and Energy 2014 Pearson Education Inc What s new in this chapter What is the interaction responsible for holding the particles together is the same interaction that makes the toner stick to a copier drum and a balloon stick to your hair as well as many other phenomena that we observe in ID: 784121
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Slide1
Electric Charge,Force, and Energy
© 2014 Pearson Education, Inc.
Slide2What's new in this chapter
What is the interaction responsible for holding the particles together is the same interaction that makes the toner stick to a copier drum and a balloon stick to your hair, as well as many other phenomena that we observe in our everyday world.
© 2014 Pearson Education, Inc.
Slide3What is Electrostatics
?
Electrostatics
is the study of the interactions between stationary electrically charged particles.
Electrostatic laws deal with the
attractive
and
repelling
forces that exist between positive and negative electric charges.
Slide4A Quick Chemistry Review
Slide5History of the Atom
Slide6A Little More Review
Particle
Proton
Electron
Neutron
Location
Nucleus
Energy Levels
Nucleus
Charge
+1.6 x 10
-19
C
-1.6 x 10
-19
CNo ChargeMass1.67 x 10-27kg9.11 x 10-31kg1.68 x 10-27kg
Slide7What’s the Origin of the Word “Electricity”?
William Gilbert, a 17
th
century physician and scientist coined the term from the Greek root “elektron” meaning amber. Amber was the material that ancient Greek philosophers had noticed would mysteriously attract small particles after it had been rubbed with fur.
Slide8A Brief History…
Since electricity from frictional sources was usually weak, electricians of the eighteenth century searched for ways to increase charge and to accumulate as much of it as possible on a substance. If charge could be accumulated the electricians could then broaden their research with the mystical phenomenon.
We needed bigger shocks
…
Slide9Stephen Gray (1666-1736)
This British chemist, is credited with discovering that electricity can flow (1729), and was the first to identify the properties of conductors and insulators. He also transmitted electricity over a wire, which eventually led to the development of the telegraph.
The figure shows that the electric force of a rubbed glass could be sent, through a wire, to the body of a person.
Slide10E.G. von Kleist (1700-1748) andPieter van Musschenbroek (1692-1761)
This German administrator and cleric, and the Dutch physicist separately and independently discovered the Leyden jar, a fundamental electric circuit element for storing electric charge, now referred to as a capacitor. Musschenbroek nearly killed his friend discharging the capacitor.
Slide11Jean-Antoine Nollet (1700-1770)
This French clergyman decided to test his theory that electricity traveled far and fast. He did the natural thing on a fine spring day in 1746, sending 200 of his monks out in a line 1 mile long.
Once aligned, Nollet hooked up a Leyden jar to the end of the line and all the monks started swearing, contorting, or otherwise reacting simultaneously to the electric shock. A successful experiment: an electrical signal can travel a mile and it does so quickly.
Slide12Charles-François de Cisternay Du Fay
(1698-1739)
Du Fay discovered two types of electric charge and was the first to suggest that electricity consisted of two fluids: "vitreous" (from the Latin for "glass"), or positive, electricity; and "resinous," or negative, electricity, and recognized that similar fluids repel, and dissimilar attract.
Slide13Benjamin Franklin (1706-1790)
Benjamin Franklin invents the theory of one-fluid electricity in which one of Nollet's fluids exists and the other is just the absence of the first. He proposes the principle of conservation of charge and calls the fluid that exists and flows ``positive''. This educated guess ensures that undergraduates will always be confused about the direction of current flow.
He also discovers that electricity can act at a distance in situations where fluid flow makes no sense.
Slide14Negative,Positive,
What’s the Difference???
The two ‘opposite’ charges may as well have been called the ying and the yang.
All that is important to know is that they are different beasts, and that opposites attract, and likes repel…
Slide15Charles Augustin de Coulomb
(1736-1806)
Coulomb developed a theory of attraction and repulsion between bodies of the same and opposite electrical charge. He demonstrated an inverse square law for such forces and went on to examine perfect conductors and dielectrics. He also is credited with creating the torsion balance.
Slide1614.1 Electrostatic Interactions
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Slide17Electrostatic interactions
If you rub two balloons the same way with a wool cloth, they will attract the wool but repel each other.
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Slide18More HistoryOver time the new property acquired by the materials caused by rubbing came to be known as electric charge
Objects that interact with each other because they are charged are said to exert electrical force on each other
When the objects are at rest, the force is called ELECTROSTATIC FORCE
Static is because the objects are not moving
© 2014 Pearson Education, Inc.
Slide19Electrostatic interactions
Materials rubbed against each other acquire electric charge.(conductors)Two objects with the same type of charge repel each other.
Two objects with opposite types of charge attract each other.
Two objects made of different materials rubbed against each other acquire opposite charges.
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Slide20Electrostatic interactions
Sometimes more vigorous rubbing leads to a greater force exerted by the rubbed objects on each other.
The magnitude of the force that the charged objects exert on each other increases when the distance between the objects decreases.
© 2014 Pearson Education, Inc.
Slide21Conceptual Exercise 14.1Take two 9-inch-long pieces of transparent tape and place them sticky side down on a plastic, glass, or wooden tabletop. Now pull on one end of each tape to remove them from the table. Bring the pieces of tape near each other. They repel each other, as shown in the figure. Can we explain the repulsion of the tapes as an electric interaction?
© 2014 Pearson Education, Inc.
Slide22Charged objects attract uncharged objects
Everyday observations show that uncharged lightweight objects, such as small bits of paper that have not been rubbed against anything, are attracted to charged objects.
WHY?????
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Slide23Charged objects attract uncharged objects© 2014 Pearson Education, Inc.
Slide2414.2 Explanation for Electrostatic Interactions
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Slide25Contemporary model for electric charge
Two objects start as neutral—the total electric charge of each is zero. During rubbing, one object gains electrons and becomes negatively charged. The other object loses an equal number of electrons and with this deficiency of electrons becomes positively charged.
Sometimes you rub nothing happens. e
-
in both materials are bound equally strong and therefore no e
-
occurs.
© 2014 Pearson Education, Inc.
Slide26Charge
Opposite charges attract and like charges repel. As a result
negatively charged electrons are attracted to the positive nucleus.
Despite the great mass difference, the charge on an electron is exactly equal in magnitude to the charge on a proton, and its magnitude is denoted by "e."
Slide27Contemporary model for electric charge
+1 charge (e+) added to -1 (e-
) charge we get no charge.
Example 2: 1 e
+
+ 1 e
+
=2e
+
The charge of 1e-
= -1.6 X 10
-19
Cq is the abbreviation used for charge© 2014 Pearson Education, Inc.
Slide28The Coulomb
We use a new SI unit, the coulomb (C), to measure charge. (More on the coulombs later)
The smallest amount of charge is the charge of 1 electron (1.6x10
-19
C).
The charge on any object, whether it is positive or negative is
quantized
, meaning that the charge consists of an
integer
number of protons and electrons.
Therefore a charge of 2e
-
can be written 2q or 2e
-
instead of 3.2x10
-19 C because charge must be a multiple of 1.6x10-19
Slide29Units of Charge
The
coulomb
(selected for use with electric currents) is actually a
very large unit
for static electricity. Thus, we often encounter a need to use the metric prefixes.
1
m
C = 1 x 10
-6
C
1 nC = 1 x 10
-9
C
1 pC = 1 x 10
-12 C
Slide30Example 1
A metal sphere has a
net charge
of –2.4 x 10
-6
C. How many excess electrons does the sphere contain?
GIVEN:
q = -2.4 x 10
-6
C
-e = -1.6 x 10
-19
C
#electrons = ???
Slide31Example 2
If 16 million electrons are removed from a neutral sphere, what is the charge on the sphere in coulombs?
1 electron: e
-
= -1.6 x 10
-19
C
q = -2.56 x 10
-12
C
Since electrons are
removed,
the charge remaining on the sphere will be
positive.
Final charge on sphere:
q = +2.56 pC+ + + + + + + + + ++ ++ +
Slide32Conceptual Exercise 14.2
You pull your sweater and shirt off together, and then pull them apart. You notice that they attract each other—a phenomenon called "static" in everyday life. Explain the mechanism behind this attraction and suggest an experiment to test your explanation.
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Slide3314.3 Conductors and Nonconductors (dielectrics)
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Slide34Conductors
In metals, some electrons can move freely throughout the metal. When we bring a positively charged rod next to a metal bar, the free electrons move closer to the positively charged rod, leaving the other side with a deficiency of electrons.
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Slide35Conductors© 2014 Pearson Education, Inc.
Slide36The electroscope
An electroscope consists of a metal ball attached to a metal rod. A very lightweight needle-like metal rod is connected on a pivot near the bottom of the larger rod. Used to study electrostatic interactions
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Slide37The electroscope
The angle of deflection is related to the magnitude of the electric charge of the electroscope.
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What would happen if I brought a positive rod near top of the electroscope?
Slide38Dielectrics/Insulators/Non-Conducting ObjectsObjects that do not have free electrons or any other charged particles that are free to move inside. (glass, wood, other nonmetal objects)
All electrons are tightly bound to their atoms or molecules
How can we explain the attraction of a neutral nonmetal object and a charge object (like the balloon on the wall or the pieces of paper)
© 2014 Pearson Education, Inc.
Slide39Dielectrics and Polarization
Plastic, glass, and other nonmetal materials do not have free electrons or any other charged particles that are free to move inside.The charge object creates a force that acts on the tightly bound nucleus that causes the charged nuclei components to separate slightly by charge (More force than the forces keeping the atom bound together)
When the atoms are in a polarized state its called an
electric dipole
.
© 2014 Pearson Education, Inc.
Slide40Electric Dipole and Polarization
Electric Dipole is any object that is overall electrically neutral but has its negative and positive charges separatedPolarization: leads to a small accumulation of charge on the surface of the object
Polarization is what causes nonconducting materials to interact with charged objects
© 2014 Pearson Education, Inc.
Slide41Polarization of conductors and dielectrics© 2014 Pearson Education, Inc.
Slide42Electric properties of materials© 2014 Pearson Education, Inc.
Slide43Is the human body a conductor or a dielectric?
As your hand approaches a positively charged cup, the free electrons on the surface of your hand and arm move toward it.
Your body is a conductor. We transfer electrons
We will talk about electric shock and sparks in a few minutes
© 2014 Pearson Education, Inc.
Slide44Grounding
If the cup has a positive charge, negative electrons in the ground are attracted toward the cup and travel from the ground to the cup, causing it to become neutral.
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Earth is a large conductor with infinite amount of electrons.
Slide45Grounding
© 2014 Pearson Education, Inc.
Slide46Law of Conservation of Charge
Like other conservation laws, the law of conservation of electric charge states that
the net charge
(which is basically the sum of the charge on each proton and electron in a system)
of an isolated system remains constant
.
Slide47Quick Overview
© 2014 Pearson Education, Inc.
Slide48What is
Net Charge?
Net charge
is the amount of
excess
charge; a neutral object has an equal number of electrons and protons, and therefore, no net charge.
No Net Charge
Positive Net Charge
Slide49How Might an Object Become Charged???
Charging by Friction
Charging by Contact (Insulators and Conductors)
Charging by Induction (Conductors Only)
?
Slide50Charging by Friction
This is called
charging by friction
. It’s basically the same phenomenon that occurs when you drag your feet across a carpet on a dry day, or rubbed a balloon through your chair. Electrons, NOT PROTONS, are, with a little bit of energy, “scraped” off, and transferred.
Slide51Triboelectric Series
Higher the material in the series the more affinity it has to take electrons.
Lower materials tend to give up electrons.
The larger the difference in position the greater the transfer of electrons.
The
Triboelectric Series
is a list of materials, showing which have a greater tendency to become positive (+) and which have a greater tendency to become negative (−). The list is a handy tool to determine which combinations of materials create the most static electricity.
Slide52Charging by Contact
Also, charging by conduction, it is the process of giving an object a net electric charge by placing it in contact with an object that is already charged.
It should be noted that it is nearly always electrons that are exchanged.
Slide53Charging by Induction
It is possible to charge a neutral
conductor
without contact or
Charging by induction involves transferring charge between two objects without them touching.
Slide54Charging by Induction (grounding wire)
The earth has an infinite amount of electronsThe earth will never increase or decrease charge when indication occurs (The electron sink is to big to notice the few electrons transferred during an induction process. So
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The electrons are being pushed down this wire into the ground.
A negatively charged rod is brought near,
but does not touch the sphere. Electrons
within the sphere are repelled by the rod,
and pass through the wire to the ground,
leaving a net positive charge on the
sphere.
Slide55Charging by Induction (grounding wire)
© 2014 Pearson Education, Inc.
While the negatively charged rod remains near the sphere, the ground is removed. Note that there can be no more movement of electrons since the sphere is isolated from the ground. Electrons cannot jump the gap between the rod and the sphere or between the ground and the sphere.
The wire is removed, disconnecting the
sphere from the ground.
Slide56Charging by Induction (grounding wire)
The rod is then removed. It is important to note that the charge on the rod remains constant (negative). The charge on the sphere is now positive as it lost electrons to Earth.
Compared to the amount of free
electrons already in Earth, the earth has gained an insignificant amount of charge and therefore the charge on the earth still stays the same (neutral).
Note: If the ground were left in place, once the initially charged object was
removed, the neutral object will pass its gained charge back to the ground.
Slide57Charging by Induction
Slide58Charging by Induction
Slide59What is Charge Polarization??
An unpolarized atom.
With an external electric field, the center of electron cloud shifts to the left, or
polarizes
.
Slide60Charge Polarization
Slide61Charge Polarization
Neutral objects may be a attracted to charged objects through charge polarization:
Slide62Charge Polarization
Slide63The Electroscope One More Time!!!
Slide6414.4 Coulombs Force Law
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Slide65Electric charge (1 More Time) © 2014 Pearson Education, Inc.
Electric Charge
Electric Charge (symbol q or Q) is a property of objects that participate in electrostatic interactions.
Electric charge is quantized-you can only change on object’s charge by increments, not continuously
Electric Charge is conserved
Unit for electric charge is the coulomb
Smalles
increment of charge is that of one electron –e = -1.6 x 10
-19
C
Slide66Coulomb's force law
In 1785, Charles Coulomb determined the relationship between distance and magnitude of charges, and the force between charges. The experimental apparatus Coulomb used is called a torsion balance.
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Slide67Coulomb
's law
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Like charges repel, opposites attract.
Slide68Gravitational and Electric Force© 2014 Pearson Education, Inc.
Gravitational force depends on mass of the objects
Electric force depends on the charge of the objects
Gravitational force is always attractive
Electrical force attractive or repulsive
Proportionality constants are much different
Slide69Gravitational and Electric Force
© 2014 Pearson Education, Inc.
Compare the gravitational force to the electrical force exerted
byt
the proton on the electron in an hydrogen atom
Slide70Comparing the magnitude of the electric force to the gravitational force
Consider a proton and an electron in a hydrogen atom. They are separated by about 10–10
m.
The electric force between the two objects is 2.3 x 10
–8
N.
The gravitational force between the two objects is 1.0 x 10
–47
N.
The electric force between these objects is about 2 x 1039
times greater than the gravitational force!
The earth gravitational pull on the e
- is 18 orders of magnitude greater than the gravitation force exerted by the proton but….it’s still 22 orders of magnitude less than the electrical forceThis why Physics confidently ignore gravitational forces when dealing with atomic size particles.© 2014 Pearson Education, Inc.
Slide71Tip
© 2014 Pearson Education, Inc.
Slide72Solving Electric Force ProblemsSuperimposition of Electrical Forces
Follow this procedure: 1. Assume all charges, other than the one that the initial net force is being calculated for
, are immobile
- this will allow the determination of the direction of the individual initial forces.
2.
Draw a free body diagram for each charge
, using the fact that opposite charges attract and like charges repel.
3. Use Coulomb's Law to find the magnitude of each force.
4.
Sum the forces
, taking into account that they are vectors with direction and magnitudes.
Use the free body diagrams to assign signs to the forces
- if they point to the right, they are positive; if they point to the left, they are negative.© 2014 Pearson Education, Inc.
Slide73Solving Electric Force Problems
Force Labeling ConventionF12 is the force that Q
1
exerts on Q
2.
F
13
is the force that Q
1
exerts on Q3.
F
23
is the force that Q2 exerts on Q3.Note that by the application of Newton's Third Law:F12 = - F21F13 = - F31F23 = - F32© 2014 Pearson Education, Inc.
Slide74Solving Electric Force Problems
A positive charge Q1 = 25 μC is located at a point x
1
= -8 m, a negative charge Q
2
= -10
μC
is located at a point x
2
= 0 m and a positive charge Q3 = 15
μC
is located at a point x
3 = 4 m.a. Draw free body diagrams for the electric force acting on Q1, Q2 and Q3.© 2014 Pearson Education, Inc.b. Find the magnitude and direction of the net force on Q2.
Slide75Example
1
A –5
m
C
charge is placed 2 mm from a +3
m
C
charge. Find the force between the two charges.
-
+
2 mm
+3
m
C-5 mCqq’Draw and label givens on figure:rFF = 3.38 x 104 N; AttractionNote: Signs are used ONLY to determine force direction.
Slide76Summary
© 2014 Pearson Education, Inc.
Slide77Summary© 2014 Pearson Education, Inc.
Slide78Summary© 2014 Pearson Education, Inc.
Slide79Summary© 2014 Pearson Education, Inc.