Chapter 21 Electric Charge
21-2 - Electric Charge:
In dry weather
Spark B produce by
o walking across certain types of carpet
o
Bringing B fingers near a metal doorknob
o
Bringing B fingers near a metal faucet
o
Bringing B fingers near a friend
Multiple sparks B produce when
o Pulling B a sweater from the body
o Pulling B clothes from a dryer
Sparks and the "static cling" of clothing
Are illustrated in
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Chapter 21 Electric Charge
FIG. 21·1 Static cling, an electrical phenomenon that
accompanies dry weather, causes these pieces of paper to stick to
one another and to the plastic comb, and your clothing
Electric charge B an intrinsic characteristic of the
fundamental particles making up those objects
Benjamin Franklin [1706 - 1790] arbitrarily B named the kind
of electricity On
Glass
Positive
Plastic
negative
Object B contains equal amounts of the two kinds of
charge B electrically neutral B no net charge
Object B contains no equal amounts of the two kinds of
charge B charge imbalance B there is a net charge
Charged objects interact by exerting forces on one
another.
.
Charges B have the same electrical sign B repel each
other
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Chapter 21 Electric Charge
Charges B have opposite
electrical signs B attract each
other.
First B charge a glass rod by rubbing one end with
silk
(Fig. 21-2a) B the two rods repel each other; that is,
each rod experiences a force directed away from the
other rod.
Second B charge a plastic rod by rubbing one end with
fur
(Fig. 21-2b), the two rods attract each other; that is,
each rod experiences a force directed toward the other
rod
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Chapter 21 Electric Charge
Attraction and repulsion between charged bodies have
many industrial applications
• Electrostatic paint spraying
• powder coating,
• fly-ash collection in chimneys
• Nonimpact ink-jet printing,
• Photocopying
Figure 21-3 B shows a tiny carrier bead in a
photocopying machine, covered with B particles of
black powder called toner B stick to it B electrostatic
forces.
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Chapter 21 Electric Charge
21-3 Conductors and Insulators
The properties of conductors and insulators are due to the
structure and electrical nature of atoms
Atoms consist of
Negatively charged
Positively charged
Neutral particle
electrons
proton
neutron
All matter is electrical interaction between electrically
charged particle
Bohr atom
Bohr atom B analogous to a solar system
The electrical forces B taking the place of gravitational
forces.
Nucleus B Protons + Neutrons
Electrons B outside the nucleus B large distance from it.
Solar System
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Chapter 21 Electric Charge
Ionization
The atom as a whole is electrically neutral
e
P
N
N
P
e
N
P
e
P
e
N
N
P
neutral atom
e
N
P
e
Negative ion
Positive ion
; How conduction can eliminate excess charge on an
object ?
Do The following experiments
6 Experiment (1)
rubbed
Observation
Hand
Fur
Metal
Earth
The rod does not charge
6 Experiment (2)
rubbed
Fur
Hand
Plastic
Observation
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Metal
Chapter 21 Electric Charge
The metal rod is charged and it attracted with the piece of fur
6 Experiment (3)
The rod of plastic is replaced by the rod of glass
rubbed
Fur
Hand
Glass
Metal
Observation
The metal rod is charged and it attracted with the piece of fur
Conclusion (1)
The metal, the human body, and the earth are conductors of
electricity.
Evidence: Permit electric charge to move from one to the other
region of material
Conclusion (2)
The glass, plastic, and etc. are insulators.
Evidence: Do not permit electric charge to move from one to the
other region of material.
Conclusion (3)
; In electricity
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Chapter 21 Electric Charge
Materials classified B
according to the ability of charge to
move through them.
Conductors B charge can move rather freely
metals Û copper Û human bodyÛ water.
Insulators B charge cannot move freely
Plastic Û glass Û chemically pure water
Semiconductors B intermediate B conductors Û
insulators
silicon Û germanium
Superconductors B charge to move without any
hindrance
materials B perfect conductors
; Demonstrates the mobility of charge in a conductor
FIG.21-4
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Chapter 21 Electric Charge
; In B Fig.21-4 ©
First
A neutral copper rod B electrically isolated from its surroundings by being suspended on a nonconducting thread
The conduction electrons B in the closer end of the
copper rod B repelled by the negative charge on the
plastic rod
Some of the conduction electrons B move to the far
end of the copper rod B leaving the near end with an
unbalanced positive charge
Negative charge B attracts the remaining positive
charge on the near end of the copper rod B rotating the
copper rod B bring that near end closer to the plastic
rod.
A copper rod B still neutral B have an induced charge B
B
some of its positive and negative charges Bseparated
due to the presence of a nearby charge
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Chapter 21 Electric Charge
Second
A positively charged glass rod B brought near one end of a
neutral copper rod B conduction electrons in B copper rod B
attracted to that end.
End B becomes negatively charged Û the other end B
positively charged
; Only conduction electrons B negative charges B moveB
positive ions B fixed in place B an object becomes
positively charged only through the removal of negative
charges.
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