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Electric Forces and Fields.
IB PHYSICS – Fields 2.
Origins: 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.
The ancient Greeks did not perform any
quantitative Electricity experiments.
Today we know that:
Electric forces hold atoms and molecules
together.
 Electrical impulses control our thinking,
feeling, muscles, and metabolic processes.
 Electronics determine much of our current
technology.
Electric Charges
• Evidence for electric charges is everywhere.
(static electricity and lightning)
• Objects may become charged by contact and
frictional forces. (clothes in dryer)
• Benjamin Franklin (1700’s) discovered that there
are two types of charges:
(positive charge and negative charge)
• Franklin also discovered the Rules of Charge.
(like charges repel and unlike charges attract)
• Electric charge was found to be conserved and
quantized. (Franklin, late 1700’s and Millikan, early 1900’s)
Classes of Materials
• CONDUCTORS are materials in which charges
may move freely (Cu, Hg, Ag, Au, Al, Fe).
• INSULATORS are materials in which charges
cannot move freely (rubber, plastic, wood, glass).
• SEMICONDUCTORS are materials in which
charges may move under some conditions (e.g. Si,
Gd, Ge, Y).
1706 - 1790
• Famous Quotations: 1. Haste makes waste.
• 2. Never leave ‘til tomorrow which you can do today.
• 3. A penny saved is a penny earned.
• 4. The sleeping fox catches no poultry.
And lesser known ones …
1. Come to class with the knowledge of the day before.
2. If you can’t calculate what you’re talking about, you don’t
know what you’re talking about.
Charles Coulomb Measured the Force
Between Charged Objects.
Charles Coulomb
1736-1806
F=k
·
(q1 q2)
d2
k = 9 x 109 N m2/C2
• Electric charge is quantized. The fundamental unit of
charge is the charge on one electron or one proton:
1e- = 1.602 x 10-19 Coulombs
Millikan’s Oil-Drop Experiment
Robert Millikan 1900, while a
professor at the University of
Chicago
Two Ways to Measure Electric Field Strength
1. Using a test charge q0 to probe the field.
2. Coulomb’s Law.
(they agree)
E = F q0
F
d
E = k · Q d2
Electric Field lines always point away from the
positive and toward the negative.
• 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
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.
LIKE CHARGES REPEL.
Charging an Object by Induction Which
Involves No Contact but Includes Grounding.
Charging by Induction (no contact
with charged object)
Charged Rod
Two Metal Spheres
(separated)
Charges Redistribute, and we have Two Charged Metal Spheres.
Another
way
Charged Rod
One Metal Sphere
(polarized)
The Opposite Method
The Theory About How an Insulator Works.
What happens if q0 is larger than a
test charge?
+ q0’ >> q0
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
Lines of force
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
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!