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LAWS AND PRINCIPLES
Verbal: The volume of a gas is directly proportional
Sometimes you can observe a series of relationships that to the (absolute) temperature for a given
seem to happen over and over. There is a popular saying, amount if the pressure is constant.
for example, that “if anything can go wrong, it will.” This is
called Murphy’s law. It is called a law because it describes a Equation: ΔV = ΔTk
relationship between events that seems to happen time after
time. If you drop a slice of buttered bread, for example, it can Graph:
land two ways, butter side up or butter side down. Accord-
ing to Murphy’s law, it will land butter side down. With this
example, you know at least one way of testing the validity of
Murphy’s law.
Another “popular saying” type of relationship seems to Increasing volume
exist between the cost of a houseplant and how long it lives.
You could call it the “law of houseplant longevity” that the
life span of a houseplant is inversely proportional to its pur-
chase price. This “law” predicts that a ten-dollar houseplant Increasing temperature
will wilt and die within a month, but a fifty-cent houseplant
will live for years. The inverse relationship is between the FIGURE 1.14 A relationship between variables can be de-
variables of (1) cost and (2) life span, meaning the more you scribed in at least three different ways: (1) verbally, (2) with an
pay for a plant, the shorter the time it will live. This would equation, and (3) with a graph. This figure illustrates the three
also mean that inexpensive plants will live for a long time. ways of describing the relationship known as Charles’ law.
Since the relationship seems to occur time after time, it is
called a “law.”
A scientifi c law describes an important relationship that principle. This principle is concerned with the relationship
is observed in nature to occur consistently time after time. between an object, a fluid, and buoyancy, which is a specific
Basically, scientific laws describe what happens in nature. The phenomenon.
law is often identified with the name of a person associated
with the formulation of the law. For example, with all other
factors being equal, an increase in the temperature of the air MODELS AND THEORIES
in a balloon results in an increase in its volume. Likewise, a Often the part of nature being considered is too small or too
decrease in the temperature results in a decrease in the to- large to be visible to the human eye, and the use of a model is
tal volume of the balloon. The volume of the balloon varies needed. A model (Figure 1.15) is a description of a theory or
directly with the temperature of the air in the balloon, and idea that accounts for all known properties. The description
this can be observed to occur consistently time after time. can come in many different forms, such as a physical model, a
This relationship was first discovered in the latter part of computer model, a sketch, an analogy, or an equation. No one
the eighteenth century by two French scientists, A. C. Charles has ever seen the whole solar system, for example, and all you
and Joseph Gay-Lussac. Today, the relationship is sometimes can see in the real world is the movement of the Sun, Moon,
called Charles’ law (Figure 1.14). When you read about a sci- and planets against a background of stars. A physical model
entific law, you should remember that a law is a statement that or sketch of the solar system, however, will give you a pretty
means something about a relationship that you can observe good idea of what the solar system might look like. The physical
time after time in nature. model and the sketch are both models, since they both give you
Have you ever heard someone state that something be- a mental picture of the solar system.
haved a certain way because of a scientific principle or law? For At the other end of the size scale, models of atoms and
example, a big truck accelerated slowly because of Newton’s laws molecules are often used to help us understand what is hap-
of motion. Perhaps this person misunderstands the nature of pening in this otherwise invisible world. A container of
scientific principles and laws. Scientific principles and laws do small, bouncing rubber balls can be used as a model to ex-
not dictate the behavior of objects; they simply describe it. They plain the relationships of Charles’ law. This model helps you
do not say how things ought to act but rather how things do see what happens to invisible particles of air as the tempera-
act. A scientific principle or law is descriptive; it describes how ture, volume, or pressure of the gas changes. Some models
things act. are better than others are, and models constantly change as
A scientifi c principle describes a more specific set of rela- our understanding evolves. Early twentieth-century models
tionships than is usually identified in a law. The difference be- of atoms, for example, were based on a “planetary model,” in
tween a scientific principle and a scientific law is usually one which electrons moved around the nucleus as planets move
of the extent of the phenomena covered by the explanation, around the Sun. Today, the model has changed as our under-
but there is not always a clear distinction between the two. standing of the nature of atoms has changed. Electrons are
As an example of a scientific principle, consider Archimedes’ now pictured as vibrating with certain wavelengths, which

1-17 CHAPTER 1 What Is Science? 17