Welcome 208266 Physics Students

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Welcome 208/266 Physics Students
Discussion Starts Today 115 SHL 116 All homework
assignments will be announced during DSC Lab
Starts Tomorrow 100 SHL 101B Read the 1st lab
in the Lab Manual Reading Assignment for
tomorrow chapter 23 sections 1-6, we should
cover 22 today. (Quiz tomorrow) You should
have 208/266 General Handout
(http//www.bartol.udel.edu/clem/208/208.htm)
Halliday/Resnick/Walker 6th Ed, Volume 3   Lab
manual (208 only)
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Electric Charge
1st Documented Observations of Electricity
Thales of Miletus in 600BC After rubbing Amber,
it will attract bits of straw The Greeks also
recorded Observations of Magnetism The mineral
lodestone, an oxide of iron, has the property of
attracting iron objects.   These two sciences
developed separately until 1820 Oersted
discovered that electric current can affect
magnetic compass needle Thus the beginnings of
a new Science Electromagnetism

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Electric Charge is stored in all objects that
surround us.

• It usually remains hidden because these objects
  contain equal amounts of two kinds of charge
  positive and negative.
•  
• With such an equality or balance, the object is
  electrically neutral, otherwise a net charge is
  present.
•  
• The imbalance is always very small to the total
  amount of positive and negative charge in the
  object.
•  
• Charged objects interact through by exerting
  forces on one another.
•  
• Charges with the same electrical sign repel each
  other, and charges with opposite electrical sign
  attract each other.
•  
• The positive and negative labels and signs
  for electric charge were chosen arbitrarily by
  Benjamin Franklin (the 1st American Physicist)

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Electric Charged may be generated by Friction

• Silk or Cloth on Glass
• -
  
• Fur on Rubber or
  Plastic
• 
  _
• During the rubbing process, tiny amounts of
  charge are transferred slightly upsetting the
  electrical neutrality of both.

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Electric Charge is stored in all objects that
surround us.
An electrical neutral Helium Atom consist of Two
electrons -2e (e electron unit of
charge) Two protons 2e Two neutrons
0 If there is an excess of electrons, it is a
negative ion and if there is a deficiency of
electrons, it is a positive ion.
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In the periodic chart, elements are sorted by the
number of charges in the Nucleus
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Materials have different Electrical
Properties (due to the structure and electrical
natural of atoms)

• Insulators - charges are fixed and cannot move.
  (Glass, Chemically pure water, Plastic)
• Conductors - charges (only electrons) can move
  around. (Metal, tap water, human body)
• Semiconductors - somewhere in between. (Adding
  certain impurities controls the degree of
  conduction.)

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An Interesting Phenomena is Charging by
InductionCompares the Difference between
Isolator and Conductor
Neutral Metal

Charged Rubber
Step 1
Step 3
Step 2
Step 4
Step 5
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Coulombs Law
The force between two charges q1 and q2 is
Coulombs Law
The force lies along the line joining the two
charges. It is repulsive if q1q2gt0 and attractive
if q1q2lt0 r is the distance between them Units
q is in Coulombs (C) F is in
Newtons (N) r is in meters
(m) k 8.99 x 109 N m2/C2
L
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The Coulomb is a very large unit
The typical amount of charge produced in a
demonstration is 1x10-6 C 1 ?C The charge on
a electron or proton is 1.6x10-19 C electron
unit of charge The total amount of charge in a
gram of protons is q 6.02x1023 x 1.6x10-19 C
96,500C (Avogadros ) If
the total charge in a gram of protons could be
extract over time span of hour 96,500/3600
26.5 amp-hr (car battery)
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Compare the Electric and Gravitation Forces
Find the attractive forces between a proton and
electron at a distance r 5.3x10-11 m (typical
separation in Hydrogen) Electric Force Fe k
e2/r2 8.99x109 (1.6x10-19
)2/(5.3x10-11)2 8.2x10-8 N Gravitational Force
Fg G Mp Me/r2 6.67x10-11
1.67x10-27 9.11x10-31/(5.3x10-11)2
3.61x10-47 N Fe/Fg 2.3x1039 Electric force
gtgtgt Gravitational force
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Characteristics of the Electric Force
Obeys the Principle of Superposition If we have N
charged particles, they interact independently in
pairs F1,Net F12 F13 F14 F15 .
F1N (Vector Sum) A shell of uniform
charge attracts and repels a charged particle
that is outside the shell as if all shells
charge were concentrated at the center. If a
charged particle is located inside a shell of
uniform charge, there is no net electric force on
the particle from the shell. If excess charge is
placed on spherical shell that is made of a
conducting material , the excess charge spreads
uniformly over the external surface.
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Additional Characteristics of the Electric
Charged

• The electric charge can only be found in integer
  number of electron charge units e
• qne, where n0, ?1, ? 2, ? 3,
• and e 1.6x10-19 C
• Hence, electric charge is Quantized
• The electric charge is conserved
• The net charge in isolated system can not change

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Fundamental Characteristics of Matter
MASS (m) CHARGE
(q) Single type of
mass Positive and
Negative Charges no need for "net mass
must deal with "net charge

most matter is electrical
neutral
no net charge
masses attract via
"Like charges repel gravitational force
unlike charges
attract."
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Fundamental Characteristics of Matter
MASS (m)
CHARGE (q)
Newtons Law of Gravity
Coulombs Law
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Fundamental Characteristics of Matter
MASS (m)
CHARGE (q)
Is mass conserved?No, not really!However, in
conjunction with energy it is through the
mass-energy equivalence Emc2.
Is charge conserved?YES! The net charge in
isolated system can not change