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Joule's Mechanical Equivalent of Heat

  1. Oct 3, 2011 #1
    As I understand it, J's MEoH is the calorific value of a substance. The standard value for this constant is given for raising 1 lb or kg of water by 1 degree.

    I am using equations which use J's MEoH but with other substances. The equations come from a book on steam turbines so they no doubt use J's MEoH constant for water.

    Like I said, if I am using these equations for other substances will J's MEoH be the same value or should I use the specific heat of the substance in question?

    Many thanks,
  2. jcsd
  3. Oct 3, 2011 #2


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    The MEoH is just the conversion rate between different units for the same thing.

    Mechanical work or energy is naturally measured in units of force and distance.

    Thermal energy was originallly measured in terms of the temperature rise of a unit volume of water.

    The two branches of physics were developed more or less indepedently, until the general ideas of conservation of energy and conversion of energy from one form to another were discovered

    In the SI system the same units (watts) are used for both, but the "old" units of calories, British Thermal Units, etc, are still used, and need the conversion factor.

    There is only value of the conversion factor, just like there are always 12 inches in 1 foot, but the specific heat of different materials is different, just like the density is different. You will find tables of specific heats taht give the value in both "thermal" and "mechanical" units.
  4. Oct 3, 2011 #3
    So if working in SI units I can just assume J to be unity?
  5. Oct 3, 2011 #4
    Can I take Joule's Mechanical Equivalent of Heat, J, as just unity when using SI considering it is just a conversion factor?
    Last edited by a moderator: Oct 3, 2011
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