Fisica

PUZZLER
Soft contact lenses are comfortable to
wear because they attract the proteins in
the wearer’s tears, incorporating the
complex molecules right into the lenses.
They become, in a sense, part of the
wearer. Some types of makeup exploit
this same attractive force to adhere to
the skin. What is the nature of this force?
(Charles D. Winters)

chapter

Electric Fields
ChapterOutline
23.1
23.2
23.3
23.4

708

Properties of Electric Charges
Insulators and Conductors
Coulomb’s Law
The Electric Field

23.5 Electric Field of a Continuous
Charge Distribution

23.6 Electric Field Lines
23.7 Motion of Charged Particles in a
Uniform Electric Field

709

23.1 Properties of Electric Charges

T

he electromagnetic force between charged particles is one ofthe fundamental forces of nature. We begin this chapter by describing some of the basic
properties of electric forces. We then discuss Coulomb’s law, which is the fundamental law governing the force between any two charged particles. Next, we introduce the concept of an electric field associated with a charge distribution and
describe its effect on other charged particles. We then show how to useCoulomb’s law to calculate the electric field for a given charge distribution. We
conclude the chapter with a discussion of the motion of a charged particle in a
uniform electric field.

23.1
11.2

PROPERTIES OF ELECTRIC CHARGES

A number of simple experiments demonstrate the existence of electric forces and
charges. For example, after running a comb through your hair on a dry day, youwill find that the comb attracts bits of paper. The attractive force is often strong
enough to suspend the paper. The same effect occurs when materials such as glass
or rubber are rubbed with silk or fur.
Another simple experiment is to rub an inflated balloon with wool. The balloon then adheres to a wall, often for hours. When materials behave in this way,
they are said to be electrified, or tohave become electrically charged. You can easily electrify your body by vigorously rubbing your shoes on a wool rug. The electric
charge on your body can be felt and removed by lightly touching (and startling) a
friend. Under the right conditions, you will see a spark when you touch, and both
of you will feel a slight tingle. (Experiments such as these work best on a dry day
because anexcessive amount of moisture in the air can cause any charge you build
up to “leak” from your body to the Earth.)
In a series of simple experiments, it is found that there are two kinds of electric charges, which were given the names positive and negative by Benjamin
Franklin (1706 – 1790). To verify that this is true, consider a hard rubber rod that
has been rubbed with fur and then suspended by anonmetallic thread, as shown
in Figure 23.1. When a glass rod that has been rubbed with silk is brought near the
rubber rod, the two attract each other (Fig. 23.1a). On the other hand, if two
charged rubber rods (or two charged glass rods) are brought near each other, as
shown in Figure 23.1b, the two repel each other. This observation shows that the
rubber and glass are in two different statesof electrification. On the basis of these
observations, we conclude that like charges repel one another and unlike
charges attract one another.
Using the convention suggested by Franklin, the electric charge on the glass
rod is called positive and that on the rubber rod is called negative. Therefore, any
charged object attracted to a charged rubber rod (or repelled by a charged glass
rod)must have a positive charge, and any charged object repelled by a charged
rubber rod (or attracted to a charged glass rod) must have a negative charge.
Attractive electric forces are responsible for the behavior of a wide variety of
commercial products. For example, the plastic in many contact lenses, etafilcon, is
made up of molecules that electrically attract the protein molecules in human...