Transcript Electric Fields

Electric Fields
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
.
What you already know
• Coulomb’s Law:
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Note:
is much smaller than
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Electric Field
• The electric field is the
that a
divided by itself.
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Note: The electric field is a
and therefore has magnitude and direction.
experiences
F
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.
The Test Charge (Cont.)
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Test Charge moves
from a
positively charged
object
Positively Charged
Object
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Test Charge moves
a
negatively charged
object
Negatively Charged
Object
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
nothing about the magnitude of charge
of the
produces the
that charge
says
, which is the
that
experiences.
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
they are, the
there are and the
the field.
together
Electric Field Lines Due to a
Point Charge
Direction of field is
Density of lines is
.
.
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Lines intersect
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Electric Dipole
Note: An electric dipole
consists of
in
and
in
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Note: Force on a
(
the field lines, and is the
of the forces due to
acting on it.
) is
of the dipole
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Examples from the Web
http://www.cco.caltech.edu/~phys1/java/phys1/EField/EField.html
http://phet.colorado.edu/sims/charges-and-fields/charges-and-fields_en.html
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.
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E-Field
inside conductor
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
.
Fe  qE
(kq)
Fe  qo 2
r
as the distance
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Electrostatic Force and Distance
• For parallel plates:
• The strength of the field is
plate to the other.
• Since
, the
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from one
will be
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Force and Electric Field Strength vs.
Distance
Point Charge
Parallel Plate
Distance
Distance
Distance
Distance
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?
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.