Electrostatics – Chap. 32

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```					CP: Electrostatics
February 1, 2010
Electrostatics – Chap. 32
Electrostatics
• Electricity at
rest
• Electric
charges
• Forces
between them
• And how they
act
Charge Model
• 1. There are two types of
charges:
Positive (+) and negative (-)
Charge Model
• 2. Particles that make up
matter have a charge.
Charge Model
• 3. Like charges repel…
while opposite charges attract
Charge Model
• 4. The force gets weaker with
increasing distance between the
charges.
Charge Model
• 5. The force increases with
increasing charge.
Conservation of Charge

• You can separate or join what
you have,
– but you can’t create charge.
Conductors and
Insulators
• Conductors allow charges to
flow through them
• Insulators allow charges to stay
put.
Three Charge States:
• 1. Neutral = equal number of
(+) and (-), evenly distributed.
Three Charge States:
• 2. Polarized = equal number of
(+) and (-), unevenly distributed.
Three Charge States:
• 3. Charged = unequal number
of (+) and (-).
Types of charge transfer
•   Contact or friction
•   Polarization
•   Induction
•   But first we should mention…
Grounding
• When we touch a surface to
drain the charges the object
becomes grounded.
• Why?
– The earth is a huge neutral
object…
– Equal + and - charges
Contact and Friction

• One object gains as much as the
other looses
• Insulators vs conductor
– Which one will hold onto more of
its charge
Polarization

• Electrostatic forces are “field” forces
– Contact is not necessary
• If you put a positively charged object
close to another object
– It will repel the positive charges
– And attract the negative charges
• What happens when you remove the
charged rod?
Induction

• Polarize the charge
• Ground the object
– Neutralizes the charge in an object
• Remove the ground
– Then remove the polarizing charge
Lab
• Grounding is critical!
• Between each step (except for
#4)
• Diagram and explain what is
going on for each step.
Electrostatics
Coulomb’s Law
February 3, 2010
Review – 3 Charge
States
• Neutral =

• Polarized =

• Charged =
• 1.
Basics of charge model

• 2.

• 3.
Polarization
• When a charged object comes
close to a neutral object
• They will be attracted
– Whether the charged object is
negative or positive.
Insulators versus
conductors
3 Ways to Charge
• 1. Friction/contact

• 2. Polarization (temporary)

• 3. Induction
1st way

2nd way
What does it mean to
“ground” something?
What can we say about these objects?
What can we say about these objects?
Ok…
• Enough review…
Charles Augustin de Coulomb
Charles Coulomb
• Looking at the force between 2
objects
• Two ideas:
• #1 The force was proportional to
the two charges
– If you double the charge on one
object the force between them
doubled
Charles Coulomb – idea
#2
• #2 The force was inversely
proportional to the distance in a
unique way…
• If he doubled the distance …
– The force decreased by 4 times
• Force is proportional to 1/d2
• This is called an inverse square
relationship
Coulomb’s Law
• F is proportional to (q1 x q2)/d2
• Or… F = k (q1 x q2)/d2
– Where k is a constant that makes
the numbers “work”
• q is the symbol for charge
– It’s units are Coulombs
Coulomb’s Law:

What
?
Coulomb’s Law
• If we increase the charge of one
or both objects …
– The force becomes stronger.
Coulomb’s Law
• If we increase the distance
between the charges…
– The force becomes less.
Electrostatic vs Gravity
• Both are field forces
– No touching necessary
• Both get smaller with distance
– “inverse square”
• Gravity gets stronger with more
mass
– Electrostatic force …with more charge
• Only one can repel, as well as
attract…
Conceptual Physics
February 5, 2010

“Ouch!! Do it again…”
Anonymous
Today’s Plan:
• Check – HW
• Electric fields
• Electrostatics Lab #2 - The
Search for Sparks
Demo Charge transfer…
• By contact / friction
• By polarization
• By induction
Review of Coulomb’s
Law
• If we increase magnitude of one
or both of the charges and leave
the distance the same the
force…
• Increases!
Coulomb’s Law
• If we increase the distance
between the charges the force
gets…
• Smaller
– Do you remember the relationship?
• Inverse square
• F = kq1q2/d2
Electrical Fields
• Around every charge there is
space that is affected by the
charge.
• We use vectors to show these
force fields
Electric Field Line Rule
#1
• The lines always point in the
direction of the force that would
act on a positive test charge.
Electric Field Line Rule
#1
• The vectors always point in the
direction of the force that would
act on a positive test charge.
Electrical Field Line
Rule #2
• The number of lines
is proportional to
the size of the
charge!

• We use lines of
force.
• Farther apart =
weaker, close =
stronger
Electrical Field Line
Rule #3
• The lines don’t
cross one
another
Charge Density
• How tightly packed the charges are
in an object. More charges = more
forceful.
Charge Density
• The charge density of objects
that are touching is the same
• The total amount of charge will
be greater in the larger object.
Charge increases in
“corners” of objects
• No repelling charge
to balance out the
force.
Lightning Rods
• Serve 2 purposes:
• 1. Attract lighting by providing
• 2. Provide a sharp point
– Charge concentrates there
– Actually allows grounding before
the charge gets too large!
Electric Shielding
• Occurs when all electrons mutually
repel each other, so the charge
inside of a conductor is normally
zero.

+
Conductor
Electric Shielding:
• Explains why it is safe to be in a car
struck by lightning, electrical
components are housed in metal
boxes, etc.
Electroscopes
Electrostatics

February 9, 2010
Homework
• Review …
– Questions?
Lightning
• Any good lightning stories?
What you need to learn
from the video:
• How is lightning formed?
• What are the characteristics of
lightning?
– Size, temperature, how does it
move…
• What is the purposes of a
lightning rod?
• How do you study lightning?
Video (cont’d)
• Where are you safe in a lightning
storm and what should you
avoid?
• What is a fulgurite?
Electrostatics

February 11, 2010
Quiz…
• You are sitting on the hood of your
car during a lightning storm
– Is this a good idea? (Why/why not?)
• The truck you are sitting on gets hit
by lightning and has an electrical
charge on it. Will there be more
charge:
– On the flat hood
– On the edge of the truck bed
– It’s all the same
Electrical Potential
Energy
• Remember gravity?
– And gravitational potential energy?
– “PE = mgh”
• The higher you go the more PE you
have…
• Consider a ladder 10 meters high.
– What’s the PE of an object with a mass
of 5 kg?
Electrical PE
• Electrical energy is very similar
• If we take a “+” charge and pull it
into away from a “-” charge
– We do “work” on it (force x distance)
• We create potential energy
• If you let it go
– Smaaackkk…
– It flies towards the “-” charge
– Making kinetic energy
Same idea …
• If you push a “+” into a “+”
charge force field…
• It acts like a spring…
– You work to get it in there…
– Let it go and…
• WHEEEE!!!
Let’s go back to the 10-
• So we could say that the potential
due to gravity at 10 meters…
– Is equal to 10 m/s2 x 10 m x the mass
• If we wanted to consider this 10
meter height…
– The PE = 100 m2/s2 x whatever mass you
have
• The gravitational “potential” is equal
to 100 J per 1 kg of mass
Electrostatics – the big
copy cat
• If you look at the potential energy
per unit charge…
– PE/# charges
• In units of Joules per Coulomb
• This is the Electric Potential
– NOT Potential ENERGY
– Just Potential
• For every Coulomb of charge at some
location
– You get so many Joules of potential
energy
Wow! Watt’s it called?
• Named after a strange Italian
– Whose name was Antonio…
• Volta!
• Note that a Volt
– Doesn’t tell us how much energy is
present
– Just how much energy per unit of
charge
Volts don’t kill
• Flow of charge through your
heart kills
• Volts just tell you how far those
charges will “fall” when you let
them go!
Volts don’t kill
• If you have a penny on top of the
school
– It has a lot of gravitational
potential
– This is like voltage
• But not much mass
– That would be like the charge
• Let’s say you jack your car up to
change a tire
– It’s gravitational potential is big or
small?
– It’s potential energy is big or small?
• If you’re unsure imagine which you’d
rather have fall on you
– The penny or the car?
Amps kill…
• The mass of the car is like a
huge electrical charge
– Even if its “potential” is low
– It packs a lot of energy…
Who can tell me…
• The difference between
– Electrical Potential and
– Electrical Potential Energy?
Capacitors:
• Devices that that can store electrical
energy. They are not batteries.
Capacitors
• Two conducting plates that are
oppositely charged with space or
some insulator separating them.
Capacitors:
• The stored electrical energy can be
discharged if a conducting path is
created between the two plates.
Capacitors:
• If you have
– Large plates (surface area)
– Close together
– Attached to a large voltage source
• Then…your capacitor has a large
stored charge.
Electrostatics

February 16, 2010
Review Day!
• Positive and negative
• Positive and positive
• 3 charges sitting side by side…
• Can a neutral balloon be
attracted to a charged balloon?
• Does a negative charged object
have any positive charges?
More…
• Insulators and conductors
• How are charges transferred?
• Explain and draw how induction
works.
– Contact transfer
• States of charge?
Review - 3
•   Field force…
•   Field strength…
•   Field lines…
•   Coulomb’s Law
•   Lightning rod
•   Electric potential
– Vs electric potential energy…

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