Chapter 16: Electric Forces and Fields (pages 557­580) Date __________
Electrostatics: Charges at rest
History
Dr. William Gilbert (1544 ­ 1603)
About 600 B.C. Thales
noticed that after rubbing
of Milan discovered
amber with wool that the
that after rubbing a
wool deflected a needle
piece of amber (petrified
on a compass.
tree resin) it would attract
Dr. Gilbert originated
bits of feathers and straw.
the term electricity.
The word electric is derived from the Greek
word for amber (elektron).
Benjamin Franklin experimented
with electricity in the 18th century.
Negative charges from clouds
leaked down the kite string
onto a key. Touching the
key produced a spark. He
determined that lightning
was static electricity.
Most of the electrical terms that we use today, battery, positive/negative, conductor, charge, and electrician were originally used by Franklin
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Properties of Electric Charge
All atoms have a positively charged nucleus
containing positively charged ___________
and uncharged _____________.
Outside the nucleus are ______________ charged electrons.
All electrons are identical in mass and amount of charge.
__________ are about 2000 times more massive than
electrons, but they have the same amount
of electrical charge, just of the ____________ sign.
___________ have about the same mass as protons but
with no electrical charge.
Normally atoms are electrically neutral. They have the same number of ___________ as __________.
Only electrons are easily transferred from one atom to another. If an electron is transferred from one atom to another, the atoms losing the electron becomes positive and the atom gaining the electron becomes negative.
Atoms that are positively or negatively charged are called ___________.
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Electric Charge
The term positive and negative refer to the electric charge.
Two single protons or two single electrons _______
each other.
A single electron and proton ____________ each other.
The attracting or repelling action exerted on protons
or electrons is an electric force.
Electric charges attract or repel because of the
electric force.
Law of Charged Objects
Objects that have the same charge repel each other.
Objects that have opposite charges attract each other. 4
Electric charge is conserved. No charge is created or destroyed
When 2 objects transfer charge from one object to the other object, the total charge of the system must be conserved.
No electrons are created or destroyed.
The electrons are transferred from one object to the
other.
When one object with a charge of +4 and another
object with a charge of ­12 touch each other the
charge is distributed evenly between the two
objects.
The electrons from the object with a ­12 charge are
transferred to the object with a +4 charge.
When the two objects separate their total charge remains ­8. Each object with have a charge of ­4.
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Conservation of Charge
+6
­8
What is the charge
that is distributed between
the two objects when they
touch each other?
What charge will each
object have when they
are separated?
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Electric Charge is quantized.
In 1909, Robert Millikan performed an experiment observing tiny oil droplet between 2 metal
plates. The oil droplets were negatively charged and then passed through a hole in the top plate. The droplets fell due to their weight. When the top plate was given a positive charge the droplets with a negative charge were attracted upwards toward the plate.
By turning the charge on the plate on and off, he was able to watch a single oil droplet rise and fall between the plates.
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Millikan repeated this process for thousands
of drops. He found that when an object is charged its
charge is a multiple of a fundamental unit
of charge, e. This means that charge is quantized. An object will have a charge of
±e, ± 2e, or ± 3e and so on.
Other experiments found that an electron has a charge of ­e and a proton has an equal and opposite charge of +e. The value of e has been determined to be 1.60 x 10­19 C, where C is the unit of charge, Coulomb. The charge of an electron is ­1.6 x 10­19 C and the charge of a proton is 1.6 x 10­19 C.
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1) Why is charge transferred by electrons? 2) What are the two parts of the Law of Charged Objects?
3) If two objects with charges of +8 and ­ 14 are brought together, what charge is distributed between them when they are brought together?
What charge will each object have when they are separated?
4) What did Millikan's experiment show about the nature of electric charge? 10
Coulomb's Law
When two charged objects are near each other they may experience an acceleration either towards or away from each other because each object exerts a force on the other object.
This force is called the electric force.
In the 1780's Charles Coulomb performed experiments to determine the magnitude of the electric force between two objects.
Coulomb found that the electric force between two charged objects is proportional to the product of the two charges (q1 and q2). He also found that the electric force is inversely proportional to the square of the distance (r) between the two objects. Coulomb's constant (kc) is 8.99 x 109 Nm2/C2.
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Sample Problem
The electrons and protons of a hydrogen atom are separated, on average, by a distance of about 5.3 x 10­11 m. Find the magnitudes of the electric force and the gravitational force that each
particle exerts on the other.
Mass of electron, me = 9.109 x 10­31 kg
Charge of electron, qe = ­1.60 x 10­19 C
Mass of proton, mp = 1.673 x 10­27 kg
Charge of proton,qp = 1.6 x 10­19 C
Universal gravitation constant, G = 6.673 x 10­11 Nm2/kg2
Coulomb's constant, kc = 8.99 x 109 Nm2/C2
Gravitational force, Fg = G memp
Electric force, Felectric = kc q1q2
2
r
r2
r = 5.3 x 10­11 m
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Solving the problem
Substitute the values into the equations and solve.
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Electric Force
Gravitational Force
The electric force between two
particles is relatively large.
The gravitational force between two
particles is extremely small.
Force may be attractive or repulsive.
Force is only attractive.
The electric force is significantly stronger than the gravitational force. Although we feel our attraction to Earth by gravity, we do not usually feel the effects of the electric force. Explain why.
An ordinary nickel contains about 1024 electrons, all repelling each other. Why don't these
electrons fly off the nickel?
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