Transcript File

Electrostatics
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A Bit of History

Ancient Greeks
–
Observed electric and magnetic phenomena as
early as 700 BC


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
–
–

1600
Found that electrification was not limited to amber
Charles Coulomb
–
–
1785
Confirmed the inverse square relationship of
electrical forces
History Final

Hans Oersted
–
–

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
–
–
They are called positive and negative
Named by Benjamin Franklin
Like charges repel and unlike
charges attract one another
Question #2
The charge on sphere 2 is three times the
charge on sphere 1. Which force diagram is
correct? (e) is none of the others.
Answer #2: (d)
The charge on sphere 2 is three times the
charge on sphere 1. Which force diagram is
correct? (e) is none of the others.
More Properties of Charge

Nature’s basic carrier of positive charge is the
proton
–

Protons do not move from one material to another
because they are held firmly in the nucleus
Nature’s basic carrier of negative charge is the
electron
Gaining or losing electrons is how an object becomes
charged
– Electrons are found in the electron cloud outside of the
nucleus
Neutrons are electrically neutral and have no charge
and are found inside the nucleus
–
Bohr Model of the Atom
More Properties of Charge

Electric charge is always conserved
–
–
Charge is not created, only exchanged
Objects become charged because
negative charge is transferred from one
object to another
Properties of Charge, final

Charge is quantized
–
–
–
–
All charge is a multiple of a fundamental unit of
charge, symbolized by e
Electrons have a charge of –e
Protons have a charge of +e
The SI unit of charge is the Coulomb (C)

e = 1.602 x 10-19 C
Conductors

Conductors are 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 are 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


The characteristics of semiconductors are
between those of insulators and conductors
Silicon and germanium are examples of
semiconductors
Charging…

Two ways
–
–
Electrification by Contact
(EBC)
Induction
Charging by Friction


Self-explanatory…
(demo)
Charging by EBC


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 Induction
Charging by Induction
Charging by Induction
1. NO physical contact
between between
charged & uncharged
object
2. OPPOSITE charged
is INDUCED
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 
Coulomb’s Law

Governs forces and charges,
q1 q 2
F  ke
2
r

ke is called the Coulomb Constant
–


ke = 8.99 x 109 N m2/C2
Typical charges can be in the µC range
Remember that force is a vector quantity
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
Vector Nature of Electric Forces



Two point charges are
separated by a distance r
The like charges produce
a repulsive force between
them
The force on q1 is equal in
magnitude and opposite in
direction to the force on q2
Vector Nature of Forces, cont.



Two point charges are
separated by a distance r
The unlike charges
produce a attractive force
between them
The force on q1 is equal in
magnitude and opposite in
direction to the force on q2
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
Electrical Field

An electric field is said to exist in the region
of space around a charged object
–
When another charged object enters this electric
field, the field exerts a force on the second
charged object
Electric Field, cont.


A charged particle, with
charge Q, produces an
electric field in the
region of space around
it
A small test charge, qo,
placed in the field, will
experience a force
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
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
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

N 
F  2x10 C 335 

C 
9
F  6.7x107 N
Electric Field Lines


A convenient aid for visualizing electric field
patterns is to draw lines pointing in the
direction of the field vector at any point
These are called electric field lines and were
introduced by Michael Faraday
Electric Field Lines, cont.

The field lines are related to the field by
–
–
The electric field vector, E, is tangent to the
electric field lines at each point
The number of lines per unit area through a
surface perpendicular to the lines is proportional
to the strength of the electric field in a given
region
Electric Field Line Patterns



Point charge
The lines radiate
equally in all directions
For a positive source
charge, the lines will
radiate outward
Electric Field Line Patterns

For a negative source
charge, the lines will
point inward
Electric Field Line Patterns


An electric dipole
consists of two equal
and opposite charges
The high density of
lines between the
charges indicates the
strong electric field in
this region
Electric Field Line Patterns




Two equal but like point
charges
At a great distance from the
charges, the field would be
approximately that of a
single charge of 2q
The bulging out of the field
lines between the charges
indicates the repulsion
between the charges
The low field lines between
the charges indicates a
weak field in this region
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
Electric Field Lines
Electric Field Lines
Electrostatics

The End…