CHAPTER 16: ELECTRIC FORCES

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CHAPTER 16 ELECTRIC FORCES FIELDS

• Section 16.1
• Electric Charge

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Electrical Charges

• Electrostatics study of electrical charges
• A charged object shows electrical effects.
• Law of Charges or Charge-Force Law
• Objects with the same charge repel each other.
• Objects with different charges attract each other.

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Electrical Charges Cont

• An atom contains electric charges.
• The electron (e-), discovered by J.J. Thompson,
  contains a negative charge.
• The proton (p), discovered by Ernest Rutherford,
  contains a positive charge.
• An atom is considered neutral because the number
  of electrons balance out the number of protons.

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Electrical Charges Cont

• A material that allows electrons to move easily
  is called a conductor.
• A material that does not allow electrons to move
  easily is called an insulator.
• Electric charges are not created or destroyed,
  instead they are separated because electrons are
  transferred.
• Electric charges are represented by the symbol, q.

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Electrical Charges Cont

• Objects can be charged 2 ways.
• Conduction charging by direct contact with a
  charged object.
• When an object is rubbed, it is known as charging
  by friction.
• Induction charging by closeness to a charged
  object. There is no direct contact.

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CHAPTER 16 ELECTRIC FORCES FIELDS

• Section 16.2
• Electric Force

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Electrical Forces

• Electric forces are created between charged
  objects.
• There are 2 types of electrical charges
  positive and negative.
• These charges exert a force on each other
  depending on the distance between them.
• Charges are measured in a unit called a coulomb
  (C). 1 C 6.25 x 1018 electrons.
• 1 electron has a charge of 1.6 x 10-19 C

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Electrical Forces Cont

• Charles Coulomb studied the relationship between
  the strength of the force that existed between
  charged objects.
• His law states that the force varies inversely
  with the square of the distance.
• F kqq1 where k 9.0 x 109 Nm2/C
• d2 q and q1 charge on
  object in C
• d distance in meters
• Look familiar????

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Principle of Superposition

• This is used when you have more than 2 charges
  which is most of the time!!!
• The resultant force on any single charge is the
  vector sum of the individual charges that are
  present.

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Using the Principle of Superposition

• To find the resultant force
• Draw a picture, label the charges
• Find the magnitude of the forces using Coulombs
  Law.
• Find the net x and net y for each force (remember
  vectors and vector components?).
• Use Pythagorean Theorem to find magnitude.
• Use tan ? to find direction.

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EQUILIBRIUM

• When the net electric force on a charge is 0,
  this is the equilibrium position.
• To find this position, set the forces equal to
  each other (use Coulombs Law).
• Solve for whatever is asked (either the charge or
  distance).

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CHAPTER 16 ELECTRIC FORCES FIELDS

• Section 16.3
• The Electric Field

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What is an Electric Field?

• It was studied by Michael Faraday who found that
  like the gravitational force of attraction, the
  electric force varies inversely with the distance
  squared.
• Electric fields are real but are represented by
  field lines that are imaginary.
• Electric Fields are measured in N/C.

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More on Electric Fields

• The formula/definition for an electric field (E)
  is E F q
• where F electric force and q charge
• The magnitude of an electric field due to a point
  charge (q) is
• E kq
• r2

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Electric Lines of Force

• We represent electric fields by electric lines of
  force or electric field lines.
• The general rules regarding electric field
  lines are
• The closer together the lines of force, the
  stronger the electric field.
• The direction of the electric field is tangent to
  the lines of force.
• The field lines start at () charges and end at
  (-) charges.
• The lines of force leaving or entering a charge
  are proportional to the magnitude of the charge.

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More on Electric Lines of Force

• Electric dipoles consist of 2 separate electric
  charges (or poles as they were known).
• The field from a dipole decreases much faster
  than that due to a point charge as we move away
  from it.
• If we have 2 oppositely charged parallel plates,
  the electric field is calculated by
• E 4pkQ
• A
• Where Q total charge on plates and A area of
  one plate.

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Conductors and Electric Fields

• Electric fields associated with charged
  conductors have several properties.
• The electric field 0 everywhere inside a
  charged conductor.
• Any excess charge resides entirely on the surface
  of the conductor.
• The electric field at the surface of a charged
  conductor is perpendicular to the surface.
• Excess charges accumulates at sharp points or
  areas of greatest curvature so that the highest
  charge accumulations occur where electric field
  is largest.