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Thread: pressure vs temp

  1. #1 pressure vs temp 
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    if temp is proportional to pressure does that mean i can take a 100 psi pump plug it in to a 2L pop bottle at stp fill it up with (air to 100) and measure a temp of -80 when i release it?


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  3. #2  
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    No. The ideal gas equation is PV=nRT where P is pressure, V is volume, n is the number of moles, R is a constant, and T is the absolute temperature.

    So you can say pressure is proportional to temperature, but that is only true if n/V stays constant. In your example, when you pressurize the 2 liter bottle, you are stuffing more moles of gas into the same volume, so n/V is not a constant.

    Where did you get the -80 from?


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  4. #3  
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    well i have seen science where you boil water by dropping pressure , so i figure the same is when you pressurize something, but opposite, isnt that why butane is cold, cos its compressed?
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  5. #4  
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    The reason why water boils when you lower the air pressure around it is not because temperature rises (it doesn't) but because the boiling point is lowered. When climbing mountains you can observe this in that the cooking time is longer as the food boils at a lower temperature.

    If I have understood it correctly the reason is following. At the water surface there is a constant exchange of vapor and liquid water, even in room temperature, however the two processes: vapor becoming water and water vaporising are in balance. Lowering the pressure reduces the amount of available water vapor shifting the balance so that water vaporises faster than vapor becomes liquid causing increased vaporisation until a new equilibrium is met. If pressure drops very quickly, as in that experiment with a glass of water in a vacuum chamber the water will appear to be boiling.

    Not 100% sure, but that is how I understood it at least.
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  6. #5  
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    well that does sound right actually, but again, what about butane when it escapes a pressured vessel, it is colder,

    or when i leave my lighter in the sun, it builds pressure


    another example i'd like to put forward is in the refrigeration cycle, in the first step CFC is compressed where it exhausts the heat,, after traveling to the lower pressure coil where it absorbs heat, thus cooling the refrigerator
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  7. #6  
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    I can answer the lighter one:

    recall that
    Volume of the lighter , number of moles and gas constant are constant in this process. (ideally no gas escapes, and lighter does not change size) what remains then is that pressure is proportional to temperature .

    edit: I am not quite familiar with what happens when a pressurised butane bottle is leaking so I skip that one. I can try to explain the refrigerator one, but I think I missed the question. Anyway, signing out now.
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  8. #7  
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    There is a decrease in temperature when an ideal gas expands and an increase when it is compressed. It is called adiabatic heating and cooling.
    http://en.wikipedia.org/wiki/Adiabat...ng_and_cooling

    At the end of the following wikipedia section there is a formula giving the final temperature in terms of the initial temperature, the ratio of final and initial pressure, and the adiabatic index, or heat capacity ratio.

    http://en.wikipedia.org/wiki/Adiabat...screte_formula

    http://en.wikipedia.org/wiki/Adiabatic_index
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  9. #8  
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    presactly, T2=T1(P2/P1)^((r-1)/r) (r=1.4 in air 1.3 in co2)

    so -80 = 10(P2/1)^(0.4/1.4)


    -80/10 = P2^0.28
    -8 =P2^0.28

    P2= - 1448psi?

    unless i use temp in kelvin then......

    190 = 280 (P2/P1) ^ (0.4/1.4)
    190/280 = (P2 ^ 0.28)
    0.678 = 0.256^0.28

    P2= 0.256psi?

    it seems more likely that i need 1500psi to freeze co2?
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  10. #9  
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    Always Kelvin in such formulas, unless the formula contains the conversion by itself or deals with temperature differences.
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