First Law of Thermodynamics

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The First Law of Thermodynamics states that:


  • The amount of heat energy(Q) which is absorbed or emitted by a thermodynamic system is equal to the sum of the system's internal energy change and the work which is generated or comsumed by it.


In equation form:


Q= \Delta U\ +W


where: Q=Qin-Qout, the net heat input; U is the system's Internal Energy; and W=Wout-Win, the net work output.

The First Law of Thermodynamics is the application of the Conservation of Energy in Thermodynamics.

       Example:
       
       A tank contains a hot fluid that is being stirred.  The initial internal energy of the fluid is 1000kJ.  During the course of 
       several hours, the fluid loses 600kJ of heat while the stirring does 250kJ of work on the fluid.  Determine the final 
       internal energy of the fluid.
       
       Solution-
        
       Use the first law of thermodynamics-
        
       \Delta U\ = Q - W
       \Delta U\ = (Q_{in}-Q_{out}) - (W_{out}-W_{in})
       \Delta U\ = (0kJ-600kJ) - (0kJ-250kJ)
       \Delta U\ = -350kJ
        
       The fluid loses 350kJ of energy for a final internal energy of 1000kJ - 350kJ = 650kJ
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