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Physics 212Electric Charge
Properties of Objects
Fundamental Particles
Atoms and Ions
Quantization of Charge
Conservation of Charge
Conductors, Insulators and Semiconductors
How to Put Charge on an Object
Polarization
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Physics 212Electric Charge
Physical Properties
That which is proper to anything;a peculiar quality of a thing;that which is inherent in a subject, or naturally essential to it;an attribute; as, sweetness is a property of sugar. Physical properties, or those which result from the relations of bodies to the physical agents,light, heat, electricity, gravitation, cohesion, adhesion, etc.,and which are exhibited without a change in the composition or kind of matter acted on. http://www.wordiq.com/dictionary/Properties
Examples of physical properties of objects include…
ColorMassHardness
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Physics 212Electric Charge
Intrinsic Physical Properties
An intrinsic physical property is a property of a set of objects that is constant throughout the entire set over space and time.
Examples of intrinsic physical properties of an NFL football include…
shape
color
texture
material
size
weight
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Physics 212Electric Charge
Intrinsic Physical Properties of Fundamental Particles
Everything in the universe is made up of a finite number of fundamental particles.
Each of these particles has the following intrinsic physical properties.
mass
spin
size
charge
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Physics 212Electric Charge
There are only two types of charge called positive and negative
Unlike charges attractand like charges repel.
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Physics 212Electric Charge
Why positive and negative?
Imagine what it would be like for the types of charges to be named in a different way.
Which of these would best lead to an understanding of the difference between these types of charge and why?
A and BGood and EvilLeft and RightJack and Jill ?21 Evil Good
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Physics 212Electric Charge
All figures shown here are from Lawrence Berkley National Laboratory“The Particle Adventure”
The universe is made of 28 fundamental particles6 quarks, 6 leptons, their antiparticles and some force particles.
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Physics 212Electric Charge
Most charge that we experience is found in the quarks and the electron.
The fundamental unit of charge is
(C) Coulombs 1910602.1 e
Each quark carries a fraction of this charge.
Electrons have a charge of -e
All figures shown here are from Lawrence Berkley National Laboratory“The Particle Adventure”
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Physics 212Electric Charge
Fundamental Charge
Quarks are only typically in combinations of three called protons and neutrons
A proton contains 1 down and 2 up quarks (+e charge)A neutron contains 1 up and 2 down quarks (0 charge)An electron is a fundamental particle (-e charge)
All figures shown here are from Lawrence Berkley National Laboratory“The Particle Adventure”
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Physics 212Electric Charge
Positive charge is created when an atom loses an electron.
This is called a positive ion.
Negative charge is created when an atom gains an electron.
This is called a negative ion.
Charge found on most objects comes typically from one place…
The object has either too many or too few electrons.
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Physics 212Electric Charge
All ordinary matter consists of …
Protons Mass = 1.67x10-27 kg Charge = +eNeutrons Mass = 1.67x10-27 kg Charge = 0Electrons Mass = 9.11x10-31 kg Charge = -e
All ordinary matter must therefore havea total charge which is an integermultiple of e.
Charge is designated by eitherQ or q.
eNeNQ electronsprotons
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Physics 212Electric Charge
Example 1
Dr. Mike puts electrons on a neutral balloon by rubbing it in his hair. The balloon gains -6.3 μC of charge. How many excess electrons did the balloon receive from Dr. Mike?
Answer
The number is given by Nelectrons in the equation from the previous slide.
electrons
QN
e
6
19
6.3 10 C
1.602 10 C
133.93 10
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Physics 212Electric Charge
Example 2
Dr. Mike puts electrons on a neutral balloon by rubbing it in his hair. The balloon gains -6.30 μC of charge. By how much did Dr. Mike’s weight change?
Answer
In Example 1, we calculated that the balloon received 3.93 x 1013 electrons. The weight of these additional electrons is then found as follows.
electrons electronM N m
13 313.93 10 electrons 9.11 10 kg 173.58 10 kg
17 163.58 10 kg 9.81 m/s 3.51 10 Nw Mg
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Physics 212Electric Charge
Example 3
Dr. Mike puts electrons on a neutral balloon by rubbing it in his hair. His hair loses 3.5 x 1016 electrons. How much charge does Dr. Mike have when he is finished?
Answer
The charge is given by Q in the equation from the previous slide.
protons electronsQ N e N e
16 19
19
3.5 10 protons 1.602 10 C
0 electrons 1 602 10 C-
- .
35.6 10 C 5.6 mC
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Physics 212Electric Charge
Conservation
The amount of charge in the universe never changes.
Example
Dr. Mike puts electrons on a balloon by rubbing it in his hair. The balloon gains -6.3 μC of charge. How much charge does Dr. Mike’s hair have after he has done this?
Answer
The electrons moved from Dr. Mike to the balloon. Thus, the charge that move to the balloon was taken away from Dr. Mike and he has +6.3 μC of charge more than he had when he began.The total charge in the universe is still the same as it was when he began.
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Physics 212Electric Charge
You can easily determine the number of electron and protons in a mole of some element.
Example
How much negative electron charge is in 3 moles of oxygen?
Answer
There are Avogadro’s number of oxygen atoms in one mole. Using the periodic table (look it up on the web or in your chemistry textbook), we see that there are 8 electrons in on oxygen atom. And there is –e charge in each electron. So we have…
electron
C10602.1
atomoxygen
electrons8
mole
atoms1002.6moles 3 1923Q
C1031.2 6Q Note: There are just as many protons. So 3 moles of oxygen is actually neutral.
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Physics 212Electric Charge
In an insulator, all of the electrons are bound to a nucleus…even the excess charges which stay right where they were forced to be by friction.
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Physics 212Electric Charge
In a conductor, some of the electrons are free to move between atoms.
Excess electrons will move freely as well, but they will stay on the surface of the conductor.
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Physics 212Electric Charge
In a semiconductor, the electrons cannot move unless given a high enough energy.
They are insulators when the energy is low.
and conductors when the energy is high
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Physics 212Electric Charge
A photoconductor is a semiconductor that uses light as its input energy.
They are insulators when the light energy is low.
and conductors when the light energy is high
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Physics 212Electric Charge
Dipoles
Dipoles are pairs of equal positive and negative charge that can rotate but not translate.
The intrinsic physical properties of the dipole are its charge and its length. (It also has mass and spin but we will almost never speak of the mass or spin of a dipole.)
The dipole is sometimes described by its dipole moment. The magnitude of the dipole moment is given by the equation
and it points from positive to negative in direction.
L
qLp
-q
+qp
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Physics 212Electric Charge
Polarization
Dipoles always attract charged objects.
1. A negatively charged object causes the dipole to rotate so that the positive charge is closer to the conductor.
2. A positively charged object causes the dipole to rotate so that the negative charge is closer to the conductor.
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Physics 212Electric Charge
Inductors and conductors sometimes exchange charge by rubbing them together.
Use the Tribolectric Table to know which way the charges move.
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Physics 212Electric Charge
In conductors, electrons are free to move…
There are two ways to add charge to a conductor.
1. Touching a charged rod to a conducting sphere transfers electrons to the sphere.
This is called charging by contact.
A negative total charge remains
What would happen if the rod were positive to begin with?
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Physics 212Electric Charge
2. If a charged rod is simply brought near a conducting sphere, the electrons will begin to move. And, some will go into the ground.
This is called charging by induction.
In conductors, electrons are free to move…
There are two ways to add charge to a conductor.
A positive total charge remains
What would happen if the rod were positive to begin with?
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Physics 212Electric Charge
In insulators, electrons are not free to move. They stick to the atom…
Insulators can shift their charges and will therefore attract any charged conductor.
1. A negative conductor repels the electrons and is then attracted by the closer protons.
2. A positive conductor attracts the electrons and is then attracted by these closer electrons.
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Physics 212Electric Charge
Dipoles are pairs of positive and negative charge that can rotate but not translate.
Like insulators they always attract the conductor.
1. A negative conductor causes the dipole to rotate so that the positive charge is closer to the conductor.
2. A positive conductor causes the dipole to rotate so that the negative charge is closer to the conductor.