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Electric Forces and Electric Fields.
AP PHYSICS B - Chapter 18.
The Ancient Greeks Found Amber.
Many myths surround the origin of amber. Ovid writes that when
Phaeton, a son of Phoebus, the sun, convinced his father to
allow him to drive the chariot of the sun across the sky for a day,
he drove too close to the earth, setting it on fire. To save the
earth, Jupiter struck Phaeton out of the sky with his thunderbolts
and he died, plunging out of the sky. His mother and sister
turned into trees in their grief but still cried mourning him. Their
tears, dried by the sun, are amber.
The Greeks called amber elektron, or sun-made, perhaps
because of this story, or perhaps because it becomes
electrically charged when rubbed with a cloth and can attract
small particles. Homer mentions amber jewelry - earrings and a
necklace of amber beads - as a princely gift in the Odyssey.
Another ancient writer, Nicias, said that amber was the juice or
essence of the setting sun congealed in the sea and cast up on
the shore.
Amber ranges in age from 30 to 50 Million years old.
Magnetite: From Magnesia (Turkey).
Formula: Fe3O4.
Description: Dark grey, slightly shiny.
Magnetite is naturally magnetic. It is also
called Lodestone. In Middle Ages, pilots
were called lodesmen. The lodestar is the
Polar star, the leading star by which
mariners are guided.
The name probably comes from
Magnesia, but there is a fable of Magnes,
a Greek shepherd, who discovered
magnetite when the nails in his shoes
stuck to the ground!
The ancient Greeks saw no connection
between Electricity and Magnetism.
Today we know that:
 Electric forces hold atoms and molecules
together.
 Electricity controls our thinking, feeling,
muscles, and metabolic processes.
 Electricity and magnetism determine much of
our current technology (e.g. computers).
 Electricity and magnetism are linked on a
fundamental level.
Electric Charges
• Evidence for electric charges is everywhere, e.g.
Ex. static electricity and lightning.
• Objects may become charged by contact and
frictional forces. Ex. clothes in dryer
• Benjamin Franklin (1700’s) discovered that there
are two types of charges:
Ex. positive charge and negative charge.
• Franklin also discovered that like charges repel
and unlike charges attract one another.
• Electric charge was found to be both quantized
(Millikan) and conserved (Franklin).
Classes of Materials
• CONDUCTORS are materials in which
charges may move freely (e.g. copper).
• INSULATORS are materials in which
charges cannot move freely (e.g. glass).
• SEMICONDUCTORS are materials in
which charges may move under some
conditions (e.g. silicon).
Charges and the Earth
• The earth acts as a near-infinite source
or sink of charges, and therefore its net
charge cannot easily be changed.
• Any conductor in contact with the earth
is said to be GROUNDED and cannot
receive a net charge. (principle of
lightning rod)
Induced Charge
• Charged objects brought close to a conductor may
cause charge to redistribute (polarize the
conductor).
• If a polarized conductor is momentarily grounded,
charge will be transferred to/from the earth, and it
may be left with a net charge (by INDUCTION).
• Objects may be charged by
– conduction (requires contact with another
charged object.
– induction (requires no contact with another
charged object).
Benjamin Franklin determined that
there are only 2 types of charge.
Rubber Rod - Negative
Glass Rod - Positive
Unlike Charges Attract.
1706 - 1790
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Famous Quotations: 1. Haste makes waste.
2. Genius without education is like silver in the mine.
3. A penny saved is a penny earned.
4. Early to bed, early to rise makes a man healthy, wealthy, and
wise.
5. Never leave ‘til tomorrow which you can do today.
6. The sleeping fox catches no poultry.
7. To find out a girl's faults, praise her to her girl friends.
LIKE CHARGES REPEL.
Charging an Object by Induction Which
Involves No Contact but Includes Grounding.
The Theory About How an Insulator Works.
Charles Coulomb
1736-1806
Discovered Coulomb’s Law
experimentally
Charles Coulomb Measured the Force
Between Charged Objects.
F=k
·
(q1 q2)
d2
k = 9 x 109 N m2/C2
Quantum of Electric Charge
• Electric charge is quantized. The
smallest possible unit is the charge on
one electron or one proton:
1e- = 1.602 x 10-19 Coulombs
• No smaller charge has ever been
detected in an experiment.
• Atoms consist of a nucleus
containing positively charged
protons.
• The nucleus of an atom is
surrounded by an equal number
of negatively charged
electrons.
• The net charge on an atom is
zero.
• An atom may gain or lose
electrons, becoming an ion with
a net negative or positive
charge.
• Polar molecules have zero net
charge but their charges are
unevenly distributed in space
(e.g. water).
Electric Charges
in Atoms
Nuclear diameter ~ 10-15 m
(femtometer)
Atomic diameter ~ 10-9 m
(nanometer)
Electric field lines
Lines of force
Field Lines Always Point Away from
the Positive and Toward the Negative.
The Electric Field Around
Two Unlike Charges.
The Electric Field Around
Two Like Charges.
Electric Field Lines: Conventions
Positive Point Charge
Negative Point Charge
Electric Field lines
Two Ways to Measure Electric Field
Strength
E = F q0
F
d
E = k · Q d2
What happens if q0 is larger than a
test charge?
+ q0’ >> q0
Charging by Induction (no contact
with charged object)
Charged Rod
Two Metal Spheres
Charged Rod
Two Metal Spheres
(separated)
Now, Two Charged Metal Spheres
Charged Rod
One Metal Sphere
(polarized)
An Electric Field Can Accelerate a
Charged Particle.
RESULT: Charge on a proton = 1.6x10-19 C
This could occur in Millikan’s Oil-drop
Experiment.
RESULT: Elementary unit of charge =
charge on an electron = -1.6x10-19 C
Setting-up a Capacitor.
Q
C=
E
---------
++++
++++
++ + +
++++
V
Q
V
V
V
E=
d
W = q ·V
The Effect of Adding an
Insulator to a Capacitor.
insulator
Three Different Types of Capacitors.
A Stud-Finder is Really a Capacitor.
A Water Molecule.
Water Molecules in an Electric Field.
No Electric Field.
Electric Field, Eo