Download Study Notes Lesson 13 Electrostatics

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Lesson 13 Electrostatics
Introduction
a.
Electrostatics— Electrostatics is the electricity at rest. It involves electric charges, the force between them,
and their behavior in the materials.
b.
Electric Field— Electric field is the aura that surrounds electric charges.
c.
Electric Current & Circuits— Moving electric charges, the voltage that produce them, and the ways that
the current can be controlled.
d.
Electromagnetism— The relationship of electric currents to magnetism.
Electrical Forces and Charges
a.
Based on the experiment situations, there must be two types of charges. Benjamin Franklin arbitrarily call
one positive and another one negative.
b.
Electrical force is much stronger than the gravity. The reason why we don’t feel it most of the time is
because that the attracting force and repelling force cancel each other.
Force
Strength
Range
c.
d.
e.
Strong
1
10-15 m
Electromagnetic
1/137
∞
Weak
10-6
10-18 m
Electrical forces arise from particles in atoms. Based on the atomic model
proposed by Rutherford and Bohr, a positively charged nucleus is surrounded
by electrons. Electrons are attracted to protons in the nucleus, but electrons
€ and repelling behavior is attributed to€a
repel other electrons. The attracting
property called charge.
By convention, electrons are negatively charged and protons positively
charged. Neutrons have no charge, and are neither attracted nor repelled by
charged particles.
Gravity
6x10-39
∞
-
++
-
Model of a helium atom
All electrons are identical; that is, each has the same mass and the same
quantity of negative charge as every other electron.
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Physics Study Notes
Lesson 13 Electrostatics
f.
The nucleus is composed of protons and neutrons. All protons are identical. All neutrons are identical. A
proton has nearly 200 times the mass of an electron, but its charge is equal in magnitude to the negative
charge of the electron. A neutron has slightly greater mass than a proton and has no charge.
g.
Atoms usually have as many electrons as protons, so the atom has zero net charge.
h.
The fundamental rule of all electrical phenomena is
Like charges repel; opposite charge attract.
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i.
The reason why don’t protons pull the oppositely charged electrons into the nucleus is in the domain of
quantum physics. An electron behaves like a wave and has to occupy a certain amount of space related to
its wavelength.
j.
The reason why the protons in the nucleus do not mutually repel and fly apart is that in addition to
electrical forces in the nucleus, there are even stronger forces (strong forces) that are non-electrical in
nature.
Conservation of Charge
a.
An object that has unequal numbers of electrons and protons is electrically charged. If it has more electrons
than protons, the object is negatively charged. If it has fewer electrons than protons, then it is positively
charged. In a neutral atom, there are as many electrons as protons, so there is no charge.
b.
A charged atom is called an ion. A positive charge lost one or more electrons. A negative charge gain one
or more extra electrons. An imbalance comes about by adding or removing electrons.
c.
Different substances have different affinity for electrons. The electrons are held more firmly in rubber than
in fur. The silk has a greater affinity for electrons than the glass or plastic rod.
d.
Conservation of charge—
i)
In an isolated system the total charge is conserved.
ii) Electrons are neither created nor destroyed but are simply transferred from one material to another.
e.
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Electrons cannot be divided into fractions of electrons. All charged objects have a charge that is whole
number multiple of the charge of a single electron.
Coulomb’s Law
a.
The electrical force between any two objects obeys an inverse-square law with distance.
b.
Coulomb’s law:
F =k
q1q2
d2
d
the distance between the charged particles
q1
the quantity of charge of one particle
q2
the quantity of charge of the other particle
k
electrostatic constant
c.
The SI unit of charge is the coulomb (C). 1 C is the charge of 6.24 x 1018 electrons.
d.
The k is a large number (k = 9.0 x 109 N•m2/C2) comparing to the universal gravitational constant G (G =
6.67 ⋅ 10–11 N•m2/kg2).
e.
Comparing to Newton’s law of gravitation, gravity is an extremely small force, while electrical force is an
extremely large force.
Fg = G
m1 m2
d2
Fe = k
and
q1q2
d2
The similarities between these two forces have made physicists think they may be different aspects of the
same thing. For the past few decades, physicists spent lot of efforts searching for a “unified field theory.”
€
€
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Physics Study Notes
Lesson 13 Electrostatics
In recent years, the electric force has been unified with nuclear weak force, which plays a role in nuclear
decay. Physics are still looking for a way to unify electrical and gravitational forces.
f.
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Because most objects have almost exactly equal numbers of electrons and protons, electrical forces usually
balance out.
Conductors and Insulators
a.
Conductor— Outer electrons of the atoms in metal are not anchor to the nuclei of particular atoms, but are
free to roam in the material. Such materials are good conductors. The good conductors for electricity are
also good conductors for heat.
b.
Insulator— Electrons in some materials, such as rubber and glass, are tightly bound and remain with
particular atoms. They are not free to wander about to other atoms in the material. They are insulators.
They are poor conductors for heat for the same reason.
c.
Semiconductor— Some materials, such as germanium and silicon, are good insulators in their pure
crystalline form but increase tremendously in conductivity when even one atom in ten million is replaced
with an impurity that add or remove an electron from the crystal structure. They can be made be behave
sometimes as insulators and sometimes as conductors. Such materials are called semiconductors. Thin
layers of semiconductors sandwiched together make up diodes and transistors.
d.
Superconductor— At temperatures near absolute zero, certain metals acquire infinite conductivity (zero
resistance). These are superconductors. Since 1987, superconductivity at “high” temperatures (about 100K)
has been found in a variety of nonmetallic compounds. Once electric current established in a
superconductor, the electrons flow indefinitely.
Charging by Frictions and Contact
a.
Charging by friction— Electrons are being transferred by friction when one material rubs against another.
b.
Charging by contact— Electrons can be transferred from one material to another by simply touching.
c.
If the object is good conductor, the charge will spread to all part of its surface because the like charge repel
each other. If the object is a poor conductor, it may be necessary to touch the rod at several places on the
object in order to get a more uniform distribution of charge.
Charging by Induction
a.
Charging by induction— If we bring a charged object near a conducting surface, even without physical
contact, electrons will move in the conducting surface.
b.
Charging by induction experiment 1:
A
c.
B
–
–
B––
–
+
–
––
+
–– +
– ++ A
–
–
B––
–
+
– –
–
– B
–
– –
+
+ +
+
+ A
+ +
Charging by induction experiment 2:
–
–
+
+
Net charge is zero
d.
–
––
+
–– +
– ++ A
––––
–
–
+
+
––––
Net charge is positive
+
+
––––
–
–
––––
Net charge is negative
Charging by induction occurs during thunderstorms. The negatively charged bottoms of clouds induced a
positive charge on the surface of Earth below. Benjamin Franklin was the first to demonstrate this in
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Physics Study Notes
Lesson 13 Electrostatics
famous kite-flying experiment, in which he proved that lightning is an electrical phenomenon. Most
lightning is an electrical discharge between oppositely charged parts of clouds.
e.
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Franklin also found that charge flows readily to or from sharp points, and fashioned the first lightning rod.
The point of the rod collects electrons from the air, preventing a large buildup of positive charge on the
building by induction. The lightning rod is to prevent a lightning discharge from occurring. If lightning
strikes, it may be attracted to the rod and short-circuited to the ground.
Charge Polarization
a.
Electrically polarized— Charging by induction is not limited to counductors. When a charged rod is
brought near an insulator, there is a rearrangement of the positions of charges within the atoms and
moleculars. On side of the atom or molecular is more positiove (or negative) than the other side.
b.
This explains why neutral bits of paper are attracted to a charged object or the negatively charged boloon
attached to the neutral wooden wall.
c.
Many molecules (H2O for example) are electrically polarized in their normal states. The distribution of
electric charge is not perfectly even. Such molecules are said to be electric dipoles. Microwave oven uses
oscillating electric field to make the water molecules oscillating. The flip-flopping H2O molecules impart
thermal motion to surrounding food molecules.
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