Ch 22 Electric Fields
2 The Electric Field. This is the electric force per unit charge at a point in space. If a test particle with
⃗ at that point. Units: [ ]
charge at a point feels net electric force ⃗ , then ⃗⃗

Table 22-1 Some Electric Fields
3 Electric Field Lines. These lines of force enable us to visualize patterns in electric fields. The density of
lines in a small region is proportional to the magnitude of ⃗⃗ , and the direction of the tangent to a line at
a point gives the direction of ⃗⃗ . The lines extend away from positive charge (where they originate) and
toward negative charge (where they terminate). Additional rules for drawing electric field lines: a) They
start at positive
charges or at infinity, b) They end at negative
charges or at infinity, and c) The
number of lines entering or leaving a charge is proportional to the magnitude of the charge.
4 The Electric Field Due to a Point Charge. The field of a positive (negative) charge points away from
| |⁄
(toward) the charge. The magnitude of the field is
. Superposition of Fields: Fields from
different charges are added in the same way as forces. ⃗⃗


⃗⃗
⃗⃗
⃗⃗
Checkpoint 1
Sample Problem: Net electric field due to three charged particles
5 The Electric Field Due to an Electric Dipole. A dipole moment ⃗ represents the combination of two
charges of equal magnitude and opposite sign, separated by a distance . The magnitude is
and the direction points from the negative to the positive charge.
Consider a point on the line joining the two charges, on the side of the positive charge, at a distance
from the dipole’s center. The electric field points away from the dipole; its magnitude is
approximately given by

Sample Problem: Electric dipole and atmospheric sprites
8 A Point Charge in an Electric Field. The electrostatic force ⃗ acting on a charged particle located in an
external electric field ⃗⃗ has the direction of ⃗⃗ if the charge of the particle is positive and has the
opposite direction if


is negative: ⃗
⃗⃗ .
Checkpoint 3
Sample Problem: Motion of a charged particle in an electric field
9 A Dipole in an Electric Field. If a dipole ⃗ is in a uniform external field ⃗⃗ , the torque acting on it is
⃗⃗ and its potential energy is
⃗ ⃗
⃗ ⃗⃗ .


Checkpoint 4
Sample Problem: Torque and energy of an electric dipole in an electric field