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0 degrees celsius + 0 degrees celsius -

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Re: 0 degrees celsius + 0 degrees celsius -

#5

Post by JangoSteve »

Digging a little deeper...

  (0 degrees Celsius) + (0 degrees Celsius) - (0 degrees Celsius) = 0 degrees Celsius

  (0 degrees Celsius) - (0 degrees Celsius) = 0 kelvin

  (0 degrees Celsius) + (0 kelvin) = 0 degrees Celsius
The last 2 of the 3 are wrong, so I'd say this is definitley a bug.

EDIT: I think I figured it out. It's converting everything after the first temperature to Kelvin, without converting the first temp to Kelvin, and then just doing the arithmetic on the numbers...

  0C + 0C => 0C + 273K => 0 + 273 = 273

  0C + 0C - 0C => 0 + 273 - 273 = 0

  0C - 0C => 0 - 273 = -273 = 0K (ok, weird this one then converts back to K)

  0C + 0K => 0 + 0 = 0
EDIT 2: While this does match the results seen by google, I think _delirium's solution is more likely the reason.

Re: 0 degrees celsius + 0 degrees celsius -

#7

Post by _delirium »

It looks like it must be using kelvin as its canonical units:

  0C + 0C = 273.15 K + 273.15 K = 546.3 K = 273.15 C
The weirdness is that conversions like that don't work for units that have different zero points. Perhaps addition of temperatures isn't really well defined? If 273.15 K + 273.15 K does not equal 0 C + 0 C, despite 273.15 K = 0 C, it's not clear what addition "means" in this context.

One restricted form of addition that makes sense (which maybe is what's intended?) is to consider the 1st temperature an actual temperature, and the 2nd temperature a temperature difference in units of degrees-C (or degrees-K, or degrees-F). So e.g.:

  "5 C warmer than 30 C"
  = 30 C + 5 C
  = 303.15 K + 5 K
  = 308.15 K
  = 35 C
But then addition isn't commutative. The problem is that "degrees C" is overloaded here, to mean both a position on a scale, and the size of a unit on that scale, with no conventional short way of distinguishing which one you mean (which is one reason Kelvin was invented, since things aren't ambiguous if you put absolute zero at 0 K).

Re: 0 degrees celsius + 0 degrees celsius -

#9

Post by mhansen »

It's meaningless to add temperatures when the zero level is at +273.

It's like adding years relative to 2010 years ago - it would be meaningless to add this year - 2010 - with (say) 1666 - the year of the Great Fire of London. It's meaningful to take differences (the Great Fire happened 344 years ago), but not to add. (Unless we converted to some kind of absolute time units e.g. years since the big bang)

I'd say what it's doing - converting to an absolute scale then adding - makes sense.

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