15.2 The First Law
Every thermodynamic system possesses an interval energy U and has the capacity to do external work
W which may be called the external energy. When heat energy is supplied (dQ) to a system then from the
law of conservation of energy it follows that the sum total of changes in the internal and external energy of
the system must equal the heat energy supplied to the system.
i.e. ........(1)
Equation (1) is the mathematical expression for the First law of Thermodynamics.
Note : (1) dU is an exact differential which just depends on the initial and the final states of the system
and not on the path connecting these states.
(2) dQ and dW are in exact differentials as they are path
dependent between the given initial and final states.
