Implications Of The First Law

• The consequence of the First Law is that in energy conversion procedures including heat to work conversion (i.e., work to heat conversion) no more work (or heat) might be generated than 1 N. m/joule or 1 joule/N.m. It will be observed later, that there is a further constraint on the conversion of heat to the work.

• The fixed rate of exchange among heat and work allows each to be stated in the units of the other. Therefore heat can be stated in N.m, whereas work can be stated in joules or kcal. It does not mean, obviously, that heat and work is similar thing. We have taken immense care differentiate among work and heat. In the SI units, with J = 1 N.m/joule, the first Law of Thermodynamics for a cyclic process becomes

    ∫ δ Q =  ∫ δ W         

• We no longer require independent process for measuring work and heat. Instead of determining the value of by cautious experimentation, either the heat or the work standard might be abandoned, and J might be stated to have a definite value, namely, 4.1868 × 103 N.m/kcal or 1 N.m/joule.

• It can be observed that the First Law, as stated by the above two equations associates the boundary interactions taking place whenever a system or its interactions with its surroundings; it is only essential that the procedure should be cyclic.

Example:

In a cyclic process the work transmits are + 20 J, − 10 J, − 10 J, and + 30 J. Determine the total transfer of heat for this cyclic process?

Solution:

Apply the First Law as specified by the equation shown below:

     ∫ δ Q =  ∫ δ W = (+ 20 – 10 – 10 + 30)J        

                           = + 30 J