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Meaning of symbols in given equation 
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#1
Jan813, 09:40 AM

P: 9

In my textbook, under the subtopic 1st law of of Thermodynamics, an equation is given as foll :
W=JQ 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
Jan813, 12:43 PM

P: 5,462

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. 


#3
Jan813, 02:28 PM

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It's mich simpler to use SI units, and measure both heat and work in the same units (joules). 


#4
Jan913, 07:12 AM

P: 9

Meaning of symbols in given equation
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. 


#5
Jan1013, 12:37 AM

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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 


#6
Jan1213, 07:52 AM

P: 9

"Work (in joules) = (4.18 joules per calorie) x Q (in calories)"
Wow really its clear now !!!! Thanks a lot 


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