Download Electrostatics

Electrostatics
History
Ancient Greeks observed electric
phenomena as early as 700 B.C.

Amber, when rubbed, attracted pieces of
straw or feather
William Gilbert (1600)

discovered electrical
charge was not limited
to amber
More History
Charles Coulomb (1785)

Confirmed the inverse square relationship
of electrical force.
Hans Oersted (1820)

Found that a compass needle deflects when
near an electrical current.
Michael Faraday (1830s)

When a wire is moved near a magnet, an
electric current is observed in the wire.
Properties of Electric Charges
Two types of charges:


Positive and negative
Named by Benjamin Franklin
Like charges repel;
unlike charges attract.
Question #1
The charge on sphere 2 is three times the
charge of sphere 1. Which force diagram is
correct?
(e) none of the above
More Properties of Charge
The positive charge carrier is the proton.

Protons do not move because they are firmly
held in the nucleus.
The negative charge carrier is the
electron.


Electrons are found in the electron cloud
surrounding the nucleus.
An object becomes charged by gaining or
losing electrons.
Neutrons are electrically neutral and are
found in the nucleus
More Properties of Charge
Electric charge is always
conserved.


Charge is not created, can only
be transferred.
Objects become charged
because a negative charge is
transferred from one object to
another.
The object that loses electrons
becomes positive.
The object that gains electrons
becomes negative.
Properties of Charge, final
Charge is quantized




Represented by the symbol e
Electrons have charge –e
Protons have charge +e
The SI unit of charge is the coulomb (C)
1e = 1.602 x 10-19 C
Conductors
Materials in which electric charges move
easily



Usually metal (copper, silver, aluminum are
good electrical conductors.
When a conductor is charged in a small
region, the charge readily (quickly) distributes
itself over the entire surface of the material.
Can be charged by induction, conduction
Insulators
Materials in which electric charges do not
move easily.


Non-metals are usually better electrical
insulators (glass, plastic, rubber).
When insulators are charged by friction, only
the area rubbed becomes charged
There is no tendency of charges to move to other
regions.

Can be charged by friction or polarization.
Semiconductors and Superconductors
Share characteristics
of both conductors
and insulators
Ex: silicon,
germanium
Below certain
temperature,
resistance is zero
Charging
By Contact


friction
conduction
Without contact


induction
polarization
Friction
Electrons will move to rubber, latex, most
plastics.
Electrons will be removed from glass.
Conduction
A charged object
(rod) physically
touches the
uncharged object
(sphere).
The same type of
charge is
CONDUCTED from
the rod to the sphere.
Induction
Induction
Induction
No physical contact
between charged and
uncharged objects
OPPOSITE charge is
INDUCED
Induction occurs in
conductors only
Polarization
Temporary charging of an insulator by
bringing a charged object close to the
surface.
Question #2
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 #2 (c)
The force on the electron is the same as
that on the alpha particle – Newton’s 3rd
law.
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 particle with the most acceleration is:
a)Alpha particle
b)Electron
c) Neither…they have the same acceleration
d)I haven’t a clue…
Answer #3 (b)
The less massive electron has more
acceleration – Newton’s 2nd law (F=ma).
Question #4
An alpha particle with two positive charges
and a less massive 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)Force
b)Speed
c) Acceleration
d)All of these
e)None of these
Answer #4
As the particles get closer the Force
increases, therefore the Acceleration
increases and the Speed increases.
Coulomb’s Law
Warmup: follow the QR codes on
page 3. Play with the apps and
answer the questions.
Coulomb’s law
Governs forces and charges,
ke is called the Coulomb Constant

ke = 8.99 x 109 N·m2/C2
Typical charges are in the μC range.
Remember that force is a vector quantity
Question #5
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)22 N
b)200 N
c) 2 x 1011 N
d)6 x 1011 N
e)I don’t have a clue
Answer #5
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?
Known
Formula
Sub
Answer
Vector Nature of Electric Charges
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 Electric Charges, cont.
Two point charges
are separated by a
distance r
The unlike charges
produce an attractive
force between them
The force on q1 is
equal in magnitude
and opposite in
direction to the force
on q2
Question #6
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)Away from each other
b)Towards each other
c) One chases the other
d)Nothing…they don’t move at all
e)I don’t have a clue
Answer #6 (a)
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)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
A charged particle,
with charge Q,
produces an electric
field in the region of
space around it
A small test charge,
q0, placed in the field
will experience a
force.
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
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 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
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 Pattern
Point charge
The lines radiate
equally in all
directions
For a positive source
charge, the lines will
radiate outward.
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
The low field lines
between the charges
indicates a weak field
in this region
The bulging out of the
field lines between
the charges indicates
the repulsion between
the charges.
Unequal charges