Up to and including Electroscopes

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Chapter 20

• Up to and including Electroscopes

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SummaryLauren Larson
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Definitions and Key Words

• Charge - A property of elementary particles that
  determines the strength of its electric force
  with other particles possessing charge. Measured
  in coulombs, or in multiple of the charge on the
  proton.
• Charged - Possessing a net negative or positive
  charge.
• Insulator - A material that does not allow the
  passage of electric charge. Ceramics are good
  insulators.
• Conductor - A material that allows the passage of
  electric charge. Metals are good conductors.
• Grounding - Establishing an electrical connection
  to the earth in order to neutralize an object.
• Conservation of Charge - In an isolated system,
  the total charge is conserved.

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Electrical Properties (p417)

• This section goes through materials tat can
  transfer charge conductors like metal, bodies,
  and moisture. Also materials that cannot
  transfer charge insulators like silk. To lose
  the charge on an object, you ground it by either
  touching the ground with the object or holding
  the object, where it is charged, while touching
  the ground. All conductors can be used to gain
  contact with the ground in order to lose charge.
  Ex. Metal wires on fuel trucks.

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Two Kinds of Charge (p418-419)

• Like charges repel and opposite charges attract.
• Ex Balloon and Wool When the balloon is
  charged by rubbing the balloon on it, the balloon
  will attract to the wool or other oppositely
  charged objects. However, if a different balloon
  is also rubbed by the wool the two balloons will
  repel as they are like charges.

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Conservation of Charge (p 419-420)

• In an isolated system the total charge is
  conserved. When you rub a piece of wool on a
  balloon no charge is lost. The negative charge
  is now on the wool and the positive charge on the
  balloon. When you add up the charge in the
  system it is the same as it was before the wool
  touched the balloon. With the modern view of
  physics this makes a lot of sense. Objects have
  electrons, protons, and neutral neutrons.
  Electrons and protons have the same amount of
  charge, (/-) 1.6 X 10-19.SO when you rub the
  wool and the balloon together, the wool gets some
  electrons form the balloon, but the total charge
  is still the same.

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Induced Attractions (p420-422)

• Not only do opposite charges attract, but objects
  with no charge are equally attracted to items
  with either a or charge.
• Ex The balloon, after charged by the wool, will
  attract to objects with no charge.

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The Electroscope (p422-424)

• Electroscopes can be used to discover if an
  object is charged or not. If an object is
  charged when it is placed on the ball the pieces
  of foil will also be charged and repel each
  other. To discover if a charge is negative or
  positive hold an object where the charge is known
  to the ball. If the foils move further apart
  they are the same charge as the object used, if
  they move closer together they are the opposite
  charge.

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Subtle PointsLeah Starr
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• Moisture is a conductor this is why electrical
  effects vary from day to day. (p417)
• Ex A person is far more likely to shock or be
  shocked in winter than in summer because the air
  is dryer.

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• We can ground a conducting object by merely
  touching it, however we must rub an insulated
  object in order to ground it. (p417)

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• Like charges are repelled by one another, however
  an uncharged object and a charged object, as well
  as oppositely charged objects will attract one
  another. (p418)

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• An object will be uncharged if it has equal
  amounts of positive and negative charges. Not
  because it has no charge. An object is
  considered to have a positive charge when it has
  an excess of protons, or deficiency of electrons.
  (p420)

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• Charges are not lost through rubbing, they are
  transferred. (p420)

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• An uncharged object still attracts a charged
  object. This occurs even within an insulated
  object, the movements of the charges are not as
  extreme.

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• An electroscope will not tell us what kind ( or
  -) of a charge is present, only that there is
  one. (p422)

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• An electroscope can be charged through induction.
  In this case then a like charge remains within
  the electroscope and the pieces of foil continue
  to repel one another. (p423)

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Conceptual QuestionsLouise Kleszyk
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Question 20.6

• Why is it easier to demonstrate electrostatic
  phenomena in Fairbanks, Alaska, than in Honolulu,
  Hawaii?

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(No Transcript)
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What are the differences between Alaska and
Hawaii?

• Hawaii is
• Close to the equator.
• Warm and Humid.
• An enjoyable Tourist Destination

• Alaska is
• Close to the North Pole.
• Cold and Dry.
• Also an enjoyable tourist destination.

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Which factor matters?

• HUMIDITY!
• Water conducts electricity.
• Humid air allows objects to lose charge.

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What does this all mean?
Although a great place to catch a tan, Hawaii is
not an ideal place to study and observe
electrostatic phenomena.
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Question 20.20

• When a charged comb is brought near bits of
  paper, the bits are first attracted to the comb
  and then repelled. Describe how the charge on
  the comb and the bits of paper changes during the
  process.

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What do we know about charges?

• LIKE charges repel one another.
• OPPOSITE charges attract.

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Heres the deal

• The comb has a charge. ( or -).
• The paper has no charge.
• Because they are opposite they attract.
• And then some of the charge transfers to the
  paper.
• Because after this they are similarly charged
  they begin to repel one another.

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And because there are no exercises for this
section

• More Conceptual Questions!
• Alicia Weller

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Question 20.1

• A handheld glass rod can be charged by rubbing it
  with silk or a plastic bag while holding it in
  your hands. Would you conclude from this that
  glass is a conductor or an insulator? Why?

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• Your body is a conductor.
• If glass was a conductor the charge would run
  through your body and be grounded.
• So, the glass rod must be an insulator!

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Question 20.21

• Why are neutral objects attracted to both
  negatively and positively charged objects?

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• The induced charges on the near side of the
  neutral object are always opposite from the
  charged object near it.
• This produces attraction no matter what the
  charge.
• This is like Sonalis example of the people in
  the restaurant when men in uniform walk by. The
  girls migrate to the window and the boys migrate
  away.

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The End!