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Electric Fields

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Title: Electric Fields


1
Electric Fields
2
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 .

3
What you already know
  • Coulombs Law


F


Note is much smaller than !
4
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


5
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.

6
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
7
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.
8
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.

_
_
9
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.

10
Electric Field Lines Due to a Point Charge
Direction of field is .
Density of lines is .
Lines intersect .
11
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.
12
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
13
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
14
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.

- - - - - - - -

15
Electrostatic Force and Distance
  • For point charges
  • The strength of the field as the distance .

16
Electrostatic Force and Distance
  • For parallel plates
  • The strength of the field is from one
    plate to the other.
  • Since , the will be .

17
Force and Electric Field Strength vs. Distance
  • Point Charge Parallel Plate

18
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?
19
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.
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