Question on unit conversion with temperatures


by sgstudent
Tags: conversion, temperatures, unit
jbriggs444
jbriggs444 is offline
#19
Apr26-13, 08:48 AM
P: 748
Quote Quote by sgstudent View Post
Hi sorry for bringing this up again, but when my teacher explained this question
Given:
[...]
Change in 1 K = Change in 1.8F
The T in PV=nRT does not denote a change in temperature. It denotes a reading on an absolute temperature scale.

So I was thinking that the better explanation was what MrAnchovy pointed one that in that question the temperature refers to T-T0. So in this case, it would make sense to use the change in conversion ratio.
Yes. Since absolute scale readings are differences between temperature T and temperature T0 (aka absolute zero) the conversion factor to change from one absolute scale reading to another absolute scale reading is the same as the conversion factor to change from one temperature difference to another temperature difference. The conversion factor of 1.8 does work.

As MrAnchovy points out, you'd better be converting between Kelvin and Rankine, not between Kelvin and Fahrenheit though. Use of the Fahrenheit scale would be wrong, wrong, wrong. It's not an absolute scale.
sgstudent
sgstudent is offline
#20
Apr26-13, 10:25 AM
P: 636
Quote Quote by MrAnchovy View Post
This is ridiculous.
  1. This is nothing to do with Mathematics
  2. This is homework and should be in the appropriate section
  3. The question you have been set is pointless - nobody uses units like those any more. There is some sense in working with conversions of things like BTUs, lb.ft, atm etc. as these are used for historical reasons in some branches of engineering. But lb-mole?
  4. You simply cannot express the gas constant in Farenheit because the product of temperature and volume is not proportional to the temperature in Farenheit. If you want to express it on this scale you MUST use the Rankine scale. Whoever set this question is a fool.
  5. 'He explained that the fact that its a per kelvin as indicated by the divide line, it would instantly mean that the temperature units conversion are of "change in"'. This explanation is rubbish. Does the "divide line" mean that there is a "change in" the number of molecules too?
Oops sorry for 1. Somehow I associated units conversion with mathematics. But also, thanks for the help! Your explanation is very clear.

Quote Quote by jbriggs444 View Post
The T in PV=nRT does not denote a change in temperature. It denotes a reading on an absolute temperature scale.



Yes. Since absolute scale readings are differences between temperature T and temperature T0 (aka absolute zero) the conversion factor to change from one absolute scale reading to another absolute scale reading is the same as the conversion factor to change from one temperature difference to another temperature difference. The conversion factor of 1.8 does work.

As MrAnchovy points out, you'd better be converting between Kelvin and Rankine, not between Kelvin and Fahrenheit though. Use of the Fahrenheit scale would be wrong, wrong, wrong. It's not an absolute scale.
Oh I think I get it now. The formula is an absolute temperature T so it wouldn't be totally correct to use T-T0 if I convert it to a Fahrenheit scale?

And I apologize for posting in the wrong sub-forums. Thanks for the help :)


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