
Currently I'm on my way to becomming an electrical engineer and I feel your pain w/ given 10 caps, inductors, and handful of resistors. I had a similar scenario as part of my final test which required figuring everything out about the circuit. Luckily, we've only been dealing with this stuff in series circuits, rather than parallel making it "easier". Thanks for the document, with my final comming up on thursday, it can only help! fing02.gif