Lecture series for Conceptual Physics, 8th Ed.

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Lecture series for Conceptual Physics, 8th Ed.
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Electrical forces p373
Electrical forces are a gazillion times stronger
than gravity.
They should crush together or tear everything
apart.
BUT THEY DONT.
Because there are two (2) electrical forces that
bunch together in atoms and are basically neutral.
Thank you atoms!
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Electrical Charges p373
1. Positive nucleusnegative electrons.
2. Electrons are all the same.
3. Nucleus protons and neutronsblah, blah.
4. Atoms are neutral until they become ions..
Electrons dont orbit in orbitals.
They exist in an area.
Think of electrons as waves that need a certain
amount of space.
The areas are called shellswhich are divided
into subshellswhich are divided into
orbitalswhich are divided into spins.
The whole periodic table is designed along these
lines.
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Actually, these lines
Shells
Subshells
s
p
d
f
spin
Why dont positive protons fly apart?
Because there are even stronger forces at short
range.
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Conservation of Charge p374
The hair and the plastic start out neutral.
Rubbing the electrons onto the plastic makes the
plastic rod negative and the hair is now positive.

Charges arent created or destroyed, but they can
be moved around.
Please remember that it takes energy to
add/remove electrons.
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Coulombs Law p376
K q1 q2
F
r2
Where k 9.0x109 N-m2/C2
Notice the similarity to Newtons Law of
Gravitation
G m1 m2
F
r2
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Conductors and Insulators p377
Conductors have a sea of electrons.
When a positive charge is put on one side of the
wire and a negative charge on the other,
then, electrons migrate toward the positive side.
Insulators have tightly bound electrons which
dont move.
Actually, theres a rainbow of conduction/insulati
on.
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Semiconductors p378
Theyre in the middle of the strong/weak electron
bond thing.
By adding impurities they can be made to conduct
or to insulate.
A whole bunch of these semiconducting thingies
make a transistor.
Superconductors p378
A material with infinite conductivity.
1987- a material that would superconduct over
100K was discovered.
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Charging p379
Charging by friction
Rubbing on/off electrons.
Then by contact
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Charging by 3 and 4
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Charging by Induction p379
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Charging by Grounding p380
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Friction of ice chunk vs ice chunk may be the
method of charging the cloud.
The negative bottom of the cloud drives electrons
away on the ground below.
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Charge Polarization p381
In an insulator electrons can only shift a little.
Fig. 21.11 shows a shift within a molecule.
A negatively charged balloon induces polarization
in a wall.
A charged rod brought near an insulator
A positive comb induces separation of charge in a
piece of paper.
OR, molecules may be polar by themselves.
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Electric Field p383
An electrical field is another kind of force
field.
A gravity field is a kind of force field
F
F
E
q
g
m
Fig 21-18 Shows some electric field lines.
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More on electric field
The lines used to represent electric fields have
direction away from positive and toward
negative.
-

Bits of cotton thread act like the piece of paper
in fig.21.13 and twist in place to line up with
the electric field.
When charges move, the disturbance in the E-field
affects other charges.
An E-field can store charge.
An E-field teamed with a magnetic field can move
through empty space.
An E-field can be channeled through metal wires.
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Electric Shielding p386
A and B push equally on the charged point.
A is 4x the charge but 2x the distance.
The NIFTY thing is that the E-field inside a
conductor is always zero.
0
0
0
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Electric Potential p388
Losing Electric PE
Losing GPE
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More on Electric Potential
Pushing creates energy in the spring.
Likewise, pushing a charge closer to another like
charge creates energy.
The closer two like charges are then, the more
electric potential energy they have.
It is convenient to define the electric potential
energy per charge as electric potential.
Electric potential is volts.
Electric potential energy
volts
Electric potential
charge
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So, lets work with electric potential energy
kqq
Ue
r
Whats the electric potential energy of two 3
Coulomb charges that are 25 cm apart?
kqq
(9.0x109)(3)(3)
3.2x1024 J

Ue
r
0.25
What is the voltage of one of these 3C charges?
Ue
3.2x1024 J
1.1x1011 J/C
electric potential


q
3C
1.1x1011 Volts
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2 charges equidistant from dome
Which charge has more electric potential energy,
Ue?
7C
Charged dome
14C
kqq
Ue
r
Which charge has the higher voltage?
They have the same voltage.
2 Ue
Ue
Joules

V

Coulomb
q
2 q
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A potential of 1 volt (V) equals 1 joule (J) of
energy per l coulomb C of charge.
Please!
So, take a 6.0 V battery.
How much energy does a 6.0 V battery give to each
coulomb of electrons that pass through it?
6.0 V
Ue is 2x as much with 2x the charge. BUT
V is the same (potential energy/charge.)
The terminal has 12V more than the terminal.
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Is this kid in mortal danger from a party balloon?
NO!
There are only a few charges.
V
So,
U
q
Theres not much energy so the kid is safe.
BUT, the energy per charge is high.
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Electric Energy Storage p390
Electric energy is stored in a capacitor.
12V
The 12V battery puts 12V of potential (V) between
the plates.
Area of capacitor
C is proportional to
Distance between them
Televisions use capacitors to store energy.
Keys on a keyboard push capacitor plates closer
together.
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Van de Graaff Generator p392
Scary, but safe. Large Vsmall charges.
The end