Physics_100_chapt_11

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Transcript Physics_100_chapt_11

Electrical forces
Physics 100
Chapt 11
Ben Franklin
Matter is filled with a mysterious
Fluid-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 normal
amount 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
Franklin hypothesized that rubbing a comb through hair
moved some “electricity” from the comb to the hair
Comb has an “electricity”
deficit & thus becomes
negatively charged
Hair has a surplus
& becomes
positively charged
_
_
_
_
+
+
+
+
Opposite charges attract
Like-sign charges repel
_
_
Modern picture of “electricity”
_
_
+
++
++
_
_
_
_
Negatively charged electrons
orbit around a positively
charged nucleus
Electrical attraction
between minus electrons
& plus nucleus keeps
electrons in orbit
Positive Ion
An atom with one or more
electrons removed
_
_
_
+
++
++
_
_
_
”net” charge is positive
“electricity” flow
“electricity” doesn’t flow from comb to hair, some
electrons from hair atoms get stuck on the comb
Comb has an excess of
of electrons & is thus
negatively charged
The removal of
each electron
leaves a positive
ions in the hair
_
_
_
_
+
+
+
+
Atomic electrons play the role of
“fluid-like electricity”
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_
+
++
++
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_
_
+
Electrical
Conductors
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_
_+ _
_
_
_
_
_
_
positive ion
(fixed in crystal)
Conduction electron
free to move around
_
_
+ _ + _ + _ + _ + _ + _ + _ +_
+ _ + _ +_ + _ +_ +_ +_ +_ +
+ _+ _ + _ + _ + _+ _+ _ + _+
_
_+ _ + _ + _ + _ + _ + _ + _ + _ +
+ + + + + + + + +
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
proportional to the
charge of each object
Inversely proportional
to the square of the
distance between them
combine:
r
q2
Fc  q1
Fc  q2
1
Fc  2
r
q1 q2
Fc  2
r
Proportionality constant:
“Coulomb’s Constant”
q 1 q2
Fc = k r2
Units & Coulomb’s constant
q 1 q2
Fc = k r2
Unit of charge = “Coulomb”(C)
Unit of distance = m
k = 9.0 x 109 Nm2/C2
This is a big number
Hydrogen atom
electron
qe = -1.6x10
qp = +1.6x10
5x10–11 m
proton
–19
C
–19
C
qe qp
Fc = k r2
=9x109Nm2/C2
(1.6x10-19C)2
(5x10-11m)2
=9.2x10-8N
Gravitational force in an atom
electron
5x10–11 m
proton
me = 9.1x10
mp = 1.7x10
–31
kg
–27
kg
memp
FN = G r2
-31kgx1.7x10-27kg
9.1x10
=6.7x10-11Nm2/kg2
(5x10-11m)2
=4.1x10-47N
Electrical vs gravitation at atomic
scales
FC=9.2x10-8N
_
FN=4.1x10-47N
5x10–11 m
+
FC
FN
=
9.2x10-8N
+39
=
2.2x10
4.1x10-47N
Electrical force is 1039x the gravitational force!!