Unit%203%20Electricity%20

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Unit 3 Electricity MagnetismElectric Fields

• Physics 5e.
• Students know charged particles are sources of
  electric fields and are subject to the forces of
  the electric fields from other charges.

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Electric Charge

• Key Question
• How do electric charges interact?

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Electric Charge

• All ordinary matter contains both positive and
  negative charge.
• You do not usually notice the charge because most
  matter contains the exact same number of positive
  and negative charges.
• An object is electrically neutral when it has
  equal amounts of both types of charge.

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Electric Charge

• Objects can lose or gain electric charges.
• The net charge is also sometimes called excess
  charge because a charged object has an excess of
  either positive or negative charges.
• A tiny imbalance in either positive or negative
  charge on an object is the cause of static
  electricity.

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Electric Charge

• Static electricity video
• http//glencoe.mcgraw-hill.com/sites/dl/free/00787
  78069/161383/00053407.html

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Electric Charge

• The innermost electrons in an atom are attracted
  strongly to positively charged atomic nucleus.
• The outermost electrons (valence electrons) of
  many atoms are attracted loosely, therefore these
  electrons can be removed more easily.

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Electric Charge

• When two neutral objects are rubbed together,
  charge is transferred from one to the other and
  the objects become oppositely charged.
• This is called charging by friction.
• Objects charged by this method will attract each
  other.

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Electric Charge

• ExampleWhen you rub a comb against your hair,
  electrons transfer from the hair to the comb.
• The comb then has excess electrons and is said to
  be negatively charged.
• Your hair, has a deficiency of electrons and is
  said to be positively charged.

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Practice Question

• Beth rubs an ebonite (polished rubber) rod with
  wool. She finds that the wool acquires a positive
  charge. Which conclusion can BEST be drawn from
  her observation?

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Practice Question

• Beth rubs an ebonite (polished rubber) rod with
  wool. She finds that the wool acquires a positive
  charge. Which conclusion can BEST be drawn from
  her observation?

Answer C. Many negatively charged particles
moved from the wool to the ebonite rod.
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Practice Question

• If James were to pick up a negatively charged
  metal ball and touch it to a neutral metal disk,
  what would happen?

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Practice Question

• If James were to pick up a negatively charged
  metal ball and touch it to a neutral metal disk,
  what would happen?

Answer D. The metal disk would become negatively
charged.
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Charge Polarization

• Since there are no free electrons to move
  throughout the material, there is a rearrangement
  of the positions of charges within the atoms and
  molecules themselves.

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Charge Polarization
This charge polarization does not last. Once the
balloon is removed, the molecules go back to
their usual positions.
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Electric Charge

• Conservation of charge
• Whenever something is charged, no electrons are
  created or destroyed.
• Electrons are simply transferred from one
  material to another.

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Electric Charge

• Electric charge is a property of tiny particles
  in atoms.
• The unit of electric charge is the coulomb (C).
• 1 charge of coulomb is 6.24x1018 electrons
• A quantity of charge should always be identified
  with a positive or a negative sign.

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Electric Forces

• Electric forces are created between ALL electric
  charges.
• Because there are two kinds of charge (positive
  and negative) the electrical force between
  charges can attract or repel.

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Electric Forces
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Electric Forces

• Charge can be transferred by conduction.
• Electrical Conduction is the movement of
  electrically charged particles through a
  transmission medium.

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Practice Question

• The diagram below shows the electric force
  between the electrons and the nucleus of an
  atom.-Which conclusion can be drawn about the
  electric force?

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Practice Question

• The diagram below shows the electric force
  between the electrons and the nucleus of an
  atom.-Which conclusion can be drawn about the
  electric force?

Answer B. The electric force between the
electrons and the nucleus acts at a distance.
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Coulombs Law

• Coulombs law relates the force between two
  single charges separated by a distance.

Constant 9 x109 N.m2/C2
Charges (C)
Force (N)
Distance (m)
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Coulombs Law

• The force between two charges gets stronger as
  the charges move closer together.
• The force also gets stronger if the amount of
  charge becomes larger

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Coulombs Law

• The force between two charges is directed along
  the line connecting their centers.
• Electric forces always occur in pairs according
  to Newtons third law, like all forces.

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Coulombs Law

• The force between charges is directly
  proportional to the magnitude, or amount, of each
  charge.
• Doubling one charge doubles the force.
• Doubling both charges quadruples the force.

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Coulombs Law

• The force between charges is inversely
  proportional to the square of the distance
  between them.
• Doubling the distance reduces the force by a
  factor of 22 (4), decreasing the force to
  one-fourth its original value (1/4).
• This relationship is called an inverse square law
  because force and distance follow an inverse
  square relationship.

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Practice Question

• The electrostatic force between two charged
  objects is influenced by the size of the charges
  and the distance between the objects. Which
  expression correctly describes the magnitude of
  the force between two charged objects?

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Practice Question

• The electrostatic force between two charged
  objects is influenced by the size of the charges
  and the distance between the objects. Which
  expression correctly describes the magnitude of
  the force between two charged objects?

Answer C. the electrostatic constant (k)
multiplied by the product of the charges divided
by the square of the distance between them
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Fields Forces

• Charge creates an electric field that creates
  forces on other charges.

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Fields Forces

• Mass creates a gravitational field that exerts
  forces on other masses.

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Electric Field Lines
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Electric Field Lines

• Show the direction of electric field
• Show the intensity of the electric field

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Fields Forces
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Practice Question

• Which diagram shows opposing positive electric
  field lines?

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Practice Question

• Which diagram shows opposing positive electric
  field lines?

Answer
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Conductors

• All materials contain electrons.
• Conductors are materials that allow charged
  particles to pass through them easily.
• The electrons are what carry the current in a
  conductor.
• Examples
• Copper, silver, and other metals

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Insulators

• The electrons in insulators are not free to
  movethey are tightly bound inside atoms.
• Examples
• Rubber, glass

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Conductors Insulators

• A semiconductor has a few free electrons and
  atoms with bound electrons that act as insulators.