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(–) (+)
Negative Test
charge charge

(+) (+)
Positive Test
charge charge

FIGURE 6.6 A positive test charge is used by convention to (+) (–)
identify the properties of an electric field. The arrow points in the
direction of the force that the test charge would experience.

near a negative charge and a positive charge. The arrow points in

the direction of the force that the test charge experiences. Th us,
when brought near a negative charge, the test charge is attracted
toward the unlike charge, and the arrow points that way. When
brought near a positive charge, the test charge is repelled, so the
arrow points away from the positive charge. A
An electric field is represented by drawing lines of force or

electric fi eld lines that show the direction of the fi eld. Th e arrows
in Figure 6.7 show field lines that could extend outward for-

ever from isolated charges, since there is always some force on
a distant test charge. Th e field lines between pairs of charges

in Figure 6.7 show curved field lines that originate on positive

charges and end on negative charges. By convention, the fi eld

lines are closer together where the field is stronger and farther

apart where the field is weaker.

Th e field concept explains some observations that were not
explained with the Newtonian concept of action at a distance. (+) (+)
Suppose, for example, that a charge produces an electric fi eld.

Th is field is not instantaneously created all around the charge,
but it is seen to build up and spread into space. If the charge is
suddenly neutralized, the fi eld that it created continues to spread
outward and then appears to collapse back at some speed, even

though the source of the field no longer exists. Consider an
example with the gravitational fi eld of the Sun. If the mass of the
Sun were to instantaneously disappear, would Earth notice this B
instantaneously? Or would the gravitational field of the Sun appear

to collapse at some speed, say, the speed of light, to be noticed by FIGURE 6.7 Lines of force diagrams for (A) a negative charge
Earth some 8 minutes later? The Newtonian concept of action at a and (B) a positive charge when the charges are the same size as the

test charge.
distance did not consider any properties of space, so according to
this concept, the gravitational force from the Sun would disappear
instantly. Th e field concept, however, explains that the disappear-

ance would be noticed after some period of time, about 8 min- emission and absorption of subatomic particles. Th is model

utes. This time delay agrees with similar observations of objects explains electrical and magnetic forces as the exchange of vir-

interacting with fields, so the field concept is more useful than a tual photons, gravitational forces as the exchange of gravitons,

mysterious action-at-a-distance concept, as you will see. and strong nuclear forces as the exchange of gluons.
Actually there are three models for explaining how gravi-
tational, electrical, and magnetic forces operate at a distance.
(1) Th e action-at-a-distance model recognizes that masses are ELECTRIC POTENTIAL
attracted gravitationally and that electric charges and magnetic Recall from chapter 3 that work is accomplished as you move an
poles attract and repel one another through space, but it gives object to a higher location on Earth, say, by moving a book from
no further explanation; (2) the fi eld model considers a fi eld to the first shelf of a bookcase to a higher shelf. By virtue of its posi-

be a condition of space around a mass, electric charge, or mag- tion, the book now has gravitational potential energy that can be

net, and the properties of fields are described by field lines; and measured by mgh (the force of the book’s weight × distance) joules

(3) the fi eld-particle model is a complex and highly mathemati- of gravitational potential energy. Using the field model, you could
cal explanation of attractive and repulsive forces as the rapid say that this work was accomplished against the gravitational fi eld
6-7 CHAPTER 6 Electricity 145