Conceptual Physics

23.3 - Interactive checkpoint: electric field

A point charge rests in an otherwise

charge-free region. Locations 1 and 2

are 0.0300 m and 0.0515 m from the

charge, respectively. If the electric

field at location 1 is 3.40×10í^5 N/C,

what is the electric field at location 2?

Answer:

E 2 = N/C

23.4 - Electric field diagrams

Electric field diagrams are used to illustrate the nature of an electric field. An electric

field diagram consists of electric field lines, as you see illustrated in Concept 1.

The left-side diagram in Concept 1 shows the electric force acting on several test

charges at various points around a central positive charge. The vectors representing the

force on each test charge point away from the positive charge, indicating the direction of

the force. The length of each vector is proportional to the amount of force on each

charge.

The field diagram, shown to the right of the force diagram, is a common and efficient

way to present similar information. The field lines have arrows pointing in the direction

of the field vector at each point, which is the same as the direction of the force that the

field would exert on a positive test charge at that point. However, the field lines do not

use length to represent magnitude. Rather, field strength is indicated by how close the

lines are to one another. Field strength is roughly proportional to the density of the lines.

The illustration for Concept 2 shows how you can determine relative field strength from

a field diagram. We have drawn two boxes at different distances from a negative

charge. The field lines are closer together in the near box than they are in the far box,

which means that the field is stronger in the region defined by the box closer to the

negative charge.

Electric field diagrams

Lines show force on positive test charge

Away from “+” charge, toward “í”

charge

Field line “density”

Proportional to field’s magnitude

(^422) Copyright 2007 Kinetic Books Co. Chapter 23