Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Must convert units?

  1. Dec 20, 2008 #1

    KFC

    User Avatar

    I am reading something about thermodynamics and everything are not given in SI units. In the material, they use Fahrenheit for temperature, BTU for heat, lb for mass and in^2 for area. Now I need to calculate something with the data given in those units, well, I need to apply equation for ideal case, to assure the result of my calculation makes sense, must I convert all unit to SI? I know ideal gas equation defined in Kelvin unit. Can I only convert the temperature to Kelvin while keeping others units unchanged ?
     
  2. jcsd
  3. Dec 20, 2008 #2

    russ_watters

    User Avatar

    Staff: Mentor

    You can google all the conversions you need. I suggest using whatever units you are comfortable with - that's what I do. I switch back and forth, depending on the situation.
     
  4. Dec 20, 2008 #3

    nicksauce

    User Avatar
    Science Advisor
    Homework Helper

    Using temperature is a bit more tricky than other quantities, since for other quantities there is usually a well-defined ratio for different for units, but for different units of temperature there is a ratio with an offset. To be safe, it's usually best to do all temperature calculations in Kelvin.
     
  5. Dec 20, 2008 #4

    mgb_phys

    User Avatar
    Science Advisor
    Homework Helper

    Remember google does automatic conversation of units - just type the question into the search box

    enter "9.81m/s^2 in furlongs/fortnight^2"
    gives the answer "9.81 (m / (s^2)) = 7.13499487 × 10^10 furlongs / (fortnight^2)"
     
  6. Dec 20, 2008 #5

    KFC

    User Avatar

    Thanks all your reply. I know how to do the units conversion, my question is just want to know if it is a must to convert temperature to kelvin before calculation? Since I was told that ideal gas equation only work in absolute temperature.
     
  7. Dec 20, 2008 #6

    mgb_phys

    User Avatar
    Science Advisor
    Homework Helper

    You can use Rankine (the Fahrenheit absolute scale) and the value of R will be different but otherwise the equation is the same.
     
  8. Jan 4, 2009 #7
  9. Jan 4, 2009 #8

    Redbelly98

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Correct. Temperature must be in absolute units. Both Kelvin and Rankine units would work. Fahrenheit and Celsius will not.

    "absolute temperature" and "Kelvin" are not synonymous.
     
  10. Jan 7, 2009 #9
    We live in a mixed world, and it is much smarter to learn to function with both SI and US Customary units. Those who think that every thing must be converted to SI units before any calculations can be made will make a lot of pointless conversions, and then will find that it is necessary to convert the final results back in many cases. Until such time as the US adopts the SI system, it is foolish not to learn to function in both systems.
     
  11. Jan 7, 2009 #10

    KFC

    User Avatar

    Thanks for reply. Well, I totally understand that. The reason I am asking this is because in some case you don't have freedom to use any unit you want. For example, in the state equation of ideal gas, we cannot plugin the termperature in Celsius. So I am just thinking it is safe to use SI units in calculation.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?