Electric Fields

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Electric Fields
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What You Will Learn

• An electric field is like a .
• An electric field is a .
• An electric field provides the that a
  will move if placed in the field.
• An electric field is composed of a series of
  imaginary .

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What you already know

• Coulombs Law

F


Note is much smaller than !
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Electric Field

• The electric field is the that a
  experiences divided by itself.
• Note The electric field is a and
  therefore has magnitude and direction.

F


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What is the

• It is enough that it does not affect the field
  due to other charged objects.
• It will move towards or away from other charged
  objects depending on whether it is charged
  similarly or dissimilarly from other objects.

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The Test Charge (Cont.)
_
_
Test Charge moves from a positively charged
object
Test Charge moves a negatively charged object
Negatively Charged Object
Positively Charged Object
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Example
A positive charge of 4.0 x 10-5 C experiences a
force of 0.36 N when located at a certain point.
What is the electric field intensity at that
point?
Note The says nothing about the
magnitude of charge , which is the of the
that produces the that charge
experiences.
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Electric Field Lines

• Electric field lines, or of , provide a
  of the at any point in space from a charge.
• Electric field lines show the that a
  would take if placed in the
  field.

_
_
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Electric Field Lines

• Electric field lines at and
  are always directed away from them towards .
• Electric field lines do or
  except at the of or charges.
• Electric field lines are ( ) to
  the where they or .
• Electric field lines .
• The strength of the field is proportional to the
  magnitude of the charge and is directly related
  to the of field lines
• the there are and the
  together they are, the the field.

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Electric Field Lines Due to a Point Charge
Direction of field is .
Density of lines is .
Lines intersect .
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Electric Dipole
Note An electric dipole consists of
in and in .
Note Force on a ( ) is the
field lines, and is the of the forces
due to of the dipole acting on it.
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Examples from the Web
http//www.cco.caltech.edu/phys1/java/phys1/EFiel
d/EField.html http//phet.colorado.edu/sims/charg
es-and-fields/charges-and-fields_en.html
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Electric Fields and Conductors

• At equilibrium, excess charges will reside on the
  of a conductor.
• The electric field is at any point within a
  conducting material at .
• Charge within a conductor is from external
  electric fields because they or on the
  where excess charges reside.

E
E-Field inside conductor
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Parallel Plate Capacitor

• The parallel plate capacitor is an device
  used in all kinds of electronics.
• Field lines in a parallel plate capacitor are
  and to one another, indicating a
  electric field.

- - - - - - - -

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Electrostatic Force and Distance

• For point charges
• The strength of the field as the distance .

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Electrostatic Force and Distance

• For parallel plates
• The strength of the field is from one
  plate to the other.
• Since , the will be .

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Force and Electric Field Strength vs. Distance

• Point Charge Parallel Plate

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Example (Millikan Oil Drop Exp.)
An oil drop is negatively charged and weighs 8.5
x  10-14 N. The drop is suspended in an electric
field intensity of 5.3 x 101 N/C. What is the
charge on the drop? How many electrons is
that?
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Key Ideas

• Electric fields exists around any conductor or
  insulator that contains a charge.
• The electric field intensity is a measure of the
  force on a test charge placed in the field.
• The strength of the field is proportional to the
  density of field lines.
• Field lines are perpendicular to all charged
  surfaces.
• The electric field is always directed away from
  positive charges and towards negative charges.