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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


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    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 :)
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