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SoS again .

  1. Nov 23, 2007 #1

    RKT

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    SoS again .........

    Hiya ,

    The commonly held view is that speed of sound (SoS) in a gas depends on temperature and not pressure. I want to know if this is always true. Consider the following two cases and let me know what you think.

    In the first case, we have cylinder made of a bad conductor of heat such some strong plastic or something. The gas is compressed in the cylinder. Here the temperature will rise and the heat more or less is confined to the cylinder. SoS will be higher than at normal temp. as the increased temperature results in higher molecular velocities inside the cylinder. Fair enough.

    In the second case, the cylinder is made of a good conductor such as a metal. Here the heat of compression will lost to the surroundings within a few minutes. The temp. inside the cylinder will be the same as the outside temp. by then. Theory says that since the temp. is the same, the SoS inside will the same as outside.

    My question : For the second case, given that the molecules are packed much more closely than at atmospheric pressure, wont any disturbance (sound) propagate quicker than at atm. pressure ? When I researched the dependence of SoS of a gas on temp. and pressure on the net, I observed that writers always cite the behavior of sound in the earth's atmosphere as proof of independence of pressure on SoS. To my mind, this is not a good example because this looks like just one particular case where pressure indeed has no bearing. In the atmosphere a change in pressure usually means a change in temp. as well and vice versa so one need not consider the two seperately.

    Regards
    RT
     
  2. jcsd
  3. Nov 23, 2007 #2

    mgb_phys

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    Only true for an ideal gas - a real gas it depends on temperature (mostly) and density (slightly)
     
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