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Celsius and Farenheit question

  1. Jan 22, 2015 #1
    It's a quick question---just trying to resolve an argument between me and my friend--to convert Celsius temp. to Farenheit, one simply multiplies Celsius temp. with 5/9 and subtract by 160/9, NOT double the Celsius temperature and add 30, right? Please tell me which one is correct, for I think my conversion is correct, but I don't know how to explain her method is wrong, other than simply using an example as in, for example, if it is 5C, it should be 41F, NOT 40F......

    Any reply would be welcome.....
  2. jcsd
  3. Jan 22, 2015 #2

    Quantum Defect

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    You can figure this out yourself with two temperatures. Water boils at sea level at 100 C (212 F). Water freezes at 0 C ( 32 F). You have two points. You can use the two-point method to determine an equation for the calibration line, either from F to C or C to F.

    For your example, F is the dependent variable (Y) and C is the independent variable (X).

    Y = MX + B ==> F = M*C + B --> B is the Y intercept (What value of F is equal to 0 C ?) M is the slope (change in F over change in C).
  4. Jan 22, 2015 #3


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    You stated that you welcome any reply.....

    Quantum Defect has already answered your question. I will, however, bring up an important aspect of this on how one tackles such a question. It is a technique and methodology used in many areas, especially in science.

    What you should have done is to compare the two different "models" with known values. When you were told by your friend of her method, I am puzzled why you can't prove that she is wrong on the spot. Use her "equation", and see what you get! Ice melts at 0 C. Double that and add 30, and you get 30 F. Is this the correct ice-melting temperature in Fahrenheit? Surely you both know this number!

    Then continue further. Water boils at 100 C. Double that and add 30, you get 230. Again, is this the correct temperature in Fahrenheit for water to boil? Without even bothering to check if your conversion is right, you have proven that her conversion is wrong.

    Moral of the story here is that always carry out something you don't know for sure, and then compare it with KNOWN scenario. You may not know if her conversion is correct, but surely you both know the temperatures in Celsius and in Fahrenheit of the boiling and freezing point of water! At the very least, you should be able to look them up, even if you don't want to look up for yourself what the exact conversion is!

    This is one such way we analyze a problem, especially when we are faced with new ideas. Do they agree with what we already know? That is a very powerful question to answer.

  5. Jan 22, 2015 #4

    Quantum Defect

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    Hear hear!

    Learning to think like a scientist is sooooo important. When you are entering the realm of the unknown, you have to have the kinds of skills that ZapperZ talks about.

    Constantly asking yourself questions: "If my model is correct, what does it say about something that I know the answer to?" If your model gets the "known" wrong, your model is wrong. The biggest discoveries occur when your model can explain a well-known "known" but can also predict the answer for a measurable (even in the distant future) unknown. Think Einstein and Mercury (planet not Freddy), here.

    In the experimental laboratory, the same kind of discipline is important. "Hmmmmm... I don't see anything with my detector. What could be wrong?...." followed by a search along some chain of apparatus to find the part that isn't working. "How do I know the power supply is working? Let me try it with something I know works..." Eventually you will find the problem. By not immediately asking one of the old crusty scientists: "Why doesn't my experiment work?" you save yourself the embarrassment of having him/her tell you that you forgot to turn on some important bit of electronics gear, unblock the laser, etc. The hard part, though, is learning when to stop beating your head against the wall and asking for help.
  6. Jan 22, 2015 #5
    Thank you, thank you!!!!!!!!!!!!!!! Finally figured out that both of us were wrong!!!!!!!
    Last edited: Jan 22, 2015
  7. Jan 22, 2015 #6
    Actually, you are both wrong (although her equation is closer than yours). The correct equation is F = (9/5)C+32.

  8. Jan 22, 2015 #7


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    You've got this all mixed up. (And Mine is easier because it uses a simple integer offset.)

    To convert C to F: multiply by 9/5 then add 32.
    To convert F to C: subtract 32 then multiply by 5/9.

    Your friend's formulae are accurate within a few degrees in normal human range AND also easy to do in your head.

    To convert C to F: multiply by 2 then add 30.
    To convert F to C: subtract 30 then multiply by 1/2.

    Attached Files:

    Last edited: Jan 22, 2015
  9. Jan 22, 2015 #8


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    Another valuable lesson to take away from this episode is: Do your research.

    The formulae for temperature conversions are hardly atomic secrets. You can derive them from known data points, sure, and this is good practice, but they have been printed innumerable times in many different reference and text books. You don't have to make up your own formulas (unless you're bored) and you don't have to go by what someone said is the correct conversion.

    You don't have to know everything. Often, it's just as good to know where to look it up.
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