flow; but you could hardly say that you added
may be unfamiliar to you, you probably know
flow.) The only kind of thermal energy that can
more about internal energy than you realize.
in any sense be added to or removed from a
Because molecules are constantly in motion, they
substance is INTERNAL ENERGY.
exert a pressure on the walls of the pipe, cylinder,
The distinction between heat and internal
or other object in which they are contained. Also,
energy must be clear in your own mind before you
the temperature of any substance arises from, and
can understand the basic principles of a gas
is directly proportional to, the activity of
turbine plant.
the molecules. Therefore, every time you read
thermometers and pressure gauges you are finding
Energy Transformations
out something about the amount of internal
energy contained in the substance. High pressures
The machinery and equipment in the engineer-
and temperatures indicate that the molecules are
ing plant aboard ship are designed either to carry
moving rapidly and that the substance therefore
energy from one place to another or to change
has a lot of internal energy.
a substance from one form to another. The
HEAT is a more familiar term than internal
principles of energy transformations and some of
energy, but may actually be more difficult to
the important energy changes that occur in the
define correctly. The important thing to remember
shipboard propulsion cycle are discussed in the
i s that heat is THERMAL ENERGY IN
following paragraphs.
TRANSITION--that is, it is thermal energy that
is moving from one substance or system to
CONSERVATION OF ENERGY.--The
another.
basic principle dealing with the transformation of
An example will help to show the difference
energy is the PRINCIPLE OF THE CON-
between heat and internal energy. Suppose there
SERVATION OF ENERGY. This principle can
are two equal lengths of pipe made of identical
be stated in several ways. Most commonly,
materials and containing steam at the same
perhaps, it is stated that energy can be neither
pressure and temperature. One pipe is well
destroyed nor created, but only transformed.
insulated; the other is not insulated at all. From
Another way to state this principle is that the total
everyday experience you know that more heat will
quantity of energy in the universe is always the
flow from the uninsulated pipe than from the
same. Still another way of expressing this principle
insulated pipe. When the two pipes are first filled
is the equation, Energy in = Energy out.
with steam, the steam in one pipe contains exactly
The energy out may be quite different in form
as much internal energy as the steam in the other
from the energy in, but the total amount of energy
pipe. We know this is true because the two pipes
input must always equal the total amount of
contain equal volumes of steam at the same
energy output.
pressure and at the same temperature. After a few
Another principle, the PRINCIPLE OF THE
minutes, the steam in the uninsulated pipe will
CONSERVATION OF MATTER, states that
contain much less internal energy than the steam
matter can be neither created nor destroyed,
in the insulated pipe, as we can tell by measuring
but onlv transformed. As vou probablv know
the pressure and the temperature of the steam in
the development of the atom bomb demonstrated
each pipe. What has happened? Stored thermal
that matter can be converted into energy; other
energy-internal energy-has moved from one
developments have demonstrated that energy can
system to another, first from the steam to the
be converted into matter. Therefore, the principle
pipe, then from the uninsulated pipe to the air.
of the conservation of energy and the principle
This MOVEMENT or FLOW of thermal energy
of the conservation of matter are no longer
from one system to another is called heat.
considered as two parts of a single law or principle
A good deal of confusion exists concerning the
but are combined into one principle. That prin-
use of the word heat. For example, you will hear
ciple states that matter and energy are inter-
people say that a hot object contains a lot
changeable, and the total amount of energy and
of heat when they really mean that it contains a
matter in the universe is constant.
lot of internal energy. Or you will hear that heat
The interchangeability of matter and energy
is added to, or removed from a substance.
is mentioned here only to point out that the state-
Since heat is the FLOW of thermal energy, it can
ment energy in must equal energy out is not
no more be added to a substance than the flow
strictly true for certain situations. However, any
of water could be added to a river. (You might
noticeable conversion of matter into energy or
add water, and this addition might increase the
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