Post on 17-Dec-2015
Electrical forces
Physics 100
Chapt 11
Ben FranklinMatter is filled with a mysteriousFluid-like substance called “electricity.”
When an object has its “normal amount”of electricity, it is electrically “neutral”And doesn’t feel electrical forces.
When an object has more than its normalamount of electricity, it is “positively charged.” When it has less than its normal amount it is “negatively charged.”
Charged objects feel electrical forces.
Friction can move “electricity” from one object to another
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Franklin hypothesized that rubbing a comb through hairmoved some “electricity” from the comb to the hair
Comb has an “electricity”deficit & thus becomesnegatively charged
Hair has a surplus& becomespositively charged
Opposite charges attract
Like-sign charges repel
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Modern picture of “electricity”
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Negatively charged electronsorbit around a positively charged nucleus
Electrical attractionbetween minus electrons & plus nucleus keeps electrons in orbit
Positive Ion
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An atom with one or moreelectrons removed
”net” charge is positive
“electricity” flow
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Comb has an excess ofof electrons & is thusnegatively charged
The removal ofeach electron leaves a positive ions in the hair
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“electricity” doesn’t flow from comb to hair, someelectrons from hair atoms get stuck on the comb
Atomic electrons play the role of“fluid-like electricity”
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ElectricalConductors
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+positive ion(fixed in crystal)
_ Conduction electronfree to move around
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Charging by induction
Charging by induction (2)
Electroscope
Electrical Force
Charles Coulomb 1736-1806
Coulomb’s Law
The electrical force between two charged objects is proportional to the charge of each object and inversely proportional to the square of the distance between them.
Coulomb’s law
q1 q2r
Fc q1
Fc q2
Fc 1r2
combine: Fc q1q2
r2 Fc = kq1q2
r2
Proportionality constant:
“Coulomb’s Constant”
proportional to thecharge of each object
Inversely proportional to the square of the distance between them
Units & Coulomb’s constant
Fc = kq1q2
r2
Unit of charge = “Coulomb”(C)
Unit of distance = m
k = 9.0 x 109 Nm2/C2
This is a big number
Hydrogen atom
qe = -1.6x10 –19 Celectron
proton
qp = +1.6x10 –19 C
Fc = kqeqp
r2
=9x109Nm2/C2(1.6x10-19C)2
(5x10-11m)2
=9.2x10-8N
5x10–11 m
Gravitational force in an atom
me = 9.1x10 –31 kg
electron
proton
mp = 1.7x10 –27 kg
FN = Gmemp
r2
9.1x10-31kgx1.7x10-27kg(5x10-11m)2
=4.1x10-47N
5x10–11 m
=6.7x10-11Nm2/kg2
Electrical vs gravitation at atomic scales
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5x10–11 m FN=4.1x10-47N
FC=9.2x10-8N
FC 9.2x10-8N
FN 4.1x10-47N= = 2.2x10+39
Electrical force is 1039x the gravitational force!!