Energy exists in many forms. For convenience,
we usually classify energy according to the size
and nature of the bodies or particles with which
it is associated. Thus we say that MECHANICAL
ENERGY is the energy associated with large
bodies or objects-usually, things that are big
enough to see. THERMAL ENERGY is energy
associated with molecules. Chemical energy is
energy that arises from the forces that bind the
atoms together in a molecule. Chemical energy
Figure 4-11.--Relationship of static and dynamic factors--
is demonstrated whenever combustion or any
Bernoulli's principle.
other chemical reaction takes place. Electrical
energy, light, X rays, and radio waves are
Figure 4-11 disregards friction and it is not
examples of energy associated with particles that
encountered in actual practice. Force or head is
are even smaller than atoms.
also required to overcome friction. But, unlike
Each of these types of energy (mechanical
inertia effect, this force cannot be recovered again
energy, thermal energy, and so forth) must also
although the energy represented still exists
be classified as being either stored energy or
somewhere as heat. Therefore, in an actual system
energy in transition.
the pressure in chamber B would be less than in
STORED ENERGY can be thought of as
chamber A. This is a result of the amount of
energy that is actually contained in or stored in
pressure used in overcoming friction along the
a substance or system. There are two kinds of
way.
stored energy: (1) potential energy and (2) kinetic
At all points in a system the static pressure is
energy. When energy is stored in a substance or
always the original static pressure LESS any
system because of the relative POSITIONS of two
velocity head at the point in question. It is also
or more objects or particles, we call it potential
LESS the friction head consumed in reaching that
energy. When energy is stored in a substance or
point. Both velocity head and friction represent
system because of the relative VELOCITIES of
energy that came from the original static head.
two or more objects or particles, we call it kinetic
Energy cannot be destroyed. So, the sum of the
energy.
static head, velocity head, and friction at any
If you do not completely understand this
point in the system must add up to the original
classification, come back to it from time to time
static head. This, then, is Bernoulli's principle;
as you read the following sections on mechanical
more simply stated, if a noncompressible fluid
energy and thermal energy. The examples and
flowing through a tube reaches a constriction, or
discussion given in the following sections will
narrowing of the tube, the velocity of fluid flow-
probably help you understand this classification.
ing through the constriction increases, and the
pressure decreases.
Mechanical Energy
Let's consider the two stored forms of
ENERGY
mechanical energy. Mechanical POTENTIAL
energy exists because of the relative positions of
Can you define energy? Although everyone
two or more objects. For example, a rock resting
has a general idea of the meaning of energy, a
on the edge of a cliff in such a position that it
good definition is hard to find. Most commonly,
will fall freely if pushed has mechanical
perhaps, energy is defined as the capacity for
potential energy. Water at the top of a dam has
doing work. This is not a very complete
mechanical potential energy. A sled that is being
definition. Energy can produce other effects which
held at the top of an icy hill has mechanical
cannot possibly be considered work. For example,
potential energy.
heat can flow from one object to another without
Mechanical KINETIC energy exists because
doing work; yet heat is a form of energy, and the
of the relative velocities of two or more objects.
process of heat transfer is a process that produces
If you push that rock, open the gate of the dam,
an effect.
or let go of the sled, something will move. The
A better definition of energy, therefore, states
rock will fall; the water will flow; the sled will slide
that energy is the capacity for producing an effect.
down the hill. In each case the mechanical
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