Chapter 21 Electric Charge
21-4 Coulomb's Law
; If two charged particles B brought near each other B
they each exert a force on the other
; If they have the same sing of charge B they repel each
other
; If the particles can move B they move away from each other.
; If the particles have opposite signs of charge B they
attract each other
; If the particles can move B they move closer to each other.
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Chapter 21 Electric Charge
The force of repulsion or attraction B electrostatic
force
(21-1)
B
along an axis extending through the
the two particles B has a magnitude of exactly 1 and no
dimension or unit B its purpose is to point
r B the distance between them
k B electrostatic constant
Eq. 21-1 B the same as that of Newton's equation B for
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Chapter 21 Electric Charge
the gravitational force between two particles with
masses ml Û m2 B separated by a distance r:
(21-2)
G B the gravitational constant
Both equations B describe inverse square laws B
involve a property of the interacting particles B the
mass in one case Û the charge in the other
The laws B differ B gravitational forces B always
attractive B electrostatic forces B attractive or
repulsive,
This difference B arises from B there is only one kind
of mass Û two kinds of charge.
Coulomb's law B survived every experimental test B no
exceptions to it have ever been found.
Coulomb's law B holds even within the atom Bcorrectly
describing the force B between the positively charged
nucleus Û each of the negatively charged electrons.
Coulomb's law B in magnitude form
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Chapter 21 Electric Charge
(21-4)
(21-5)
εο B permittivity constant Ð
(21-6)
; The gravitational force Û the electrostatic force Bboth
obey the principle of superposition
(21-7)
; shell theorem
A shell of uniform charge
B attracts or repels B
charged particle B outside the shell as if all the shell's
charge B concentrated at its center.
If a charged particle B located inside a shell of uniform
charge B no net electrostatic force B on the particle
from the shell.
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Chapter 21 Electric Charge
Spherical Conductors
If excess charge is placed on a spherical shell that is made of conducting
material, the excess charge spreads uniformly over the (external) surface
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