23.0 - Introduction
An electric charge can exert force on another charge at a distance. No direct
contact between the charges is required. The fact that the electrostatic,
gravitational, and magnetic forces act at a distance puzzled early scientists who
studied them. They speculated about the mechanism that would allow one body to
push or pull on another without touching it. To explain how this could occur, the
British scientist Michael Faraday (1791-1867) pioneered the concept of fields. As
time has passed, this concept has assumed an increasingly important role in
physics.
Today, physicists say that an electric field surrounds an electric charge, and that the
electric field of one charge exerts a force on another charge. Investigating the
fundamentals of fields is the central topic of this chapter.
To begin your study of fields, try the simulation to the right. Here, the charged
particle you see is creating an electric field, represented by the symbol E. The field
is invisible but you can observe it using a field meter.
In essence, the field supplies a “road map” to calculate the force that will be exerted
on any second charge that enters the region surrounding the charged particle you see. The stronger the field at a point in space, the greater
the force that will be exerted on a given second charge when it is placed there. The field points in the same direction as the force that would be
exerted on a positive test charge.
Locate your mouse pointer anywhere on the screen. When you click, you will see an electric field vector that points in the direction of the field,
and a readout that tells you the strength of the electric field at that point.
The initial charge of the visible particle is 1.00 nC, but you can use the controller in the simulation to change this to other values, both positive
and negative.
Initially, keep this particle’s charge constant in the simulation, and try to answer the following questions: How does the field strength vary with
the distance from the charge? Does it seem to increase linearly as you move closer, or do you see great increases in the field at points near
the charge? You should see similarities between the electric field and the force that would be exerted on a second, positive charge.
Next, try changing the charge of the visible particle in the simulation. Does the amount of its charge affect its field strength at a given point? In
what direction does the field of the charged particle point when it is positive? Does the field point in the same direction if you give the source
particle a negative charge?
Using the simulation, you can experiment with some of the fundamentals of electric fields. In the rest of this chapter, you will continue your
exploration of this topic.
23.1 - Electric fields
Electric field: An electric
field describes the nature of
the electric force that a
charge will encounter at a
given location. Fields provide
the model for forces acting at
a distance.
Electrically charged objects exert forces on other
charged objects at a distance: The forces occur without direct contact. Charged objects exert forces on each other analogous to the
gravitational forces exerted by the Earth and the Moon on one another. Electric, gravitational and magnetic forces all act at a distance. To
describe their behavior, scientists have developed the concept of fields, which have become a fundamental tool for explaining the nature of the
universe.
Charges establish electric fields around themselves. A small, positive test charge is often used to establish the nature of an electric field. A test
charge is assumed to be weak enough that it does not alter the field being analyzed. In diagrams, we represent a test charge as a white sphere
with a red plus sign.
You see in Concept 1 a test charge placed in a field, which in this case is generated by a negatively charged particle. The direction of the field
is the direction of the force on the test charge. In the diagram, the field is represented as a vector and labeled with E, the symbol for an electric
field. This field exists at every point whether or not a test charge is present.
As we explain the nature of electric fields, we first review the fundamentals of the electrostatic force. Coulomb’s law states that the force
The muscles and heartbeats of fish generate telltale electric fields,
perceptible to organs in the wide head of this hammerhead shark.