18.5 Coulomb’s Law
So charged objects exert forces on one another:
Let’s consider two positive
point charges
separated by some distance r:
Point charges
:
the charges are much smaller than the distance between them.
x
+
+
q
1
q
2
+F
F
r
*Since they are like charges, they repel each other, and Newton’s 3
rd
Law tells
us they do this with equal and opposite force.
So what do we know about this force?
grows weaker with distance
1.
It grows weaker with distance.
2.
It gets stronger for increasing charges.
3.
It’s directed along the line containing the two charges.
1
Coulomb determined that:
2
2
1
r
q
q
k
F
=
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View Full DocumentThis is Coulomb’s Law
2
2
1
r
q
q
k
F
=
k
is a constant:
2
2
9
10
99
.
8
4
1
C
m
N
k
o
⋅
×
=
=
πε
10
8.85
2
2
12

C
o
×
=
ε
This is the
permitivity of free space
.
m
N
⋅
Coulomb’s Law, like Newton’s Universal Law
of Gravitation, is an
inverse square law
.
2
2
1
r
m
m
G
F
=
*Nature likes inverse square laws.
They will
show up again.
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 Spring '08
 SPRUNGER
 Charge, Force, Electric charge, 2 Coulomb, 18.5 Coulomb

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