Ch 21 Electrostatics

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Ch 21 Electrostatics

• The study of the properties of stationary charges

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Properties of Charge

• Fundamental Property of Matter
• Two types of charge
• positive
• negative
• Like charges repel
• Unlike charges attract
• Charge is quantized (distinct fundamental units)
• e 1.602 10-19 C
• Coulomb (C) is the SI unit of charge
• Law of Conservation of Charge The total charge
  of any isolated system is a constant

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Charge Conservation
There are 92 protons on either side of this
nuclear reaction equation.
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Charge Conservation Particle Physics
The decay of a neutral photon into and electron
and a positron (positively charge electron) in
GREEN !!!
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Method of generating static charge

• Process of inducing a positive charge on a
  conducting sphere.
• Contact or induction on isolated (insulated)
  conductors

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Material Behaviors

• Insulator - large effort required to move charges
  through material
• Examples wood, plastic, rubber, glass, dry air,
  vacuum
• Conductor - very little effort required to move
  charges through material
• Examples most metals, ionic solutions, plasmas
• Semi-conductor - intermediate effort required to
  move charges through material
• Examples germanium, silicon

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Charge and Force
Insulators
Conductor and Inuslator
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Coulombs Law (Force Law between charges)

• Charles Auguste Coulomb - 1785
• Force between two point charges is proportional
  to each charge
• Force between two point charges is inversely
  proportional to the distance separating them
• Force between charges obey the Law of
  Superposition (vector addition)

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Coulombs Law (point Charges)
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Example 1

• A positive 1.0 mC charge is located at the
  origin and a -0.3 mC charge is located at x
  2.0 cm. What is the force on the negative charge?

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Example 2

• A positive 0.1 mC charge is located at the
  origin, a 0.2 mC charge is located at (0.0 cm,
  1.5 cm), and a -0.2 mC charge is located at (1.0
  cm, 0.0 cm). What is the force on the negative
  charge?

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Determine the force between the negative charge
and each positive charge.
The force between charges 1 and 2.
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The force between charges 3 and 2.
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Determine the components of each force.
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a
Determine the sum of the components of the forces.
Determine the resultant force
from x axis
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Sample prob 21-3

• What is force between spheres
• b) Ground sphere A, then what is the force ?

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Field

• Region of space influenced by some physical
  phenomena e.g., temperature, gravity, electric,
  magnetic behaviors

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Electric Field

• Region of space where another charge would be
  influenced by a charge or distribution of charges
• Units of electric field is Newtons/Coulomb (N/C)
• Electric Field points away from positive charge
  and toward negative charge

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Field of a point charge
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Effect of a finite test charge on a conductor