Electrostatics

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Electrostatics
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Explore 1
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You slide out of your car, grab the door handle,
and ZAP you get a jolt!

• What happened?
• You have rubbed electrons off of your car seat
  onto your booty. Then the electrons travel from
  your booty around your body to your hand to the
  door seeking a positive charge. When your hand
  touches the door, the shock you get is from
  static electricity.
• Static electricity occurs when charges on the
  surfaces of objects transfer to or react with
  another object because of its charge.
• Static electricity occurs without current (flow
  of electrons).

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Challenge!

• Move all of the salt from plate A to
• plate B.
• Rules
• - do not touch the salt with your hand
• - do not touch the plates with your hand
• - you may use the balloon given

Record your solution/progress in your journal
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Magic Straw

• Tear a piece of paper into small pieces. Place
  the straw close to the pieces of paper. What
  happens?
• 2. Now rub the straw briskly with wool or fur
  and place it near the strips of paper. What
  happens?

Record all observations/question answers in your
journal
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Explain 1
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Everything is made of atoms

• Electrons orbiting the nucleus are free to move
  from atom to atom.
• Opposite charges attract
• electrons (negative) are attracted to
  protons (positive)
• Like charges repel (remember the hair separating
  when charged by Van de Graaf generator)
• Note static electricity works even with
  insulators because electrons dont have far to
  move.

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Magic Straw Questions

• What can you say about the size of this
  interaction compared to that of the Earths
  gravitational interaction with the paper bits?
• What can you say about the relationship between
  distance and this interaction?

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Coulombs Law

• Coulombs law states that the magnitude of the
  force between charges is directly proportional to
  the product of the charges and inversely
  proportional to the square of the distance
  between the charges.

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About q

• The units used for charge are coulombs.
• An electron or proton has a charge of 1.60218 x
  10-19 coulombs.
• Electric charge is conserved within a system.

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About F

• Electrostatic force the attraction or repulsion
  of particles due to electric charge.
• Force is still measured in NEWTONS.

About k

• It is an experimentally determined constant.
• Its value is 8.99755 x109 Nm2/C2

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Applying Coulombs Law
0.05 m


1.15 x10-6
4.95 x10-6

• What is the direction and magnitude of the force
  the left charge exerts on the right charge?

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Applications of Electrostatic Force

• Air Filters use synthetic fibers to create an
  electrostatic charge that attracts particles in
  the air
• Laser Printers use electrostatic force as part
  of the process to transfer an image from the
  computer to a sheet of paper

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Electric Field

• The concept of electric field is used to
  describe how charges affect each other at a
  distance
• E F/qtest
• qtest is a positive test charge
• Electric field is measured in N/C

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Calculating Electric Field
F1.2 N

1.15 x10-6
-

• What is the electric field at the location of the
  test charge?

E F/qtest
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Electric Field and Coulombs Law

• Combining the coulombs laws and the equation
  for electric field allows for calculating the
  electric field of a point charge.
• E q
• r2

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Practice

0.112 m
1.15 x10-6
A

• What are the magnitude and direction of the
  electric field at point A.

E q r2
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Applications of Electric Field

• NASA uses an ion propulsion system for their
  spacecraft.
• An electric field is appolied to ionized gas.
• The field causes the charged particles of exhaust
  to move with great velocity.
• Greater velocity means greater momentum (pmv)
• Because of conservation of momentum, the rocket
  is propelled forward.

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Charging Objects
Explore 2
http//www.glenbrook.k12.il.us/gbssci/Phys/Class/e
statics/u8l1c.html
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Activity 1

• Electroscope

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Building an Electroscope

• An electroscope is a device which can help you
  determine whether or not an object is charged.
• Materials
• Cup Electroscope Styrofoam cup, aluminum foil,
  glue stick, flexible straw, tape, straight pin

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Electroscope Final Product
Straw
Pith ball
Tape
Cup
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Building the Electroscope

• Pith ball
• 1. Cut a 1 cm section off of the end of the long
  side of the flexible straw.
• 2. Apply glue (glue stick recommended) to the
  straw section.
• 3. Place the end of a 4 cm piece of string on the
  straw section, and wrap a 1 cm wide strip of
  aluminum foil around the string and the straw
  section.

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Building the Electroscope (continued)

• Stick the straight pin through the straw, 0.5 cm
  from the end of the short section of the flexible
  straw.
• Thread the pith ball string through the hole
  created by the straight pin. Tie off string.
• Tape the long section of the straw to the top
  half of an upside down styrofoam cup.

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Using the Electroscope

• Rub a straw with fur.
• Bring the straw close to the pith ball. What
  happens?
• Touch the pith ball to transfer the charge. What
  is the charge on the pith ball?
• Re-deposit electrons on your straw using the fur.
• Bring the straw close to the pith ball again.
  What happens now? Explain.

Record all observations/question answers in your
journal
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Activity 2

• Sticky Balloon

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Sticky Balloon

• Rub an inflated balloon on your hair.
• Hold the balloon up to the wall
• What do you see? Explain.

Record all observations/question answers in your
journal
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Activity 3

• Electrophorus

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Building an Electrophorus

• Glue a Styrofoam cup upside down on an aluminum
  pie plate.

Cup
Pie plate
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The Electrophorus

• Rub the bottom of the styrofoam plate with the
  fur. Place the plate bottom-side up on your
  table.
• Hold the pie plate by the cup handle and lower
  the pie plate to just above the styrofoam plate.
  Dont touch the pie plate.
• Raise the pie plate and hold it up to your
  electroscope. Is the pie plate charged?
• Lower the pie plate to just above the styrofoam
  plate. Touch the pie plate with your finger. What
  happens?
• Raise the pie plate and hold it up to your
  electroscope. Is the pie plate charged? By what
  method?

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Explain 2
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How can you charge an object?

• Friction (i.e. straw rubbed with fur)
• Contact (i.e. electroscope)
• Polarization (i.e. balloon and wall)
• Induction (i.e. electrophorus)

http//www.glenbrook.k12.il.us/gbssci/Phys/Class/e
statics/u8l1c.html
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Charging by Friction

• TRIBOELECTRIC SERIES
• the items higher on the list should give up
  electrons and become positively charged
• your handglassyour hairnylonwoolfursilkpape
  rcottonhard rubberpolyesterpolyvinylchloride
  plastic

Rubbing the straw with the wool or fur like in
the Magic Straw activity is an example of
charging by friction.
http//www.arborsci.com/CoolStuff/balloon_charge.j
pg
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Charging by Contact

• A charged object transfers electrons when it
  comes in contact with another object

Charged Straw
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Charging by Polarization
The charged object on the left aligns charges on
the surface of the insulator on the right.
http//www.glenbrook.k12.il.us/gbssci/Phys/Class/e
statics/u8l1e.html
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Induction

• Charging by induction requires no contact between
  the objects.

http//www.glenbrook.k12.il.us/gbssci/Phys/Class/e
statics/u8l2b.html
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Lightning and Van de Graaff Generator
/

• In an electrical storm, the storm clouds are
  charged like giant capacitors in the sky. The
  upper portion of the cloud is positive and the
  lower portion is negative. This induces a
  positive charge on the earth's surface below the
  cloud. All that is needed now is a conductive
  path for the negative cloud bottom to contact the
  positive earth surface. Dont be that path!!!
• A Van de Graaff generator works the same way.