IB 5.2 Electric Field

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IB 5.2 Electric Field Potential
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Electric Field

• Just like mass create gravitational fields,
  charges create electric fields
• With gravity the field strength is measure as
  Newton per kilogram
• What do you think Electric fields are measured
  in?
• Newton's per Coulomb

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Measuring the Electric Field

• If the unit is Newton's per Coulomb, what is the
  equation?
• E F / qo
• Simple enough, right.
• E Electric Field
• F Force
• qo charge in the field

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Summing electric Fields

• Alternative Equation
• It is the surrounding charges that creates an
  electric field at a given point in space.
• Look at Example 8

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Picturing the Electric Field
Click on the picture to open an applet
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Electric Field Rules

• Fields start at positive and end at negative, or
  start and end at infinity.
• This is by convention. The field is said to
  predict the movement of a positive charge.
• The density of lines should represent the
  strength of the field.
• A positive charge will have a velocity tangent to
  a field line.
• Field lines do not actually exist since the are
  an infinite number of paths a test charge can
  take.

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Parallel Plate Capacitors

• Used to store charge.
• The most common type is a parallel plate
  capacitor.
• Charge is spread uniformly, not really,
  throughout the plate
• The electric field between two parallel plates is
  100 N C-1.
• What acceleration would a 2.0 mC charge with a
  mass of 10-3 kg experience if placed in the field?

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Electric Potential

• Consider a positive charge Q and a test charge q.
• Work must be done to move q closer to Q since
  like charges repel.
• V Voltage (Potential measured in Volts)

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Work and Route

• Regardless of the path taken to move the charge
  the work is always the same.
• Electric Potential Energy is U qV
• Potential Difference is DU

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Electric Fields Inside Conductors

• Excess electric charge moves to the surface of a
  conductor
• At equilibrium the electric field inside a
  conductor is zero
• This comes from the fact that free electrons will
  always be on the outside of the conductor.
• So the electric field lines dont penetrate the
  conductor
• The electric field outside a conductor is
  perpendicular to the surface

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