Download Ch16_2008

Chapter 16
Electric Charge and Electric Field
© 2008
Giancoli, PHYSICS,6/E © 2004. Electronically reproduced by
permission of Pearson Education, Inc., Upper Saddle River, New Jersey
Structure of the atom (protons, neutrons and electrons)
• Nucleus contains protons and neutrons
•Protons have positive charge, neutrons are neutral
•Mass of proton ≈ mass of neutron
•Mass of proton (and neutron)  1800x mass of electron
• Electrons have negative charge and are attracted to nucleus
• Charge of electron is equal in magnitude to that of proton
• Normal atom is neutral
• Ion is atom that has gained or lost one or more electrons
Ch16
2
Conductors and Insulators
Conductor: metal atoms in solids have one or more “free” electrons per atom
which move freely through the material
Insulator: no free electrons so it will not conduct electricity
Ch16
3
Static Electricity
Static Electricity:
• Rubbing certain materials together can separate electrons from
their atoms
• Removing electrons from a material makes it positive
• In solids, it is always the free electrons that move
• Electrical charge on the plastic rod induces a separation of charge
in scraps of paper and thus attracts them.
Ch16
4
Induced Charge
(a) If you bring a + charge near a conductor, it will attract electrons to it
leaving the other half of the metal positive.
(b) If they touch, then electrons move to the positively charged object, leaving
the conductor positively charged.
Ch16
5
Coulomb’s Law
Q1Q2
F  k
r2
•Forces are equal in magnitude but opposite in direction
•For spherical charges, r is the center to center distance
•This equation gives the magnitude of the force--you have to
figure the direction from the signs of the charges
Ch16
6
Coulomb’s Law
F  k
Q1Q2
r2
•k  9.0  109 N·m2 / C2
•C is Coulomb -- the unit of charge
• e = 1.60x10-19 C electronic charge (positive)
Ch16
7
Coulomb’s Law
Ch16
8
Example 1. Particles of charge Q1 = +5.00 C, Q 2 = -6.0 C
and Q3 = +8.0 C are placed in a line separated by 0.40 m
between each pair. Calculate the force on Q2.
 Q2
F21
Q1
_
+
Q Q
F21  k 2 2 1
r21
F21 1.7 N
Q Q
F23  k 2 2 3
r23

F23
Q3
+
 9.00 10  9
N m 2 6.0 10 6 C 5.0 10 6 C 
C2
0.40 m2
This is the magnitude, we get direction from charges.
 9.00 10
9
N m 2 6.0 10 6 C 8.0 10 6 C 
C2
0.40 m2
F23  2.7 N
F  F23  F21
Ch16
 2.7 N  1.7 N  1.0 N
Force is directed to the right.
9
Example 2. Particles of charge Q1 = +5.00 C, Q 2 = -6.00 C and Q3 =
+8.00 C are placed on the corners of a square of side 0.400 m as shown
below. Calculate the force on Q2 (Magnitude and direction).
Q1
Note that the charges and distances are
+
the same as in Example 1, so we do not

need to use Coulombs Law again.
F

F21
Q2
_
F21 1.7 N

+

F23
F
F212  F232
tan  
F23  2.7 N
Q3

(1.7 N ) 2  ( 2.7 N ) 2  3.2 N
F21
F23
 F21 
1  1.7 N 
  32o
  tan 
  tan 
 2.7 N 
 F23 
1
Ch16
10
The Electric Field
•Graphical representation of electrical forces
•Electrical force “acts at a distance” like gravity
•Electric field E surrounds every charge
•We investigate the field with a small positive
charge called a “test charge” q
•The field is given by:

 F
E
q
Ch16
11
The Electric Field

 F
E
q
•Units are N/C
•E is a vector = direction of force experienced by positive
test charge
•The magnitude of q is so small that it does not disturb the
charges that cause the field
•To plot the field, move the test charge around the charges
that cause the field
•Since q is positive the field points away from a + charge
and towards a - charge
Ch16
12
Field of a Point Charge Q
Q1Q2
Qq
F k 2 k 2
r
r
Qq
F k r2
E 
q
q
Ek
Q
r2
This is the field created by a point charge or a
spherical charge distribution Q
Ch16
13
Electric Field is a Vector
•Field thus points toward a negative charge and
away from a positive charge
•Since test charge is positive, the direction of the
electric field is the direction of the force felt by a
positive charge
•If there are two or more charges creating the
field then the field at any point is the vector sum
of the fields created by each of the charges
•The test charge does not contribute to the field
and it is too weak to cause any of the charges
creating the field to move.
Ch16
14
Example 3A. A +100 C point charge is separated from a
-50 C charge by a distance of 0.50 m as shown below. (A) First calculate the
electric field at midway between the two charges. (B) Find the force on an
electron that is placed at this point and then calculate the acceleration when it is
released.
Q1
Q2
E1
E2
_
+
Q
E1  k 21
r
Q2
E2  k 2
r
 9.0 10 9
N m 2 100 106 C 
7 N

1
.
4

10
C
C2
0.25 m2
9
N m 2 50 106 C 
6 N

7
.
2

10
C
C 2 0.25 m 2
 9.0 10
E  E1  E2  1.4  10 7 N C  7.2  10 6 N C  2.1 107 N C
Ch16
15
Example 3B. A +100 C point charge is separated from a
-50 C charge by a distance of 0.50 m as shown below. (A) First calculate the
electric field at midway between the two charges. (B) Find the force on an
electron that is placed at this point and then calculate the acceleration when it
is released.
Q1
Q2

E
+
_
In part A we found that E = 2.1x107 N/C and is directed to the right.


F
E 
q
F  q E  eE

F  1.6  1019 C
  2.110
7
N

C
 3.4 1012 N
( to left )


F  ma
F
a 
m
Ch16
3.4  10 12 N

 3.7 10 18 m 2
31
s
9.110 kg
( to left )
16
Example 4. A +100 C point charge is separated from a
-50 C charge by a distance of 0.50 m as shown below. Sketch
the electric field at the point x as shown.

E1
X

E

E2
Q1
+
Ch16
Q2
_
17
Electric Field Lines
• Graphical way of showing the electric field.
• You have seen graphical representations of the earth’s
magnetic field-the electric field maps are similar.
• Sometimes called lines of force.
• Arrow on field line gives direction of force.
• The closer together the lines of force are, the stronger the
electric field.
• Electric field lines are directed out from positive charges
(a) and in toward negative charges (b).
Ch16
18
Electric Field of Point Charges
Ch16
19
Electric Field of Point Charges
Ch16
20
Electric Field of Parallel Plates
Ch16
21
Electric Fields and Conductors
•In the static situation, the field outside the conductor is perpendicular to
the surface of the conductor
• if the field had a component parallel to the surface, it would cause the
electrons in the conductor to move until there was only a perpendicular
component.
Ch16
22
Electric Fields and Conductors
•If a conductor is placed in an electric field, the electrons will rearrange
themselves until the field inside the conductor is zero
•The field inside a hollow conductor shell is zero (Fig 16-33)
•This makes a metal car a relatively safe place in an electrical storm.
Ch16
23