1. Norah Ali Al-moneef king Saud unversity
Chapter electric field
23.1 Properties of Electric Charges
23.2 Charging Objects By Induction
23.3 Coulomb’s Law
23.4 The Electric Field
23.6 Electric Field Lines
23.7 Motion of Charged Particles in a Uniform Electric Field
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2. Norah Ali Al-moneef king Saud unversity
Electric Charge
Types:
Positive
Glass rubbed with silk
Missing electrons
Negative
Rubber/Plastic rubbed with fur
Extra electrons
Arbitrary choice
convention attributed to ?
Units: amount of charge is measured in [Coulombs]
Empirical Observations:
Like charges repel
Unlike charges attract
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3. Norah Ali Al-moneef king Saud unversity
Charge in the Atom
Protons (+)
Electrons (-)
Ions
Polar Molecules
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4. 23.1 Properties of Electric Charges
Conservation
electricity is the implication that electric charge is always conserved.
• That is, when one object is rubbed against another, charge is not created in the process. The electrified state is due to a transfer of charge from one object to the other.
• One object gains some amount of negative charge while the other gains an equal amount of positive charge.
Quantization
The smallest unit of charge is that on an electron or proton. (e = 1.6 x C)
It is impossible to have less charge than this
It is possible to have integer multiples of this charge
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5. Conductors and Insulators
23.2 Charging Objects By Induction
Conductors and Insulators
Conductor transfers charge on contact
Insulator does not transfer charge on contact
Semiconductor might transfer charge on contact
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6. Charge Transfer Processes
Conduction
Polarization
Induction
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7. Norah Ali Al-moneef king Saud unversity
23-3 Coulomb’s Law
Empirical Observations
Formal Statement
Direction of the force is along the line joining the two charges
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8. Norah Ali Al-moneef king Saud unversity
Consider two electric charges: q1 and q2
The electric force F between these two charges separated by a distance r is given by Coulomb’s Law
The constant k is called Coulomb’s constant and is given by
The coulomb constant is also written as
0 is the “electric permittivity of vacuum”
A fundamental constant of nature
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9. Norah Ali Al-moneef king Saud unversity
Double one of the charges
force doubles
Change sign of one of the charges
force changes direction
Change sign of both charges
force stays the same
Double the distance between charges
force four times weaker
Double both charges
force four times stronger
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10. Norah Ali Al-moneef king Saud unversity
Example:
What is the force between two charges of 1 C separated by 1 meter?
Answer: 8.99 x 109 N,
Blackboard calculation
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11. Norah Ali Al-moneef king Saud unversity
Coulomb’s Law Example
What is the magnitude of the electric force of attraction between an iron nucleus (q=+26e) and its innermost electron if the distance between them is 1.5 x m
The magnitude of the Coulomb force is
F = kQ1Q2/r2
= (9.0 x 109 N · m2/C2)(26)(1.60 x 10–19 C)(1.60 x 10–19 C)/(1.5x10–12 m)2
= x 10–3 N.
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12. Example - The Helium Nucleus
Part 1: The nucleus of a helium atom has two protons and two neutrons. What is the magnitude of the electric force between the two protons in the helium nucleus?
Answer: 58 N
Part 2: What if the distance is doubled; how will the force change?
Answer: 14.5 N
Blackboard calculation
Inverse square law: If the distance is doubled then the force is reduced by a factor of 4.
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13. Example - Equilibrium Position
Consider two charges located on the x axis
The charges are described by
q1 = 0.15 C x = 0.0 m
q2 = 0.35 C x = 0.40 m
Where do we need to put a third
charge for that charge to be at an equilibrium point?
At the equilibrium point, the forces from the two charges will cancel.
Here the forces from q1 and q2 can balance. q3
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Norah Ali Al-moneef king Saud unversity

14. Zero Resultant Force, Example
Two fixed charges, 1mC and -3mC are separated by 10cm as shown in the figure (a) where may a third charge be located so that no force acts on it? 
The magnitudes of the individual forces will be equal
Directions will be opposite
Will result in a quadratic
Choose the root that gives the forces in opposite directions
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15. Example - Charged Pendulums
Consider two identical charged balls hanging from the ceiling by strings of equal length 1.5 m (in equilibrium). Each ball has a charge of 25 C. The balls hang at an angle  = 25 with respect to the vertical. What is the mass of the balls?
Step 1: Three forces act on each ball: Coulomb force, gravity and the tension of the string. x y
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16. Example - Charged Pendulums (2)
Step 2: The balls are in equilibrium positions. That means the sum of all forces acting on the ball is zero!
d=2 l sin θq
Blackboard calculation
Answer: m = 0.76 kg
A similar analysis applies to the ball on the right.
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Norah Ali Al-moneef king Saud unversity

17. Electric Force and Gravitational Force
Coulomb’s Law that describes the electric force and Newton’s gravitational law have a similar functional form
Both forces vary as the inverse square
of the distance between the objects.
Gravitation is always attractive.
k and G give the strength of the
force.
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18. Norah Ali Al-moneef king Saud unversity
Example:
An electron is released above the surface of the Earth. A second electron directly below it exerts an electrostatic force on the first electron just great enough to cancel out the gravitational force on it. How far below the first electron is the second? Fe
5.1 m e mg
r = ? e
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Norah Ali Al-moneef king Saud unversity

19. Norah Ali Al-moneef king Saud unversity
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.
Compare the electrostatic and gravitational the forces
Fe/Fg = 2 x 1039  The force of gravity is much weaker than the electrostatic force
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Norah Ali Al-moneef king Saud unversity

20. Norah Ali Al-moneef king Saud unversity
1/10/2006
Norah Ali Al-moneef king Saud unversity

21. Electric Forces and Vectors
Electric Fields and Forces are ALL vectors, thus all rules applying to vectors must be followed.
Consider three point charges, q1 = 6.00 x10-9 C (located at the origin),q3 = 5.00x10-9 C, and q2 = -2.00x10-9 C, located at the corners of a RIGHT triangle. q2 is located at y= 3 m while q3 is located 4m to the right of q2. Find the resultant force on q3.
Which way does q2 push q3?
Which way does q1 push q3? 4m q2 q3 3m
Fon 3 due to 1 5m q q1
Fon 3 due to 2
q = 37 q3
q= tan-1(3/4)
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Norah Ali Al-moneef king Saud unversity

22. Norah Ali Al-moneef king Saud unversityq2 q3 3m
Fon 3 due to 1 5m q q1
F3,1sin37
Fon 3 due to 2
q = 37
q= tan-1(3/4) q3
F3,1cos37
5.6 x10-9 N
7.34x10-9 N
1.1x10-8 N
above the +x
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23. Norah Ali Al-moneef king Saud unversity
Coulomb’s Law Example
Q = 6.0 mC
L = 0.10 m
What is the magnitude and direction of the net force on one of the charges?
We find the magnitudes of the individual forces on the charge at the upper right corner:
F1= F2 = kQQ/L2 = kQ2/L2
= (9 x109 N · m2/C2)(6 x10–3 C)2/(0.100 m)2 = 3.24 x107 N.
F3= kQQ/(L√2)2 = kQ2/2L2
= (9 x109 N · m2/C2)(6 x10–3 C)2 /2(0.100 m)2
= 1.62 x107 N.
We find the magnitudes of the individual forces on the
charge at the upper right corner:
F1= F2 = kQQ/L2 = kQ2/L2
= (9.0 ´ 109 N · m2/C2)(6.00 ´ 10–3 C)2/(0.100 m)2
= 3.24 ´ 107 N.
F3= kQQ/(L√2)2 = kQ2/2L2
= (9.0 ´ 109 N · m2/C2)(6.00 ´ 10–3 C)2/2(0.100 m)2
= 1.62 ´ 107 N.
The directions of the forces are determined from the signs
of the charges and are indicated on the diagram. For the
forces on the upper-right charge, we see that the net force
will be along the diagonal. For the net force, we have
F = F1 cos 45° + F2 cos 45° + F3
= 2(3.24 ´ 107 N) cos 45° ´ 107 N
= 6.20 ´ 107 N along the diagonal, or away from the center of the square.
From the symmetry, each of the other forces will have the same magnitude and a direction away from the center: The net force on each charge is ´ 107 N away from the center of the square.
Note that the sum for the three charges is zero.
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Norah Ali Al-moneef king Saud unversity

24. Norah Ali Al-moneef king Saud unversity
along the diagonal, or away from the center of the square.
From the symmetry, each of the other forces will have the same magnitude and a direction away from the center: The net force on each charge is= ء 107 N away from the center of the square. .
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25. Norah Ali Al-moneef king Saud unversity
Example - Four Charges
Consider four charges placed at the corners of a square with sides of length 1.25 m as shown on the right. What is the magnitude of the electric force on q4 resulting from the electric force from the remaining three charges?
Answer:
F (on q4) = N
… and the direction?
Blackboard calculation
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Norah Ali Al-moneef king Saud unversity

26. Norah Ali Al-moneef king Saud unversity
HOMEWORK :
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.
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?
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27. Norah Ali Al-moneef king Saud unversity
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?
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 = m. (a) Determine the electric field on the y axis at y =0.500 m. (b) Calculate the electric force on a uC charge placed on the y axis at y = m.
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.
23-44; The electrons in a particle beam each have a kinetic energy of 1.60 x J. What are the magnitude and direction of the electric field that stops these electrons in a distance of 10.0 cm?
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Norah Ali Al-moneef king Saud unversity