kilowatt question

Discussion in 'Homework Help' started by bwd111, Aug 15, 2013.

  1. bwd111

    Thread Starter Member

    Jul 24, 2013
    117
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    Single phase load with current draw of resistive element is 38 A at an impressive volatage of 235. My answer was 8.9 kw and I was told this was wrong. Ive done the calc over and over and still come out with same answer. Here is my work 38 × 235/ 1000=8.9kw
     
    Last edited: Aug 15, 2013
  2. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Is there another part of the load which is not resistive which would have a power factor calculation?

    Is the 235 V an RMS voltage?
     
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  3. bwd111

    Thread Starter Member

    Jul 24, 2013
    117
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    That was all that was in equation
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,007
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    If no other information is given then it's reasonable to assume that both the current and voltage are RMS and your calculation is correct. If you stated that when you gave your answer then I don't see how it can be called wrong. :confused:
     
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  5. assembler_C

    New Member

    Mar 19, 2008
    7
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    The answer is 8.93 kVA.

    Transformer ratings always are in Volt-Amperes, not Watts.
     
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  6. WBahn

    Moderator

    Mar 31, 2012
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    Where in the OP's post did he mention or imply that he was talking about transformer ratings?

    bwd111: It looks, once again, like your school has a bad question (either insufficient information or a wrong answer) and your instructors don't know enough to recognize either.

    And, again, UNITS!

    What is so hard about going?

    P = 38A * 235V = 8930W = 8.93kW [EDIT: Fixed type -- was stated as 8.39kW originally. Thanks The RB]
     
    Last edited: Aug 20, 2013
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  7. LDC3

    Active Member

    Apr 27, 2013
    920
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    Must be some of those old fashion watts, not the new metric watts. You know those ones that aren't 1000W per kilowatt. ;)
     
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  8. WBahn

    Moderator

    Mar 31, 2012
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    You're thinking of a kibiwatt (KiW). :D
     
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  9. bwd111

    Thread Starter Member

    Jul 24, 2013
    117
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    Some people need the / 1000 to learn how to move the (.) I guess
     
  10. WBahn

    Moderator

    Mar 31, 2012
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    You are completely missing the point.

    38 x 235 / 1000 is NOT kilowatts! It is a number. Just like 72 in NOT my height, but 72 inches IS.

    38 AMPS x 235 VOLTS is a power, namely 8930 WATTS.

    8930 and 8.93 are NOT the same! They are two different numbers. The first is 8921.07 larger than the second. If you just divide a number by 1000 you end up with a number that is a different number than you started with.

    If you want to move the decimal point explicity, then multiply that quantitiy by one so that you don't change the value.

    1000W = 1kW

    So divide both sides by 1000V and you have that

    1kW/1000W = 1

    Thus

    8930W * (1kW/1000W) = (8930/1000) * (W*kW)/(W) = 8.93kW

    I'm not going to stop harping on this. If nothing else, someone reading it may take the hint.

    I fully realize that you likely won't, at least not until you are trying to convince a jury that you shouldn't be found guilty of criminal negligence for getting a bunch of people killed when your very notes show that had you simply bothered to look at the units of your own work you would have realized that the system you designed was guaranteed to fail.
     
    Last edited: Aug 16, 2013
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  11. bwd111

    Thread Starter Member

    Jul 24, 2013
    117
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    Thats how i did the calc 38*235/1000=8.93
     
  12. LDC3

    Active Member

    Apr 27, 2013
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    But you are missing the units. Therefore, it is wrong!
     
  13. WBahn

    Moderator

    Mar 31, 2012
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    Consider this analogy:

    A woman goes to the doctor because she isn't feeling well. The doctor examines her and determines that she has an infection that can be treated with a particular drug and looks in the Physicians Desk Reference to get the proper dosage, sees that it is given per unit of body weight and so he writes 10 on the chart, tells the nurse to weigh her and give her chart to the P.A. who is to then write a presciption. The nurse weighs her and writes down 100 and gives the chart to the P.A., who multiplies the two numbers together, gets 1000, and writes a prescription for 1000mg. She dutifully takes the drugs as prescribed and after a week she falls seriously ill, is rushed to the hospital, and dies. When the autopsy is performed, it is determined that she died from a massive overdose of that drug. The medical examiner requests her records and gets the chart. Upon looking in the Physician's Desk Reference, he sees that dosage was given as 10μg/kg. He sees her weight written as 100, but having examined the body of this very petite woman realizes immediately that the weight was written down in pounds. So he does the following calculation:

    <br />
\text{dosage} \ = \ 10\frac{\mu g}{kg} \, \cdot \, 100lb \, \cdot \, \frac{0.454kg}{1lb} \ = \ 454 \mu g \, \cdot \, \frac{1mg}{1000 \mu g} \ = \ 0.45mg  \ \approx \ 0.5mg <br />

    He then sees that she was prescribed a dosage of 1000mg, more than 2000 times more that she should have been taking.

    The family then sues the clinic for malpractice leading to wrongful death. On the stand, the doctor is asked why he didn't put the units on his notation of the dosage he commented that he always writes it that way and it's always in μg/kg. When asked why the patient's weight was written in pounds with no units indicated, the nurse remarked that that's the number that the scale showed. When asked why the units weren't tracked when calculating the prescription dosage, the P.A. stated that you always just multiply the dosage by the weight and the answer is always in milligrams.

    In closing, the plaintiff's lawyer says, "Ladies and gentlemen of the jury, my clients have suffered a grevious loss. That man has lost his wife of nearly ten years, that couple has lost their only daughter, and those four young children have lost there mother. And why? Because a doctor couldn't be bothered to write five characters, "μg/kg", after a number. Because a nurse couldn't be bothered to write just two characters, "lb", after a number, and because a physician's assistant could be bothered to insist that those characters be there or to perform the simply computation which, as you saw the medical examiner do from scratch and in his head, took less than 30 seconds to perform properly, tracking the units, to ensure an unambiguous and correct answer. Yes, people can and do make mistakes. Yes, sometimes honest mistakes have catastrophic consequences. But my clients aren't asking you to punish this clinic because of a series of honest mistakes. My clients are asserting that these mistakes should never have happened. That paying attention to units when reporting and using data isn't an inconvenience that can be ignored. That, in fact, paying attention to units is perhaps the single most effective means of catching and correcting mistakes before they have catastrophic consequences and the fact that it is so easy to do and do properly only underscores the negligence shown by failing to do so. Ladies and gentlemen of the jury, it is incumbent upon members of the health profession to go above and beyond the norm in their efforts to avoid mistakes and be held accountable when they don't, because their mistakes cost lives."

    How are you, as a member on this jury, going to vote?
     
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  14. Ramussons

    Active Member

    May 3, 2013
    557
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    Maybe the answer is wrong because of Decimal Place Accracy :D

    Should have been 8.93 KW instead of 8.9 KW

    Ramesh
     
  15. WBahn

    Moderator

    Mar 31, 2012
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    Getting back to the original question. Were you told WHY 8.9kW was wrong? Did you ask? After redoing the problem and getting the same answer, where you just told again that the answer was wrong? Did you ask?

    You (or someone on your behalf) is paying a considerable amount of money for this school to teach you something. You have a reasonable expectation to guidance from your instructors.
     
  16. WBahn

    Moderator

    Mar 31, 2012
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    Actually, if going for proper sig figs, 8.9kW would have been correct since the input values that were multiplied together included one that only had 2 sig figs.

    Oh, and KW is kelvin-watts. :eek:

    :p
     
  17. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Clearly not based on many of the questions I have answered here.....
     
  18. WBahn

    Moderator

    Mar 31, 2012
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    Having a reasonable expectation to something doesn't mean that he will exercise it or that he will receive it, no matter how reasonable it might be.
     
  19. Ramussons

    Active Member

    May 3, 2013
    557
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    Thanks. That's something I never knew.

    Learning Never Ends!

    Ramesh
     
  20. WBahn

    Moderator

    Mar 31, 2012
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    Yep. It's all part of sloppiness with units aggravated by the fact that people that will end up as techies generally get thoroughly mistaught by constantly being inundated with units bandied about by marketers and other technological illiterates. On top of that, it is dealt with piecemeal in formal education and is usually in the "Day 1" material that is presented and then assumed to be learned from that point on, but that is seldom graded with much rigor.

    In general, the use of uppercase and lowercase mean different things. In some cases one is defined and the other isn't, but I can't think of any units off the top of my head in which it is correct to just use either for the same thing. They also try to avoid using the same character with the same case for both a prefix and a unit, but that is not universal -- the obvious example is 'm' for milli and 'm' for meters. The latter being lowercase is the consequence not only of history, but also the protocol of using capital letters for units named after people and lowercase for other units.

    One that I was always getting wrong because I didn't know any better was that when using a unit named after a person, you use uppercase for the unit, but you do not capitalize it when you spell it out, so you have 10F or 10 farads; you do not have 10 Farads.

    Here is a guide from NIST that I just found. It is interesting reading. I learned a couple of things from it that I didn't know and it raised a couple of questions that I need to look into further.

    http://physics.nist.gov/Pubs/SP811/sec07.html

    But I was very happy to see the very first sentence because it explicitly states the position I have been espousing for decades: The value of a quantity is its magnitude expressed as the product of a number and a unit.
     
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