Transcript 2- Chapter 2305phys

Chapter 2
Electric Field
2.1 Coulomb’s Law
2.2 Electric Field
2.3 Electric field lines
Slide 1
Fig 23CO, p.707
‫‪INTRODUCTION‬‬
‫‪ Electric Charge‬الشحنة الكهربية‬
‫– تتكون المادة من جزيئات‪ ،‬التي تتكون من ذرات‪ ،‬تحتوي كل ذرة على‬
‫جسيمات ذات شحنة كهربية موجبة – البروتونات‪ -‬داخل النواه‬
‫وجسيمات ذات شحنة كهربية سالبة ‪ -‬االلكترونات‪ -‬حول النواه‪.‬‬
‫‪Slide 2‬‬
Figure 23.2
When a glass rod is
rubbed with silk, electrons
are transferred from the
glass to the silk. Because
of conservation of charge,
each electron adds
negative charge to the
silk, and an equal positive
charge is left behind on
the rod. Also, because the
charges are transferred in
discrete bundles, the
charges on the two
objects are +/-e, or +/-2e,
or +/-3e, and so on.
Slide 3
Fig 23-2, p.708
(a) The charged object on the
left induces a charge
distribution on the surface of an
insulator due to realignment of
charges in the molecules.
Slide 4
Fig 23-5a, p.710
Slide 5
Fig 23-5b, p.710
Two point charges separated by a distance r
exert a force on each other that is given by
Coulomb’s law. The force F21 exerted by q2
on q1 is equal in magnitude and opposite in
direction to the force F12 exerted by q1 on q2.
(a)When the charges are of the same sign,
the force is repulsive.
(b) (b) When the charges are of opposite
Slide 6
signs, the force is attractive.
Fig 23-7, p.713
Coulomb’s experiments showed that the electric
force between two stationary charged particles
• is inversely proportional to the square of the
separation r between the particles
and directed along the line joining them;
• is proportional to the product of the charges q1
and q2 on the two particles;
• is attractive if the charges are of opposite sign
and repulsive if the charges have the same sign.
Slide 7
Charles Coulomb French
physicist (1736–1806)
F q1 q 2
F
1
r2
F  ke
q1 q 2
r2
The value of the Coulomb constant ke depends
on the choice of units. The SI unit of charge is
the coulomb (C). The Coulomb constant ke in SI
units has the value ke = 8.9875 x109 N.m2/C2
Slide 8
Example 23.2
Consider three charges located at the
corners of a right triangle as shown in
figure where q1=q3= 5 C, q2= -2  C
and a = 0.1 m. Find the resultant force
exerted on q3
q2 exert q3 by force F23 on negative x-axis
F23
6
6
q2 q3
(
2
x
10
)(
5
x
10
)
9
 ke
 9N
2  (8.99 x10 )
2
a
(0.1)
q1 exert q3 by force F13 on negative x-axis
6
6
(
5
x
10
)(
5
x
10
)
q1q3
9
 (8.99 x10 )
 11N
F13  ke
2
2
2(0.1)
( 2a )
F13 divided into x-axis = F13x= 11 cos45= 7.9 N
F3x = F13x+F23=7.9 - 9 = -1.1 N
F3y= F13y= 7.9
Slide 9
F3= -1.1 i+7.9j
and F13y= 7.9N
Slide 10
Example :
Tow protons in an atomic nucleus are typically separated by distance of 2x10-15 m.
The electric repulsion force F between the protons is
(note that k=9x109 N.m2/C2, charge of proton q=1.6x10-19 C)
q
q
r
Slide 11
Example:
The electric force of two electrons separated by r = 20x10-9 m
(note that k=9x109 N.m2/C2, charge of electron q=1.6x10-19 C)
q
q
r
Slide 12
* An electric field is said to exist in the region of space around a charged object.
When another charged object enters this electric field, an electric force acts on it.
The vector E has the SI units of newton per coulomb (N/C),
Slide 13
Slide 14
Slide 15
Example :
In the figure, the electric field of the charge q= 24 x10-6 C at a distance r = 2 m
(Note K=9x109 N.m2/C2)
q
r
Slide 16
Example:
An electric field force is acting on a charge of q=4nC by F=16N. What is the
magnitude of the electric field
Slide 17
Example:
The electric filed at a point r=1km far from a point charge of q=3 C is
(Note K=9x109 N.m2/C2)
Slide 18
Slide 19
q1
N1

q2
N2
Slide 20
Example:
The figure shows the electric field lines for two charges separated By a
small distance. The ratio (q2/q1) equal
a
-q2
+q1
b
-q2
-q1
c
+2q1
Slide 21
-q2
Electric charges have the following important properties:
• Unlike charges attract one another, and like charges repel one another.
• Charge is conserved.
• Charge is quantized—that is, it exists in discrete packets that are some
integral
multiple of the electronic charge.
Conductors are materials in which charges move freely. Insulators are
materials in which charges do not move freely.
Slide 22
where ˆr is a unit vector directed from the charge to the point in question.
The electric field is directed radially outward from a positive charge and
radially inward toward a negative charge.
The electric field due to a group of point charges can be obtained by using
the superposition principle. That is, the total electric field at some point equals
the vector sum of the electric fields of all the charges:
Slide 23
Electric field lines describe an electric field in any region of space. The
number of lines per unit area through a surface perpendicular to the lines is
proportional to the magnitude of E in that region.
Slide 24
23-7; Three point charges are located at the
corners of an equilateral triangle. Calculate the
net electric force on the 7.00 uC charge.
F1
60
60
F2
Slide 25
23-8: Two small beads having positive charges 3q
and q are fixed at the opposite ends of a
horizontal insulating rod extending from the
origin to the point x =d. a third small charged
bead is free to slide on the rod. At what position
is the third bead in equilibrium? Can it be in
stable equilibrium?
Slide 26
3.The electric force between an electron and a proton separated by 3nm is
q1q2
F k 2
r
Slide 27