Transcript Chapter23 english

Chapter 23: Electric Field
23-3 Coulomb’s Law
23-4 Electric Field
23-6 Electric field lines
23-7 Motion of charged particles in a
uniform electric field
Slide 1
Fig 23CO, p.707
INTRODUCTION
Slide 2
Slide 3
Fig 23-2, p.708
(a) Negatively charged rubber rod suspended by a thread is attracted to a
positively charged glass rod.
(b) (b) A negatively charged rubber rod is repelled by another negatively charged
rubber rod.
Slide 4
Fig 23-1, p.708
Charging a metallic object by induction (that is, the two
objects never touch each other).
(a) A neutral metallic sphere, with equal numbers of
positive and negative charges.
(b) The electrons on the neutral sphere are redistributed
when a charged rubber rod is placed near the sphere.
(c) When the sphere is grounded, some of its electrons
leave through the ground wire.
(d) When the ground connection is removed, the sphere
has excess positive charge that is nonuniformly
distributed.
Slide 5
(e) When the rod is removed, the remaining electrons
redistribute uniformly and there is a net uniform
distribution of positive charge on the sphere.
Fig 23-4, p.710
(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 6
Fig 23-5a, p.710
Slide 7
Fig 23-5b, p.710
Slide 8
Table 23-1, p.712
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 signs, the force is
attractive.
Slide 9
Fig 23-7, p.713
Slide 10
Fig 23-7a, 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 11
Charles Coulomb French
physicist (1736–1806)
F  q1 q 2
F
1
r2
F  ke
q1 q 2
r
2
r
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 12
The electron and proton of a hydrogen atom are separated (on the average) by a
distance of approximately 5.3 x10-11 m. Find the magnitudes of the electric force
and the gravitational force between the two particles.
Using Newton’s law of gravitation
Slide 13
Thus, the gravitational force between charged atomic
particles is negligible when compared with the electric
force.
Slide 14
* The gravitational field g at a point in space, g = Fg / m.
* 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 newtons per coulomb (N/C),
Slide 15
Slide 16
Slide 17
Slide 18
Slide 19
When a particle of charge q and mass m is placed in an electric field E, the electric
force exerted on the charge is qE. If this is the only force exerted on the particle, it
must be the net force and so must cause the particle to accelerate. In this case,
Newton’s second law applied to the particle gives;
Fe = qE = ma
The acceleration of the particle is therefore
a = qE/m
•If E is uniform (that is, constant in magnitude and direction), then the acceleration
is constant.
• If the particle has a positive charge, then its acceleration is in the direction of the
electric field.
•If the particle has a negative charge, then its acceleration is in the direction
opposite the electric field.
Slide 20
A positive point charge q of mass m is released from rest in a
uniform electric field E directed along the x axis. Describe its
motion.
we can apply the equations of kinematics in one dimension
Taking xi = 0 and vxi = o
Slide 21
The kinetic energy of the charge after it has moved a distance x = xf-xi, is
We can also obtain this result from the work–kinetic energy theorem because
the work done by the electric force is Fex = qEx and W = ∆K
Slide 22
An electron enters the region of a uniform electric
field with vo=3.00x106 m/s and E= 200 N/C. The
horizontal length of the plates is l = 0.100 m.
(a) Find the acceleration of the electron while it is
in the electric field.
(b) Find the time it takes the electron to travel through the field.
(c) What is the vertical displacement y of the electron while it is in the field?
If the separation between the plates is less than this, the electron
will strike the positive plate.
Slide 23
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 24
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 25
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.
A charged particle of mass m and charge q moving in an electric field E has an
acceleration
Slide 26
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.
Slide 27
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.
Slide 28
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 29
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 30
Problem 23-12; An object having a net charge of 24.0 C is placed in a uniform
electric field of 610 N/C that is directed vertically. What is the mass of this
object if it “floats” in the field?
Slide 31
Problem 23-12; An object having a net charge of 24.0 C is placed in a uniform
electric field of 610 N/C that is directed vertically. What is the mass of this
object if it “floats” in the field?
Slide 32
3-18; Two 2.00uC point charges are located on the x axis. One is at x = 1.00
m, and the other is at x =- 1.00 m. (a) Determine the electric field on the y
axis at y =0.500 m. (b) Calculate the electric force on a - 3.00uC charge
placed on the y axis at y = 0.500 m.
Slide 33
3-18; Two 2.00uC point charges are located on the x axis. One is at x = 1.00
m, and the other is at x =- 1.00 m. (a) Determine the electric field on the y
axis at y =0.500 m. (b) Calculate the electric force on a - 3.00uC charge
placed on the y axis at y = 0.500 m.
Slide 34
23-41; An electron and a proton are each placed at rest in an electric field of
520 N/C. Calculate the speed of each particle 48.0 ns after being released.
Slide 35
23-41; An electron and a proton are each placed at rest in an electric field of
520 N/C. Calculate the speed of each particle 48.0 ns after being released.
Slide 36
23-44; The electrons in a particle beam each have a kinetic energy of 1.60 x
10-17 J. What are the magnitude and direction of the electric field that stops
these electrons in a distance of 10.0 cm?
Slide 37
23-44; The electrons in a particle beam each have a kinetic energy of 1.60 x
10-17 J. What are the magnitude and direction of the electric field that stops
these electrons in a distance of 10.0 cm?
Slide 38
‫مثال ‪:13-23‬يتحرك جسم مشحون بشحنة موجبة مقدارها ‪ q‬وكتلته ‪m‬‬
‫خالل مجال كهربي منتظم ‪ ،E‬كما هو موضح بالرسم فإذا كانت سرعته‬
‫االبتدائية ‪ vo‬أوصف حركة الجسم؛‬
‫تعتبر حركة الجسم حركة خطية بسيطة ‪Simple Linear Motion‬‬
‫‪F = qE = ma → a = qE/m‬‬
‫‪v = vo + at‬‬
‫‪X = vot + ½ a t 2‬‬
‫‪V2 = vo2 + 2ax‬‬
‫‪vo = 0‬‬
‫األزاحة‬
‫‪x= ½ a t2 = qEt2/2m‬‬
‫‪V2=2ax = 2qEx/m‬‬
‫الطاقة الحركية‬
‫‪Fig 23-25, p.726‬‬
‫→‬
‫‪x=0‬‬
‫&‬
‫‪v=at = qEt/m‬‬
‫‪K = ½ mv2 = ½ m 2qEx/m = qEx‬‬
‫‪Slide 39‬‬
‫‪ 7-23‬حركة جسيم مشحون بمجال كهربي منتظم ‪Motion of charged particles in a‬‬
‫‪uniform electric field‬‬
‫تكافئ حركة الجسم الموضحة بالرسم حركة المقذوفات بمجال الجاذبية‬
‫‪F = qE = ma‬‬
‫ويكون تسارع الجسم المشحون هو‬
‫‪a= qE/m‬‬
‫* يكون تسارع الجسم باتجاه المحال للشحنة الموجبة وعكس اتجاه المجال للشحنة السالبة‬
‫يتحرك ألكترون خالل مجال كهربي منتظم ‪ E‬فأن‪:‬‬
‫’‪a’= - eE/m j‬‬
‫‪vy=at= eEt/m‬‬
‫‪y = ½ at2 = - eEt2/2m‬‬
‫‪Fig 23-26, p.726‬‬
‫& ثابت=‪vx= v0‬‬
‫&‬
‫‪x=vot‬‬
‫‪Slide 40‬‬
Figure 23.27 Schematic diagram of a cathode ray tube. Electrons leaving the
cathode C are accelerated to the anode A. In addition to accelerating
electrons, the electron gun is also used to focus the beam of electrons, and
the plates deflect the beam.
Slide 41
Fig 23-27, p.728