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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
  access to “ground”
• 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?
    • How about 10 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-
m ladder
• 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
Contrast with your car
• 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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