Fisica

P U Z Z L E R
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)

c h a p t e r

Electric Fields
ChapterOutline
23.1 23.2 23.3 23.4
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

708

23.1 Properties of Electric Charges

709

T

he electromagnetic force between charged particles is one of thefundamental 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, you will findthat 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 to have becomeelectrically 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 an excessive amount ofmoisture 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 a nonmetallic thread, asshown 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 states of electrification. On thebasis 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 anycharged 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 tears. These protein molecules are...