Grade 11ADV&ASP – PHY71 AND PHY60A -Term 1-2019-2020
Teacher: F@tima (Fatima.alsawafteh@aths.ac.ae)
Course Description
The Physics C: Electricity and Magnetism course is a calculus-based, college-level physics course,
especially appropriate for students planning to specialize or major in physical science or engineering.
The course explores topics such as electrostatics; conductors, capacitors, and dielectrics; electric
circuits; magnetic fields; and electromagnetism. Introductory differential and integral calculus is used
throughout the course.
* 7 periods weekly
References and resources: In this course you will use the following resources:
1- (Physics for scientists and Engineers- Serway/Jewett- College Board)
2- The AP Physics C: Electricity & Magnetism (2014)
3- SAT topics (2016)
Course assessment:
Grade 12ADV – PHY71 -Term 1-2018-2019
Teacher : F@tima (Fatima.alsawafteh@aths.ac.ae)
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Week2: Sunday 9/9/2018
Ch23: Electric Fields
LEARNING OUTCOMES & KPIS: L1: APPLY THE CONCEPT OF ELECTRIC CHARGES AND USE COULOMB’S LAW WITH SUPERPOSITION
PRINCIPLE TO FIND THE NET FORCE ON A CHARGE DUE TO OTHER CHARGES AROUND IT
OBJECTVES: 1.1, 1.2, 1.3
1.1 Describe the types of charge and the attraction and repulsion of charges
1.2 Describe polarization and induced charges
1.3 Calculate the magnitude and direction of the force on a positive or negative charge due to other
specified point charges
23.1 Properties of Electric Charges (P. 690)
There are two kinds of electric charges, which were given the names positive and negative by
Benjamin Franklin (1706–1790)
Charges of the same sign repel one another and charges with opposite signs attract one
another.
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q is the standard symbol used for charge as a variable.
The SI unit of electric charge is coulomb (C)
Electric charge is quantized: means that electric charge always occurs as some integral
multiple of a fundamental amount of charge electron e.
Any electric charge = integer number x the charge of the electron
q=N e
where:
q of the electron = e = -1.6x10-19 c
Electric charge is always conserved in an isolated system: means that if an object gains some
amount of negative charge the other gains an equal amount of positive charge in the same
system.
Example:
when a glass rod is rubbed with silk, the silk obtains a negative charge that is equal in
magnitude to the positive charge on the glass rod. Electrons are transferred from the glass to
the silk in the rubbing process. Similarly, when rubber is rubbed with fur, electrons are
transferred from the fur to the rubber, giving the rubber a net negative charge and the fur a net
positive charge. This process is consistent with the fact that neutral, uncharged matter contains
as many positive charges (protons within atomic nuclei) as negative charges (electrons).
Properties of electric charge
• There are two kinds of charges in nature; charges of opposite sign attract one another and
charges of the same sign repel one another.
• Total charge in an isolated system is conserved.
23.2 Charging Objects By Induction(P. 692)
1
Electrical conductors are materials in which some of the electrons are free electrons that are
not bound to atoms and can move relatively freely through the material; electrical insulators
are materials in which all electrons are bound to atoms and cannot move freely through the
material.
When a negatively charged rubber rod is brought near the sphere, electrons in the region
nearest the rod experience a repulsive force and migrate to the opposite side of the sphere. This
leaves the side of the sphere near the rod with a positive charge because of the diminished
number of electrons, as in the Figure 23.4b. (The left side of the sphere in Figure 23.4b is
positively charged as if positive charges moved into this region, but remember that it is only
electrons that are free to move.) This occurs even if the rod never actually touches the sphere.
If the same experiment is performed with a conducting wire connected from the sphere to the
Earth (Fig. 23.4c), some of the electrons in the conductor are so strongly repelled by the
presence of the negative charge in the rod that they move out of the sphere through the wire
and into the Earth. The symbol at the end of the wire in Figure 23.4c indicates that the wire is
connected to ground, which means a reservoir, such as the Earth, that can accept or provide
electrons freely with negligible effect on its electrical characteristics. If the wire to ground is
then removed (Fig. 23.4d), the conducting sphere contains an excess of induced positive
charge because it has fewer electrons than it needs to cancel out the positive charge of the
protons. When the rubber rod is removed from the vicinity of the sphere (Fig. 23.4e), this
induced positive charge remains on the ungrounded sphere. Note that the rubber rod loses none
of its negative charge during this process.
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polarization: induction in insulators
A process similar to induction in conductors takes place in insulators. In most neutral
molecules, the center of positive charge coincides with the center of negative charge. However,
in the presence of a charged object, these centers inside each molecule in an insulator may shift
slightly, resulting in more positive charge on one side of the molecule than on the other. This
realignment of charge within individual molecules produces a layer of charge on the surface of
the insulator, as shown in Figure 23.5a.
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Knowing about induction in insulators, you should be able to explain why a comb that has
been rubbed through hair attracts bits of electrically neutral paper and why a balloon that has
been rubbed against your clothing is able to stick to an electrically neutral wall.
Check your understanding:
1- If you rub an inflated balloon against your hair, the two materials attract each other. Is the amount of
charge present in the system of the balloon and your hair after rubbing
(a) less than
(b) the same as
(c) more than the amount of charge present before rubbing
2- Three objects are brought close to each other, two at a time. When objects A and B are brought
together, they repel. When objects B and C are brought together, they also repel. Which of the
following are true?
(a) Objects A and C possess charges of the same sign.
(b) Objects A and C possess charges of opposite sign.
(c) All three of the objects possess charges of the same sign.
(d) One of the objects is neutral.
(e) We would need to perform additional experiments to determine the signs of the charges.
3- How many electrons gained by an object carrying a charge q = -8x10-18 C?
4- Assume that an object loose 1x109 electrons, then what will be the charge of this object?
5- Two identical metallic spheres, one has an electric charge equal +10mc, and the other is neutral. If
the two spheres conducted with each other. What will be the charge on each sphere in coulombs?
5- Agree or disagree? Justify.
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When two objects charged by rubbing, after charging they will attract each others.
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When two objects charged by induction, after charging they will attract each others.
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When two objects charged by conduction, after charging they will attract each others.
23.3 Coulomb’s Law
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;
• is a conservative force.
From experimental observations on the electric force, we can express Coulomb’s law
as an equation giving the magnitude of the electric force (sometimes called the
Coulomb force) between two point charges:
vector form for the electric force exerted by a charge q1 on a second charge q2, written
F12, is
where ˆr is a unit vector directed from q1 toward q2.
! =
F
k eq1q1
r2
where ke is a constant called the Coulomb constant.
The value of the Coulomb constant 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
9
2
ke = 8.987 5 x 10 N.m /C
2
This constant is also written in the form
ke=
1
4πϵ ∘
where the constant ϵ0 (lowercase Greek epsilon) is known as the permittivity of free space
and has the value
-12
ϵ0 = 8.854 2 x 10
2
2
C /N . m
Note: We will use the term point charge to mean a particle of zero size that carries an
electric charge. The electrical behavior of electrons and protons is very well described
by modeling them as point charges.
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Check your understanding:
1- Object A has a charge of + 2 !μC, and object B has a charge of+ 6 μ
! C. Which statement is true about
the electric forces on the objects?
(a) FAB = -3FBA
(b) FAB = -FBA
(c) 3FAB = -FBA
(d) FAB =3FBA (e) FAB = FBA (f) 3FAB = FBA
2- The electron and proton of a hydrogen atom are separated (on the average) by a distance of
-11
approximately 5.3 x 10 m. Find the magnitudes of the electric force and the gravitational force
between the two particles.
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AP practice problems(Class activity)
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1. A 20 µC and a 10 µC charge are placed 1.0 m apart as shown in the diagram above. The
10 µC charge experiences a force of 1.8 N directed to the right. The 20 µC charge will
experience a force of
A) 0.9 N to the right
B) 0.9 N to the left
C) 1.8 N to the right
D) 1.8 N to the left
E) 3.6 N to the right
2. If the 20 µC charge is moved to a distance of 2 m from the 10 µC charge, the new
force on the 10 µC charge will be
A) 0.45 N
B) 0.9 N
C) 1.8 N
D) 3.6 N
E) 7.2 N
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3. Two charges of +4 µC and -2µC are placed on a line 1 m apart. At which point could
a small positive charge be placed such that the net force exerted on the small positive
charge is zero?
A) At a point halfway between the two charges
B) At a point between the two charges but closer to the -2µC charge
C) At a point to the left of the +4 µC charge
D) At a point to the right of the -2 µC charge
E) None of the above
4. Consider two charged spheres of equal size carrying a charge of +8 C and – 4 C, respectively.
The spheres are brought in contact with one another for a time sufficient to allow them to
reach an equilibrium charge. They are then separated. What is the final charge on each sphere?
A) -4 C
B) -2 C
C) -1 C
D) +1 C
E) +2 C
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5. Two charges each with charge +Q are located on the y – axis, each a distance a on either side of the
origin. Point P is on the x-axis at a distance 2a from the origin. A small negative charge q is now placed at
point P. On the diagram above, sketch an arrow indicating the direction of the
i.
electric force acting on the negative charge
ii. Determine the magnitude of the electric force at point P in terms of the given quantities and any
fundamental constants.
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2- Two identical small charged spheres, each having a mass of 3.0 x 10 kg, hang in equilibrium as
shown in the figure. The length of each string is 0.15 m, and the angle θ is 5.0°. Find the magnitude of
the charge on each sphere.
How to express the resultant force acting on q in unit vector form ?
1- Consider three point charges located at the corners of a right triangle as shown in the figure, where
q1 = q3 = 5.0 μC, q2 = -2.0 μC. a= 0.10 m. Find the resultant force on q 3?
Where Is the Resultant Force Zero?
3- Three point charges lie along the x axis as shown in the figure . The positive charge q1 =15.0 μC is
at x = 2.00 m, the positive charge q2 = 6.00 μC is at the origin, and the resultant force acting on q3 is
zero.
a. What is the x coordinate of q3?
HW1W1(1-7)