Chapter hvac engineering Fundamentals: Part 1 Introduction

17.3
First Law of Thermodynamics
The first law of thermodynamics sounds like the law of the conser-
vation of mass, with different vocabulary. If the law of mass conser-
Engineering Fundamentals: Part 2
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Engineering Fundamentals: Part 2
455
vation asserts that
matter
can be neither created nor destroyed, then
the first law of thermodynamics states that
energy
cannot be created
or destroyed. This implies that the various forms of energy may be
converted, one to another. It means that we can account for all energy
conversions in a system with accuracy.
Energy in
⫺
Energy out
⫽
Change in stored energy
Entropy
is used to define the unavailable energy in a system. In
another sense, entropy defines the relative ability of one system to act
on another. As things move toward a lower energy level, where one is
less able to act upon the surroundings, the entropy level is said to
increase. If we look at the universe as a whole, things are running
down, so the entropy of the universe is said to be increasing.