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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
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
“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
ElectricalConductors
+ + + + + ++ + +
+ + + + + ++ + ++ + + + + ++ + +
+ + + + + ++ + ++ + + + + ++ + +
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+positive ion(fixed in crystal)
_ Conduction electronfree to move around
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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!!Fc = 22,000,000,000,000,000,000,000,000,000,000,000,000,000 x FN
N-poles & S-poles can never be isolated
NS
NS NS
Break a magnet into two pieces:
A new N-pole & S-pole are formed
Compass needles
If left to its own accord, the N-pole of amagnet will try to point towards the North
The Earth is a magnet
SouthPole
NorthPole
N
S
Earth’s geographicNorth pole is a
Magnetic S-pole
geographicSouth pole is a
magnetic N-pole
Coulomb’s law
q1
M1
q2
M2r
Fc = kq1q2
r2
FN = GM1M2
r2
Newton’s law of Gravitation
All possible
distances!
Electroscope
How do the electronsin the foil know thatsomeone is moving a charged object up here?
& that it’s negativelycharged?
& how much it ischarged by?
Faraday’s method for computingE fields
+
Imaginary small positive “test” charge qtest
Ftest=coulomb force on test charge
Ftest
E = Ftest/qtestx+
RulesE-field lines start on + charges and end on - charges
Direction of E-field lines = direction of force on a + charge placed at that point (opposite for – charge)Where E-field lines are close together, the E field is large; where they are far apart, the E-field is small.
The bigger the charge, the more E-field lines start (or stop) on it.
E-field lines never cross
Magnetic Fields
NS
xImagine a smallN-pole is placedat the point
NDetermine themagnetic forceon the imaginaryN-pole
B-field is inthe directionof the netforce
Rules
NS
B-field lines come out of N-poles & go into S-poles
B-field lines never end
Density of B-field strength of the field
The stronger the pole, the more B-field exit (or enter)
Gravitational Fields
mg =
Fm
m
Fmg Rules:
•g-field lines start at infinity & end on mass
•The more dense the lines, the stronger the g-field
•The bigger the mass, the more g-field lines end on it
•At any point in space, g is the acceleration due to gravity there. (On the surface of the earth, g = 10 m/s2.)
test particle force on test particle
mass of test particle
E-field RulesE-field lines start on + charges and end on - charges
Direction of E-field lines = direction of force on a + charge placed at that point (opposite for – charge)
Where E-field lines are close together, the E field is large; where they are far apart, the E-field is small.The bigger the charge, the more E-field lines start (or stop) on it.
E-field lines never cross