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Physics 2B Summer Session 1
Unit 1 Lecture Notes
Motivation
The Electromagnetic Force (EM) is one of the
F
our
F
undamental
F
orces
found in nature:
1.
Gravitational Force
2.
Electromagnetic Force
3.
Strong Nuclear Force
4.
Weak Nuclear Force
The last two don’t affect our daily life on earth (see
Physics 2D
)
The EM is much, much
stronger
than Gravity
E&M determines the
structure
of matter, both atomic & molecular
So E&M is an
integral part
of Chemistry, Biology, Engineering, Etc.
Prolific use of electronic
devices
is found in all branches of science
Properties of Electric Charge
±
Charge is a
fundamental property
of matter, like mass
so charge doesn’t exist apart from some kind of particle
±
Charge was first discovered by the Greeks (
ελεκτροη
)
±
Charge comes in two flavors – positive & negative
±
Like
charges repel –
Unlike
charges attract
±
Charge is
Quantized
– electron (
e
−
) & proton (
e
+
)
±
Charge appears to be
balanced
in nature
There are an equal number of plus & minus charges
So we get Neutral Atoms and a Neutral Universe
±
Conservation – charge is neither created nor destroyed
MKS Unit =
Coulomb
(C)
+1
9
18
+
1 e
1.6 10
C
1 C
6.25 10 e
−
=×
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View Full DocumentFirst a quick word about
units
,
symbols
and the like
.
Take a look at the PDF file “MKS Units” on the course website under the
“Handouts” section. It is
important to know
that this system is
closed
under
all of the formulas and equations in this course. In other words, your solution
will come out in the correct fundamental units
provided
that you input
all
your data in the other appropriate fundamental units of the system.
For example, you wouldn’t expect the distance formula
s
vt
=
to yield an
answer in
miles per hour
if you put in
20 meters/second
v
=
and
22 minutes
t
=
. It was fairly easy in mechanics (
Physics 2A
) to cancel
physical units – remembering, say, that a
Joule
is equal to a kilogrammeter
per second squared. Here, however, there are many new and colorfully
named units which can be described in terms of other fundamental units in a
variety
of ways. So be careful with units.
Coulomb’s Force Law
Let’s begin with two electric point charges,
q
1
and
q
2
separated by a distance
r
and calculate the force
on
q
2
by
q
1
. Note that we have drawn a line
from
q
1
to
q
2
and extended that line with a
unit vector
which points
away
from the
source charge
q
1
.
The force is found using
12
2
ˆ
qq
k
r
=
12
Fr
Coulomb Force Equation
where the constant
2
9
0
1
9.00 10
4
N m
k
C
πε
==
×
i
Note the
resemblance
to the gravitational force equation
2
ˆ
mm
G
r
=
12
which also
Is an inverse square law with respect to distance,
Depends on the product of a fundamental particle property
and has Force Symmetry, i.e.,
= −
12
21
FF
For a numerical exercise, try to follow example 231 on page 574.
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 Summer '08
 schuller
 Force

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