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Electrostatics
ELECTROSTATICS

Electricity at rest
A Bit of History

Ancient Greeks
–
Observed electric and magnetic phenomena as
early as 700 BC
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
Found that amber, when rubbed, became electrified and
attracted pieces of straw or feathers
Magnetic forces were discovered by observing
magnetite attracting iron
A Bit More History

William Gilbert
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–

1600
Found that electrification was not limited to amber
Charles Coulomb
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–
1785
Confirmed the inverse square relationship of
electrical forces
History Final
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Hans Oersted
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–

1820
Compass needle deflects when placed near an
electrical current
Michael Faraday
–
A wire moved near a magnet, an electric current
is observed in the wire
Properties of Electric Charges

Two types of charges exist
–
–
positive and negative
Named by Benjamin Franklin
•Like charges repel
•Opposite charges attract
+
+
+
-
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+
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-
Charges continued
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+
The natural order is balanced charges
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-
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+
+
- +
-
-
•Unbalanced
charges are
possible
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-
+
+
+
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-
Net charge of zero
+
-
+
+
+
+
+
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- +
-
Question #2
•The
charge on sphere 2 is three times the
charge on sphere 1
•
Which force diagram is correct?
•
A,B,C,D, or E (none of them)
More Properties of Charge

Positive charge carrier is the proton
–
Protons do not move from one material to another


Held in Nucleus
Negative charge carrier is the electron (e-)
–
An object becomes charged (+ or -) by gaining or
losing electrons
More Properties of Charge

Electric charge is always conserved
–
–
Charge is not created, only exchanged
Charging occurs through the exchange of
electrons
 Lose
–
Gain a positive charge
 Gain
–
an electron
an electron
Gain a negative charge
Properties of Charge, final


The SI unit of charge is the Coulomb (C)
Charge is quantized
–
All charges are a multiple of the fundamental unit
of charge, symbolized by (e)
–
Electrons have a charge of e e- = -1.602 x 10-19 C
Charge of -2 = 2* e- = 2* (-1.602 x 10-19 C)
– Protons have a charge of e+
–
 e+ = 1.602 x 10-19 C
–
Charge of +2 = 2* e+ = 2* (1.602 x 10-19 C)
Conductors

Conductors: materials in which the electric
charges move freely
–
–
-
-
Copper, aluminum and silver are good conductors
When a conductor is charged in a small region,
the charge readily distributes itself over the entire
surface of the material
- - - -
- - - -
Insulators

Insulators : materials in which electric
charges do not move freely
–
–
Glass and rubber are examples of insulators
When insulators are charged by rubbing, only the
rubbed area becomes charged


There is no tendency for the charge to move into other
regions of the material
semiconductors : characteristics between
those of insulators and conductors
–
Silicon and germanium are examples
Charging…

Three ways
–
Friction
 Mechanical
–
Conduction (or Contact)
 Direct
–
motion (rubbing)
contact (no rubbing)
Induction
 Charge
alteration without any contact
Charging by Friction


Self-explanatory…
(demo)
Charging by Conduction


A charged object (the
rod) is physically
touches the other
uncharged, object (the
sphere)
The same type of
charge is CONDUCTED
from the rod to the
sphere
Charging by Conduction
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-
-
-
-- -- - - - - - - -- - - - --
--
--
--
--
- - --
Charging by Induction
•
Induced charge - NO physical contact between
charged & uncharged object
•
OPPOSITE charge is INDUCED
Temporary charge Induction
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+
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+
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- +
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ELECTRICALLY POLARIZED
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+
+
+
-
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+ +
- -
Permanent charge Induction
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+
+
- +
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GROUNDING
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+
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Another way to Induce a charge
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- +
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++
+
+
+
+
++
+
-
+ -
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+
Net Charge of Zero
+ -
- ---
--
Question #3
An alpha particle with two positive charges and
a less-massive electron with a single negative
charge are attracted to each other.
The force on the electron is:
a) Greater than that on the alpha particle
b) Less than that on the alpha particle
c) Same as that on the alpha particle
d) I haven’t a clue…
Answer #3: (c) Same
The force on the electron the same
as that on the alpha particle Newton’s Third Law.
Question #4
An alpha particle with two positive charges and
a less-massive electron with a single negative
charge are attracted to each other.
The particle with the most acceleration is the
a) Alpha particle
b) Electron
c) Neither - they have the same acceleration
d) I haven’t a clue…
Answer #4: (b) Electron
The particle with the most
acceleration is the ELECTRON.
Newton’s Second Law (F=ma)
Question #5
An alpha particle with two positive charges and a lessmassive electron with a single negative charge are
attracted to each other. As the particles get closer to
each other, each experiences an increase in:
a)
b)
c)
d)
e)
force
speed
acceleration
All of these
None of these
Answer #5: (d) ALL
As the particles get closer, the FORCE
 and thus the ACCELERATION  and
also the SPEED 
Electrical Field

Gravitational Field - A force field that exists
around any object with mass
–

Interacts with mass
Electric field - A force field that exists around
a charged object
–
–
Interacts with charges
How do we know it exists?


If another charged object enters this electric field, the
field exerts a force on the second charged object
direction of movement determines charge of the field
Visualizing an Electric Field

Michael Faraday developed the concept of
drawing Electric Field Lines
–
–
–
Vector quantity
Proximity of field lines indicates field strength
Arrows indicate direction of field

Direction indicates the charge
–
–
Out of positive
Into negative
Electric Field Lines

Point Charge
–
Field lines radiate
equally in all directions

–
Radiate out on positive
Proximity to each other
indicates field strength
+

Negative Point Charge
–
Lines point inward

Towards the charge
-
Electric Field Line Patterns

Electric dipole consists of two equal
and opposite charges
–
Add field lines

–

Connected field lines
indicates opposite charge
Matching numbers of
field lines indicates
similar charge values
The high density of
lines between the
charges indicates the
strong electric field in
this region
+
-
Electric Field Lines
Electric Field Line Patterns


Two equal but like point
charges
Zoomed out (far away)
–



the field would be appear
to be one charge
Zoom in (close-up)
No connections indicate
like charges; (repulsion)
Low density of field lines
between the charges
indicates a weak field in
region “C”
Electric Field Lines
Electric Field Patterns


Unequal and unlike
charges
Note that two lines
leave the +2q charge
for each line that
terminates on -q
Electric Field
Lines, cont.
Electric Field, cont.

How do we know they are there?
–

Interact with charges
How do we know what charge they are?
–
–
Experimenting (testing)
test charge, placed in the field, will experience a force
Electric Field Testing, cont.
+
+
+
+ + +
+ + +
+
+ + +
Electric Field Testing, cont.
+
Direction of Electric Field

The electric field
produced by a negative
charge is directed
toward the charge
–
A positive test charge
would be attracted to the
negative source charge
Direction of Electric Field, cont

The electric field
produced by a positive
charge is directed away
from the charge
–
A positive test charge
would be repelled from
the positive source
charge
Electric Field

Mathematically,
F = q 0E
q
E pt chg = k 2
r

The electric field is a vector quantity
Question #9

What is the magnitude of the electric field
0.50 meters away from a -3C point charge?
a)
b)
c)
d)
e)
1.08 x 105 N/C
-1.08 x 105 N/C
5.4 x 104 N/C
-5.4 x 104 N/C
I don’t have a clue…
F = q 0E
q
E pt chg = k 2
r
Answer #9: (a) 1.08x105 N/C

What is the magnitude of the electric field
0.50 meters away from a -3C point charge?
a)
b)
c)
d)
e)
1.08 x 105 N/C
-1.08 x 105 N/C
5.4 x 104 N/C
-5.4 x 104 N/C
I don’t have a clue…
F = q 0E
q
E pt chg = k 2
r
Question #9

What is the magnitude of the electric field
0.50 meters away from a -3C point charge?
a)
b)
c)
d)
e)
1.08 x 105 N/C
-1.08 x 105 N/C
5.4 x 104 N/C
-5.4 x 104 N/C
I don’t have a clue…
F = q 0E
q
E pt chg = k 2
r
Electrostatic Forces
•
If like charges repel and opposites attract…
•
•
That means there is motion
If there is motion there must be a force (F = ma)
•
•
Newton’s Second Law
There must be a way to calculate the electrostatic
force!
Coulomb’s Law
q1 q 2
F = ke 2
r
F = electrostatic force or electrical force
ke = electrostatic force constant
aka – proportionality constant
aka – Coulomb’s Constant
= 9.0x109 Nm2/C2
q1 = charge for particle 1
q2 = charge for particle 2
r = radius (distance between charges)
Question #4
Two charges (+20 C and -10 C) are 3 m apart.
What is the magnitude of the force between them?
F=?
q1 = +20uC
q2 = -10uC
r = 3m
ke = 9x109 Nm2/C2
a)
b)
c)
d)
0.2 N
0.6 N
22.22 N
2.0 x 10 11 N
q1 q 2
F = ke 2
r

9 x10 Nm
F
9
F  0.2 N
2


/ C 2 20 x106 C 10 x106 C
(3m) 2

Balloon on a Ceiling

Which is stronger the force of gravity, or
electrical force?
Vector Nature of Electric Forces



Two point charges are
separated by a distance r
The like charges produce
a repulsive force between
them
EQUAL
The FORCE
force on q1 is
equal in
magnitude andOPPOSITE
opposite in
direction to the force on q2
Vector Nature of Electric Forces



Two point charges are
separated by a distance r
The unlike charges
produce a repulsive force
between them
EQUAL
The FORCE
force on q1 is
equal in
magnitude andOPPOSITE
opposite in
direction to the force on q2
r
Question #6
If q1 = +20 C and q2 = +10 C and the two
charges are 3 meters apart, what is the
MAGNITUDE of the force between them?
a)
b)
c)
d)
e)
0.2 N
0.6 N
22.22 N
2.0 x 10 11 N
I don’t have a clue
q1 q 2
F = ke 2
r
Answer #6: (a) 0.2 N
q1 q 2
F = ke
r2
9
-6
-6
9x10
20x10
10x10
(
)
(
)
(
)
F=
2
3
F = 0.2N
Question #7
If q1 = +20 C and q2 = +10 C and the two
charges are 3 meters apart, what is the
DIRECTION of the force between them?
a)
b)
c)
d)
e)
Away from each other
Towards each other
One chases the other
Nothing - they don’t move at all
I don’t have a clue
q1 q 2
F = ke 2
r
Answer #7: (a) Away
If q1 = +20 C and q2 = +10 C and the two
charges are 3 meters apart, what is the
DIRECTION of the force between them?
Like charges repel
Question #10

What is the electrostatic force acting on a 2 nC
charge placed in a 335 N/C electric field?
a)
b)
c)
d)
e)
0N
6.7 x 10-4 N
6.7 x 10-7 N
6.7 N
I don’t have a clue…
F = q 0E
q
E pt chg = k 2
r
Answer #10: (c) 6.7 x 10-7 N

What is the electrostatic force acting on a 2 nC
charge placed in a 335 N/C electric field?
F = qE
æ
Nö
F = (2x10 C )ç 335 ÷
è
Cø
-9
F = 6.7x10-7 N
Electrical Shielding

Electrical charges spread over the surface of
a conductor in such a way that the net
charge INSIDE (at the center) of the
conductor is zero
Electrical Potential Energy

Energy possessed by a charge by virtue of its
location

A function of the charge sizes involved and their
proximity to one another
+ +
+ + +
+ + +
+
+ ++
+
Electrical PE >> KE
Electric Potential
electrical potentialenergy
electricpo tential 
ch arg e
joule
1Volt  1
coulomb
Electrostatics

The End…