Chapter 21 Electric Charge and Electric Field
•Electrostatics - interaction between electric charges that are at
rest
•Electron - negatively charged particle that revolves around the
nucleus of an atom with mass approx. equal to 9.1 x 10-31 kg.
•Proton - positively charged particle inside the nucleus of an atom
with a mass approx. equal to 1.67 x 10-27 kg.
•Neutron - uncharged particle inside the nucleus of an atom with a
mass approx. equal to 1.67 x 10-27 kg.
•http://www.youtube.com/watch?v=FquL0GG9RGI
Chapter 21
1
Chapter 21 Electric Charge and Electric Field
•Unlike charges attract and like charges repel
•Positive Ion - an atom that lost one or more electrons.
•Negative Ion - an atom that gained one or more electrons.
•Ionization - gaining or losing electrons
•Conductor - permits the easy movement of charge
•Insulator - does not permit the easy movement of charge
•Conservation of Charge Principle - The algebraic sum of all
electric charges in a closed system is constant.
•Magnitude of charge on an electron or proton (1.6 x 10-19
coulombs) is a natural unit of charge (all observable charge is an
integral multiple).
Chapter 21
2
Atom to Nucleus Ratio
Tennis
ball
6 cm
diameter
(10-15 m)
6km
diameter
(10-10 m)
Chapter 21
3
Chapter 21 Electric Charge and Electric Field
Charge by Induction - give another body a charge of opposite
sign without losing its own charge
Chapter 21
4
Coulombs Law (Chapter 21, Sec 3)
Force between two point charges (diameter of q much less than r)
q2
q1
F
+
+
F
r
1 q1q 2
F
(21.2)
2
4π 0 r
Coulomb’s law:
force between
two point charges)
where
F = force in newtons
q = charge in coulombs (disregard sign)
r = distance between charges in meters
 0  8.854 x10 12
1
where 0 = permittivity of free space
Chapter 21
40
 k  9 x109
5
Superposition Principle (Chapter 21, Sec 3)
q1
+
r1
F2
q0
+
F
F1
q2
r2
+
q1q0
F1  k 2
r1
q2 q0
F2  k 2
r2
F = F1 + F2 (Vector Sum)
Chapter 21
6
Electric Field (Chapter 21, Sec 4)
Definition of an electric field (E) - The electric force on a
charged body is exerted by the electric field created by other charged bodies.
Figure 21-13
F
q0  0 q
0
E  lim
Chapter 21
in newtons/coulomb
7
Electric Field (Chapter 21, Sec 4)
Force on a Charge in an Electric Field
F0 = q0E
F0 = – q0E
Chapter 21
8
Electric Field (Chapter 21, Sec 4)
Electric field (E) at point p distance r away from point charge q
q
Coulomb’s law
Test charge q0
+
+
qq0
F k 2
r
r
F
q
1 q
E
k 2 
2
q0
r
40 r
(where q0 is much less than q)
q
p
+
•
r
E
1
q
40 r 2
Chapter 21
(21-6)
9
Electric Field (Chapter 21, Sec 4)
Electric field (E) distance r away from point charge q (vector notation)
Figure 21-15
Chapter 21
10
Electric Field (Chapter 21, Sec 4)
Electric field (E) of a point charge q
Figure 21-16
Chapter 21
11
Electric Field (Chapter 21, Sec 4)
Example 21-8
Figure 21-19
qE  eE
ay 


m m
m
Fy
x  v0t
Charge on electron
q = – e = – 1.6 x 10-19 coulombs
1 2
1 eE 2
y  a yt  
t
2
2 m
2
x
t2  2
v0
Chapter 21
1 eE x 2
y
2 m v0 2
12
Electric-Field Calculations (Chapter 21, Sec 5)
Superposition Principle for an Electric Field
q1
+
r1
p
E2
•
E
E1
q2
r2
+
q1
E1  k 2
r1
q2
E2  k 2
r2
E = E1 + E2 (Vector Sum)
Chapter 21
13
Electric-Field Calculations (Chapter 21, Sec 5)
Example 21-9
q1 = q2 = 12 nC
Figure 21- 20
Chapter 21
14
Electric-Field Calculations (Chapter 21, Sec 5)
Electric Field of a Line Charge (Example 21-11)
Chapter 21
15
Electric-Field Calculations (Chapter 21, Sec 5)
Electric Field of a Line Charge
Electric field distance r from an infinite line charge  coul/m
 coul/m
p
•
r
E
1

20 r
Line out of page
line length >> r
Chapter 21
16
Electric-Field Calculations (Chapter 21, Sec 5)
Chapter 21
17
Electric-Field Calculations (Chapter 21, Sec 5)
Electric Field of an Infinite Plane Charge

E
2 0
(Eq 21-12)
 = surface charge density in coul/m2
Note that E is is constant (not dependent on the
distance from the charged surface).
Chapter 21
18
Electric-Field Calculations (Chapter 21, Sec 5)
Electric Field of two oppositely charged infinite plates (Ex 21-13)
Figure 21-24

E1  E2 
2 0

E  E1  E2 
0
Chapter 21
19
Electric Field Lines (Chapter 21, Sec 6)
Figure 21-26
Chapter 21
20
Q21.1
When you rub a plastic rod with fur, the plastic rod becomes
negatively charged and the fur becomes positively charged.
As a consequence of rubbing the rod with the fur,
A. the rod and fur both gain mass.
B. the rod and fur both lose mass.
C. the rod gains mass and the fur loses mass.
D. the rod loses mass and the fur gains mass.
E. none of the above
Q21.2
A positively-charged piece of plastic exerts an attractive force
on an electrically neutral piece of paper. This is because
A. electrons are less massive than atomic
nuclei.
B. the electric force between charged particles
decreases with increasing distance.
C. an atomic nucleus occupies only a small part
of the volume of an atom.
D. a typical atom has many electrons but only
one nucleus.
Q21.4
Three point charges lie at the
vertices of an equilateral triangle as
shown. All three charges have the
same magnitude, but Charge #1 is
positive (+q) and Charges #2 and #3
are negative (–q).
The net electric force that Charges
#2 and #3 exert on Charge #1 is in
Charge #2
–q
Charge #1
+q
y
x
A. the +x-direction.
B. the –x-direction.
C. the +y-direction.
D. the –y-direction.
E. none of the above
–q
Charge #3
Q21.5
A positive point charge +Q is released from rest in an electric
field. At any later time, the velocity of the point charge
A. is in the direction of the electric field at the position
of the point charge.
B. is directly opposite the direction of the electric field at
the position of the point charge.
C. is perpendicular to the direction of the electric field at
the position of the point charge.
D. is zero.
E. not enough information given to decide
Q21.7
Two point charges and a point P lie
at the vertices of an equilateral
triangle as shown. Both point
charges have the same negative
charge (–q). There is nothing at
point P.
The net electric field that Charges
#1 and #2 produce at point P is in
Charge #1
–q
P
y
x
A. the +x-direction.
B. the –x-direction.
C. the +y-direction.
D. the –y-direction.
E. none of the above
–q
Charge #2
Q21.8
The illustration shows the electric
field lines due to three point
charges. The electric field is
strongest
A. where the field lines
are closest together.
B. where the field lines
are farthest apart.
C. where adjacent field
lines are parallel.
D. none of the above
Q21.9
Positive charge is uniformly
distributed around a semicircle.
The electric field that this
charge produces at the center
of curvature P is in
A. the +x-direction.
B. the –x-direction.
C. the +y-direction.
D. the –y-direction.
E. none of the above
Q21.10
Three point charges lie at the
vertices of an equilateral triangle
as shown. Charges #2 and #3
make up an electric dipole.
The net electric torque that
Charge #1 exerts on the dipole is
Charge #2
+q
Charge #1
+q
y
x
A. clockwise.
B. counterclockwise.
C. zero.
D. not enough information given to decide
–q
Charge #3