Electric Charge, Force and Field

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Electric Charge, Force and Field
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Properties of Charge

• Unlike charges attract, like charges repel
• Force between charges varies as an inverse square
  (same as gravity)
• Charge is conserved
• Charge is quantized
• p is 1800 heavier than e-

Benjamin Franklin
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Electrical Properties

• Conductor
• Allows easy current
• Metals
• Insulator
• Prevents current flow
• Plastics, glass, air
• Semiconductor
• Intermediate behavior
• Carbon, silicon, germanium

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Coulombs Law
e0 - Permittivity of free space
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Superposition

• The net force from multiple charges is additive.
• Ex. A charge of 0.02 C is located at the origin
  and a charge of -0.05 C is located at (10 m, 0
  m). What is the force exerted on a 0.03 C
  charge located at (5 m, 5 m)?

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Electric Field

• The force per unit charge experience by a charge.
• For a point charge the electric field is

Units (N/C)
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Drawing Electric Fields

• Lines begin on positive charges and end on
  negative charges.
• of lines leaving a charge is proportional to
  the magnitude of the charge.
• Therefore, the strength of the electric field is
  proportional to the density of the electric field
  lines.
• No two field lines can cross.

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Field Lines

• How would you draw the field lines for the
  following sets of charges?

Point Charge
Dipole
Like Charges
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Continuous Charge

• Use superposition to add all the infinitesimal
  charges, dq.
• If charge is uniformly distributed, then

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Electric Field on a Wire

• Ex. A wire has a length of 2 m and a charge of
  20 mC. What is the electric field at a point 4
  cm away from the center of the wire? (Assume the
  charge is uniformly distributed.)

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Charge in a Field

• Charge will accelerate in the direction of the
  electric field lines.
• Ex. What is the acceleration of an electron in a
  25 N/C electric field?

e-
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Torque on a Dipole

• Remember
• then

is a unit vector going from the negative
to the positive charge