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3
Principle of Conservation
of Energy
3.1 FIRST LAW OF THERMODYNAMICS
In Chapter 2 it was shown that energy transfers from one system to another as either
work or heat. Energy transfer from one system to another is the only way that a
system gains or loses energy. The rst law of thermodynamics states that energy
is not generated or destroyed; it only changes from one form to another or transfers
from one system to another. This important physical principle can be expressed by
a simple mathematical equation. Consider a system with total initial energy E
i
as
shown in Figure 3.1. The total energy of this system after some time, during wh ...