Download Electrostatics (Mr. P`s PPT)

Electrostatics
KWL – It’s Electric!
Make a 3 column chart in your notebook and title the three
columns K, W, L:
K (know)
Write
everything
you know
about
electricity
here.
W (want)
L (learned)
Leave Blank
Write anything
for Now!
you want to
Later
you
will
learn about
write
electricity
everything
here.
that you
learned from
your study in
electricity!
Electrostatics
Electrostatics is the study of electrical
charges at rest.
Did you know?: The force
due to the charges of one
proton and one electron
are on the scale of 1040
times bigger than the
force due to gravity
between these particles!
Static Electricity Continued…
Side Note:
The word electricity comes
from the Greek word,
elektron, which means
“amber.”
The Greeks found that if
you rub amber with cloth,
and bring it near dust or
small leaves, it would
attract these things.
Static Electricity
Electrons can be added or taken away from an
atom when a strong enough force is applied.
(Friction can do the trick!)
Once electrons are either lost or gained by an
object. The object is said to be “charged.”
Unlike charges attract; like charges repel.
Benjamin Franklin was the first to distinguish
charge as either + or -
Where do the Charges Come
From?
Electrons can be pulled from their
orbits in an atom.
Normally, there is the same number of
electrons as protons in each atom.
Ions are atoms that do not have equal
numbers of electrons and proton.
The Atom
• The nucleus, made
up of protons and
neutrons, is orbited
by electrons.
• The electrons on
the outermost shell
can be pulled off of
the atom if the
proper amount of
force is applied.
The Parts of the Atom
Summary of Subatomic Particles:
Proton
In nucleus
Tightly Bound
Positive Charge
Massive
Neutron Electron
In nucleus
Tightly Bound
No Charge
Massive
Outside nucleus
Weakly Bound
Negative Charge
Not very massive
https://www.youtube.com/watch?v=xqNSQ
3OQMGI
https://www.youtube.com/watch?v=q2QY6
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Charges
Charge is conserved, meaning the total
amount of charge in the universe is
constant.
Therefore =>
“The net amount of electric charge in any
process is zero.”
Law of Conservation
of Electric Charge
Question 1
Jean Yuss makes the following observations:
Object C Object D Object E Object F
attracts B repels C attracts D attracts A
repels F
Jean knows that object A is negatively charged.
What are the charges on objects C, D, E, and F? Jean
Yuss, knows… Explain how...
Conductors
Conductors are materials which allow
electrons to flow freely from atom to atom.
A conductor will allow charge to spread
out equally across the entire surface of the
object.
Conductors
Insulators
In contrast to conductors, insulators are
materials which impede the free flow of
electrons from atom to atom.
Semiconductors
A semiconductor is a material
that behaves like both a
conductor and an insulator.
At room temp. S.C.’s conduct
slightly more than an insulator but
less than a conductor.
At very low temp. S.C.’s behave
as insulators.
At high temp. or under direct
light, S.C.’s act as great
conductors.
Source: www.siliconfareast.com/whatissemicon.htm
Weekly HW. Due 3-19-10
Section Review Questions Pg. 563 (ALL)
& Practice Problems A on Pg. 566
Electrical Forces
Electrical Forces are forces due to charged
particles or objects.
Coulomb’s Law
Fe = k
Where:
.
q1 q2
2
d
Fe = the electrical force
k = constant = 9x109 N.m2/C2
q1 = the charge on object 1
q2 = the charge on object 2
d = the distance between the objects or charges
Charge!
Note that in this equation the k is a very
large constant! Electric forces are strong!
Coulombs: SI Unit of Charge
Also note, the charge of an electron is:
e or qe= -1.6x10-19 C
What is the charge of 1 proton?
qp= +1.6x10-19 Coulombs
Question 1
Two charged objects have a repulsive
force of 0.80 N. If the distance separating
the objects is doubled, then what happens
the force?
A. It becomes less
B. It increases
C. It remains the same
Answer:
It Becomes Less!
.2 N
Question 2
Determine the electrical force of attraction
between two balloons if one is charged
with 6.0 x 10-7 C of charge and the other is
charged with -6.0 x 10-7 C. The balloons
are measured to be a distance of 0.50 m
apart.
Solution
Step 1: Identify known values in variable
form:
q1 = -6.0 x 10-7 C
q2 = +6.0 x 10-7 C
d = 0.50 m.
Step 2: Identify what you are trying to find.
F = ???
Step 3: Substitute and solve.
F = (9x109)(-6.0 x 10-7)(+6.0 x 10-7)
0.502
Answer:
-0.0130 N
Question 3
If a charge on one sphere +3.0 x 10-8 C
and another charge of +2.0 x 10-8 C is
brought nearby, what is the strength of the
electrical force when they are a distance of
0.6 m apart?
Draw a picture that includes the force
vectors for this situation. Be sure to
indicate proper direction.
Solution
Step 1: Identify known values in variable
form.
q1 = +3 x 10-8 C
q2 = +2 x 10-8 C
d = 0.6 m
Step 2: Identify what you are trying to find
F = ???
Step 3: Substitute and solve
F = (9x109)(3x10-8)(2x10-8)
.62
Answer:
1.5x10-5 N
Coulomb’s Law – Note on the
Sign of the Force
When you use coulomb’s law the sign of
the force will indicate if it is an attractive or
a repulsive force.
If F is + Then the charges Repel
If F is - Then the charges Attract
What does the Sign of a Force Vector Normally Indicate???
Answer: Direction
Challenge Problem
Charging by Induction
Another Method for charging is induction.
Induction only occurs in conductors because
electrons have to be free to move.
Lightning
(a.k.a. charging by induction!!)
+
_
+
_
+
_
Collisions produce
charged rain
particles. The
heavier particles
are (-) because
they have more
electrons. Thus
they sit near the
bottom of the cloud
and the lighter
particles (+) sit near
the top.
Negative charge
Attraction
in the clouds
As the
induces a
“stepped
positive charge leader” nears
on the earth
the ground, it
below.
draws a
“Stepped
“streamer” of
Leader”
positive
Negatively
charge
charged electrons
upward.
begin zigzagging
downward.
Flowing
Contact!
Charge
Intense wave
As the leader
of positive
and the
charge, a
streamer
“return stroke,”
come
travels upward
together,
at 108 m/s
powerful
electric
current
begins
flowing
Charge Polarization
Occurs in Insulators and not in conductors.
Happens when an object is brought near a strong
charge.
Individual atoms line up so that they all have their +
side and – side facing the same way.
Electric Fields
An Electric Field
surrounds any particle
that has charge.
An electrical field exists
because electric force
will act on any charged
particle nearby.
“Stinky Analogy!!!
E=F
q
Where q is the test
charge.
Electric Field Lines
Electric field lines are shown moving out of
positive particles and into negative particles.
More lines, closer together indicate strong
fields! Less lines spread further apart
represent weaker fields.
Electric Field Hockey
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Review: Potential Energy

Review Questions:
1. What is work?
2. What is gravitational potential energy?
Work and Potential Energy
The amount of work done on a particle to
move it through an electric field is equal to
the Potential Energy gained or lost.
Remember Conservation of Energy!
Recall lifting
a rock of the ground.
Electrical Potential Energy

A charged object has Electrical Potential
Energy because of its location in an electric
field!!!!
• To move a charge in
an electric field
against its natural
direction of motion
would require work.
Electrical Potential Energy
Which Positive Test
Charge has more
Electric Potential
Energy?
Charges always want to be
at the lowest potential
energy possible.
+
q2
+
q1
Electric Potential
We know that Electrical Fields exist, even when no test
charge is present. (remember the stinky analogy!)
Using similar logic, we can reason that different P.E.
levels exists around a charge at different locations in
space, regardless of the size of the test charge used.
Electric Potential or just Potential is simply
Electric Potential Energy divided by test
charge. (this is also called voltage.)
V = PEelec. / q
V = Electric Potential at some point in space
Potential Difference
Only differences in electrical potential are
important!!!
If point a has high potential (Va), and point b has
low potential (Vb), then a positive test charge will
move from a towards b.
A negative test charge will do the opposite.
If there is no difference in electric potential, then
a test charge will not move if released!
Electric Potential And Potential
Energy
Va
Vb
V = PEelec. / q
+
q2
And by algebra:
+
q1
PE = q x V
DPE = PEb – PEa
= q x (Vba )
This also equals the work
done to move the particle
from A to B.
Relationship between E and V
For a constant Electric Field, E, the electric
potential between two points is easily
discovered:
Vba = E.d
Where d is the distance between the
charges.
Equations for Electrical Force, E
Field, Electric Potential
E = k .Q
F = k.q.Q
r2
r2
V = k .Q
r
SCALAR!
Not a VECTOR!
These equations only apply when a point charge (Q) is responsible for
creating the electric field.