1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Understanding ratios

  1. Jan 14, 2008 #1

    I'm currently in my 2nd year of physics and one thing I've noticed is how often problems can be solved using ratios, for example I had a problem in my thermodynamics class that involved finding a temperature after knowing its pressure and using the triple-point of water and its pressure as a reference point, I had put so much effort into this question and could not understand it, then a friend in my class solved in very easily using ratios. This is something that happens frequently with me (I think I over think the issue), anyways.... does anyone have any tips in recognizing this type of problem solving using a ratio?

  2. jcsd
  3. Jan 14, 2008 #2


    User Avatar
    Science Advisor
    Homework Helper

    Look for closed systems.
    Know general laws like the gas law.
    Recognise opposing effects, eg. temperature and pressure.

    Dimensional analysis is also a big help.
  4. Jan 14, 2008 #3
    I guess the problem I have with it is, if I have two equations, like with this example it was PV = nRT , but in this case I would have that equation twice (one for the totally known system and one for the "half" known system), and since n,R and V are constant I can just ignore them, but what is it that allows me to then say P1/T1 = P2/T2, which is what I did to solve for the unknown temp... does my question make sense, I hope it does

    Thanks for the reply
  5. Jan 15, 2008 #4
    I am not sure if understood your question.
    For this example, it is quite simple:
    1st state : P1V1/T1 =nR and 2nd state: P2V2/T2 =nR so
    P1V1/T1 = P2V2/T2
    That equation holds for any fixed amount of gas (ideal)
    So you can have some other derivative equations :
    T = const ==> P1V1=P2V2
    V=const ==> P1/T1=P2/T2 etc..
  6. Jan 15, 2008 #5
    oh, ok, yeah I get it now. thanks :)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook