Physics 212
Charge
Properties of Objects
Fundamental Particles
Atoms and Ions
Quantization of Charge
Conservation of Charge
C d t
Conductors,
I
Insulators
l t
and
d Semiconductors
S i
d t
Polarization
How to Put Charge on an Object
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Physics 212
Charge
Physical Properties
That which is proper to anything;
a peculiar quality of a thing;
th t which
that
hi h is
i inherent
i h
t in
i a subject,
bj t or naturally
t
ll essential
ti l to
t it;
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
http://www.wordiq.com/dictionary/Properties
Examples of physical properties of objects include…
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Color
Mass
Hardness
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Physics 212
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
material
weight
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texture
color
size
Physics 212
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.
size
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charge
mass
spin
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Physics 212
Charge
There are only two types of charge called positive
and negative
Unlike charges attract
and like charges repel.
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Physics 212
Charge
Why positive and negative?
Imagine what it would be like for the types of charges
to be named in a different way
way.
Which of these would best lead to an understanding
of the difference between these types of charge and
why?
A and B
Good and Evil
Left and Right
Jack and Jill
1 Good + 2 Evil = ?
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Physics 212
Charge
The universe is made of 28 fundamental particles
6 quarks, 6 leptons, their antiparticles and some force
particles.
All figures
shown here
are from
Lawrence
Berkley
National
Laboratory
“The Particle
Adventure”
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Physics 212
Charge
Most charge that we experience is found in the quarks
and the electron.
The fundamental unit of charge is
e = 1.602 × 10 −19 Coulombs (C)
All figures
shown here
are from
Lawrence
Berkley
National
Laboratory
“The Particle
Adventure”
Each quark carries
a fraction of this
charge.
h
Electrons have a
charge of -e
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Physics 212
Charge
Fundamental Charge
Quarks are only typically in combinations of three called protons and
neutrons
All figures
shown here
are from
Lawrence
Berkley
National
Laboratory
“The Particle
Adventure”
9
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)
Physics 212
Charge
Charge found on most objects comes typically from one place…
The object has either too many or too few electrons.
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Positive charge is created
when an atom loses an
electron.
Negative charge is created
when an atom gains an
electron.
This is called a positive ion.
This is called a negative ion.
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Physics 212
Charge
All ordinary matter consists of …
Protons
Neutrons
Electrons
Mass = 1.67x10-27 kg
Mass = 1.67x10-27 kg
Mass = 9.11x10
9 11x10-31 kg
Charge = +e
Charge = 0
Charge = -e
e
All ordinary matter must therefore have
a total charge which is an integer
multiple of e.
Charge
g is designated
g
by
y either
Q or q.
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Q = N protons (+ e ) + N electrons (− e )
Physics 212
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
f
Dr. Mike?
Answer
The number is given by Nelectrons in the equation from the previous
slide.
N electrons =
Q
−6.3 × 10−6 C
=
= 3.93 × 1013
−19
−e
−1.602 ×10 C
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Physics 212
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.
∆M = N electrons melectron
= ( 3.93 ×1013 electrons ) ( 9.11× 10−31 kg )
= 3.58 × 10−17 kg
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∆w = ∆Mg = ( 3.58 × 10−17 kg ) ( 9.81 m/s ) = 3.51× 10−16 N
Physics 212
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.
Q = N protons ( e ) + N electrons ( −e )
= 3.5 × 1016 protons (1.602 × 10−19 C ) +
0 electrons ( -1.602 × 10-19 C )
= 5.6 ×10−3 C = 5.6 mC
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Physics 212
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 212
Charge
You can easily determine the number of electron and
protons in a mole of some element.
Example
p
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),
textbook) we see that there are 8 electrons in on
oxygen atom. And there is –e charge in each electron. So we
have…
atoms 
electrons 
C 

 − 1.602 × 10 −19
Q = (3 moles) 6.02 ×10 23
 8

mole
oxygen
atom
electron




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Q = −2.31×106 C
Note: There are just as many protons. So 3
moles of oxygen is actually neutral.
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Physics 212
Charge
In an insulator, all of the electrons are bound to a nucleus.
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Physics 212
Charge
In a conductor, some of the electrons are free to move between
atoms.
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These electrons stay on the surface of a conductor.
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Physics 212
Charge
In a semiconductor, the electrons cannot move unless given a
high enough energy.
They are insulators when the energy is low.
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and conductors when the energy is high
Physics 212
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A photoconductor is a semiconductor that uses light as its
input energy.
They are insulators when the light energy is low.
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and conductors when the light energy is high
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Physics 212
Charge
Dipoles
Dipoles are pairs of equal positive and negative charge that can rotate
but not translate.
-q
q
L
r
p
+q
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
p = qL
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and it points from positive to negative in direction.
Physics 212
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 212
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 212
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.
A negative
ti
total
charge
remains
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What would
happen if the
rod were
positive to
begin with?
This is called charging by contact.
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Physics 212
Charge
In conductors, electrons are free to move…
There are two ways to add charge to a conductor.
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.
What would
happen if the
rod were
positive to
begin with?
A positive
total
charge
remains
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This is called charging by induction.
Physics 212
Charge
In insulators, electrons are not free to move. They stick to the atom…
Insulators can shift there charges and will therefore attract any
charged conductor.
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1.
A negative conductor repels
the electrons and is then
attracted by the closer
protons.
2
2.
A positive conductor attracts
the electrons and is then
attracted by these closer
electrons.
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Physics 212
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.
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Physics 212
Charge
Properties of Objects
Fundamental Particles
Atoms and Ions
Quantization of Charge
Conservation of Charge
C d t
Conductors,
I
Insulators
l t
and
d Semiconductors
S i
d t
Polarization
How to Put Charge on an Object
28
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