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Meaning of symbols in given equation

  1. Jan 8, 2013 #1
    In my textbook, under the subtopic 1st law of of Thermodynamics, an equation is given as foll :
    where W=work done ;Q=heat supplied
    but they have not mentioned what is J.

    plz could someone tell me what exactly 'J' stands for in this equation
  2. jcsd
  3. Jan 8, 2013 #2
    Is this an old textbook?

    J is the symbol for a quantity called 'The mechanical equivalent of heat'.

    Both heat and work are forms of energy (as are all the other terms in the First Law) but we need to ensure that they are all in the same units.

    At one time, the metric measure for heat was the calorie, whilst the unit for work was the joule. Today we measure it all in joules.

    J is simply the conversion constant and is equal to 4.18 joules per calorie.
  4. Jan 8, 2013 #3


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    Homework Helper

    And if it was an old American textbook, it might be using Brithsh Thermal Units and horsepower...

    It's mich simpler to use SI units, and measure both heat and work in the same units (joules).
  5. Jan 9, 2013 #4
    well yeah I know in S.I. 'J' stands for joule...

    but in the equation W=JQ ; if we consider W(work) and Q(energy) in the same units, then here J seems to be some constant with no units and no dimensions ! [as 'J' relates W & Q with a multiplication and not addition or substaction]

    the equation was not given while solving a problem , but was stated like it is a basic formula.
  6. Jan 10, 2013 #5


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    Staff: Mentor

    By looking through other worked problems in that text book you should be able to deduce the system of units that book uses.

    I believe Studiot provided you with a very good explanation:

    Work (in joules) = (4.18 joules per calorie) x Q (in calories)

    illustrating how J has a value and units
  7. Jan 12, 2013 #6
    "Work (in joules) = (4.18 joules per calorie) x Q (in calories)"

    Wow really its clear now !!!!
    Thanks a lot
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