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Thread: Under Pressure

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    Under Pressure

    In general, if you have a gas in a container and you double the amount of gas, the new pressure will be double the old pressure.
    So my question is this: If you have a tyre filled with the standard 32 psi and you double the amount of air molecules in the tyre, what pressure will your tyre gauge now read? Assume that the tyre does not expand, and that the first sentence is exactly true.
    <font color=blue><font face="Script MT Bold"><big>Rob</big></font face=script></font color=blue>

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    Re: Under Pressure

    <span style="background-color: #FFFF00; color: #000000; font-weight: bold"><font color=yellow>78.7 psi

    32 psi = 32psi (gauge). It is gauge pressure where 0 psig = atm pressure = 14.7 psi absolute. (which is where you see the dbling). so the tire is 32+14.7 = 46.7 psia. Dbl the molecules = double the pressure (assuming the same volume, and temperature)= 93.4 psia = 78.7 psig</font color=yellow></span hi>


    Steve

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    Re: Under Pressure

    Clearly, you were not under pressure with this one, Steve.

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    Re: Under Pressure

    Oops. Forgot to log in.
    <font color=blue><font face="Script MT Bold"><big>Rob</big></font face=script></font color=blue>

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    Re: Under Pressure

    <span style="background-color: #FFFF00; color: #000000; font-weight: bold"><font color=yellow>Just tried it and the gauge reads 64. Mind you, I carried out the test on my moon buggy. </font color=yellow></span hi> <img src=/S/grin.gif border=0 alt=grin width=15 height=15>

    Alan

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    Re: Under Pressure

    That must be why it's a little on the low side.
    <font color=blue><font face="Script MT Bold"><big>Rob</big></font face=script></font color=blue>

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    Re: Under Pressure

    I would bet that you didn't use tires that did not "expand" with the added pressure!

    Steve

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    Re: Under Pressure

    Steve
    I don't understand the significance of this. <span style="background-color: #FFFF00; color: #000000; font-weight: bold"><font color=yellow>There's only a 14 psi difference between the two situations.</font color=yellow></span hi>

    Alan

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    Re: Under Pressure

    By "this" are you refering to the question itself or the remark about the tire expanding or a difference in the pressure?

    Steve

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    Re: Under Pressure

    The issue of whether or not the tyre "expanded". Obviously, if it does expand appreciably it will have a significant effect on the answer. But I suspect you have another reason for making the comment - that's my query.

    Alan

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    Re: Under Pressure

    I don't know if you read my original response.

    <span style="background-color: #FFFF00; color: #000000; font-weight: bold"><font color=yellow>When you double the amount the double the absolute pressure not the gauge pressure: 64 psig is not twice 32 psig since the 0 psig has pressure: it is not a true "no pressure" to relate to. 0 psig is atm pressure, where doubling needs to be related to a system that has a true 0. It is the same with Temperature. 50

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    Re: Under Pressure

    I'd say my attempted humour has gone astray (again). I meant to suggest that the test I supposedly carried out on my moon buggy was actually carried out on the moon. <span style="background-color: #FFFF00; color: #000000; font-weight: bold"><font color=yellow>In this case, the "atmospheric" pressure could be approximated to zero and the gauge pressure (difference) could be approximated to the absolute pressure in the tyre.</font color=yellow></span hi> Maybe I should rezero/ recalibrate my own humour gauge.

    Alan

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    Re: Under Pressure

    AHH,
    I missed the "joke". I thought you actually did the experiment (on earth). I guess he didn't stipulate where the testing was done. Your answer would be correct if done on the moon.

    Email notes are such an inefficient means for humor sometimes.

    Steve

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