Energy is a property of the system
By rearranging
we can have
∫2b1 (dQ – dW) = ∫2c1 (dQ – dW)
It shows that the integral is the same for the paths 2-b-1 and
2-c-1, connecting the states 2 and
1. That is, the quantity ∫ (dQ – dW) does not depend on the path
followed by a system, but depends only on the initial and the final states of
the system. That is ∫ (dQ – dW) is an exact differential of a property. This
property is called energy (E). It is given by
dE = dQ-dW E = KE + PE
+U
where U is the internal
energy. Therefore,
dE = d(KE) + d(PE) + dU = dQ-dW
Quit often in many situations the KE or PE changes are negligible.
dU = dQ – dW
An isolated system does not exchange energy with the
surroundings in the form of work as well as heat. Hence dQ = 0 and dW = 0. Then the first law of thermodynamics reduces to dE
= 0 or E2 = E1 that is energy of an isolated
system remains constant.
