Page 260 - Schaum's Outline of Theory and Problems of Applied Physics

CHAPTER 21

Thermodynamics

FIRST LAWOF THERMODYNAMICS

Converting internal energy to mechanical energy is much more difficult than the reverse, and perfect efficiency is
impossible. A heat engine is a device or system that can perform this conversion; the human body and the earth’s
atmosphere are heat engines, as are gasoline and diesel motors, aircraft jet engines, and steam turbines. All heat
engines operate by absorbing heat from a reservoir of some kind at a high temperature, performing work, and
then giving off heat to a reservoir of some kind at a lower temperature (Fig. 21-1).
High-temperature reservoir

T 1

Heat Q 1
input

Heat Work
engine output
W

Heat
Q
exhaust 2
T 2

Low-temperature reservoir
Fig. 21-1

Two general principles apply to all heat engines. The first law ofthermodynamics is an expression of the
principle of conservation of energy. According to this law, in any process that a system of some kind (such as a
heat engine) undergoes, we have
Q = U + W

Net heat input = change in internal energy + net work output
Here Q is the net heat added to the system during the process; if the system gives off heat, Q is negative. When
the internal energy of the system U increases, U is positive; when U decreases, U is negative. The net work
done by the system during the process is W; if work is done on the system, W is negative.
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